Dehydration Reaction

8/19/2019 Dehydration Reaction

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Dehydration reactionThis article is about chemical reactions resulting in the loss of water from a molecule. For the

removal of water from solvents and reagents, see Desiccation.

In chemistry and the biological sciences, a dehydration reaction is usually defined as a chemicalreaction that involves the loss of a water molecule from the reacting molecule. Dehydration reactions

are a subset of condensation reactions. Because the hydroxyl group (–O! is a poor leaving group,

having a Br"nsted acid catalyst often helps by protonating the hydroxyl group to give the better

leaving group, –O#$. %he reverse of a dehydration reaction is a hydration reaction. &ommon

dehydrating agents used in organic synthesis include concentrated sulfuric acid,

concentrated phosphoric acid, hot aluminium oxide and hot ceramic.

Dehydration reactions and dehydration synthesis have the same meaning, and are often used

interchangeably. %wo monosaccharides, such as glucose and fructose, can be 'oined together (to

form sucrose! using dehydration synthesis. %he new molecule, consisting of two monosaccharides,

is called a disaccharide.

%he process of hydrolysis is the reverse reaction, meaning that the water is recombined with the two

hydroxyl groups and the disaccharide reverts to being monosaccharides.

In the related condensation reaction water is released from two different reactants.

Dehydration reactions

In organic synthesis, there are many examples of dehydration reaction, for example dehydration of

alcohols or sugars.

Dehydration reactions

Reaction Equation

&onversion

of alcohols toethers

# )*O + )*

O*) $ #O

&onversion of

alcohols toalenes

)*&#*&O*)

+ )*&-&*)

$ #O

for example the conversion of glycerol to acrolein/0

or the dehydration of 2-methyl-1-cyclohexanol to

(mainly! 1-methylcyclohexene #0

https://en.wikipedia.org/wiki/Desiccationhttps://en.wikipedia.org/wiki/Desiccationhttps://en.wikipedia.org/wiki/Chemistryhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Waterhttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Leaving_grouphttps://en.wikipedia.org/wiki/Br%C3%B8nsted_acidhttps://en.wikipedia.org/wiki/Br%C3%B8nsted_acidhttps://en.wikipedia.org/wiki/Br%C3%B8nsted_acidhttps://en.wikipedia.org/wiki/Hydration_reactionhttps://en.wikipedia.org/wiki/Hydration_reactionhttps://en.wikipedia.org/wiki/Sulfuric_acidhttps://en.wikipedia.org/wiki/Sulfuric_acidhttps://en.wikipedia.org/wiki/Phosphoric_acidhttps://en.wikipedia.org/wiki/Phosphoric_acidhttps://en.wikipedia.org/wiki/Aluminium_oxidehttps://en.wikipedia.org/wiki/Aluminium_oxidehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Disaccharidehttps://en.wikipedia.org/wiki/Disaccharidehttps://en.wikipedia.org/wiki/Hydrolysishttps://en.wikipedia.org/wiki/Hydrolysishttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Organic_synthesishttps://en.wikipedia.org/wiki/Alcoholhttps://en.wikipedia.org/wiki/Alcoholhttps://en.wikipedia.org/wiki/Etherhttps://en.wikipedia.org/wiki/Alkenehttps://en.wikipedia.org/wiki/Glycerolhttps://en.wikipedia.org/wiki/Glycerolhttps://en.wikipedia.org/wiki/Acroleinhttps://en.wikipedia.org/wiki/Dehydration_reaction#cite_note-1https://en.wikipedia.org/wiki/Dehydration_reaction#cite_note-2https://en.wikipedia.org/wiki/Chemistryhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Chemical_reactionhttps://en.wikipedia.org/wiki/Waterhttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Leaving_grouphttps://en.wikipedia.org/wiki/Br%C3%B8nsted_acidhttps://en.wikipedia.org/wiki/Hydration_reactionhttps://en.wikipedia.org/wiki/Sulfuric_acidhttps://en.wikipedia.org/wiki/Phosphoric_acidhttps://en.wikipedia.org/wiki/Aluminium_oxidehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Disaccharidehttps://en.wikipedia.org/wiki/Hydrolysishttps://en.wikipedia.org/wiki/Condensation_reactionhttps://en.wikipedia.org/wiki/Organic_synthesishttps://en.wikipedia.org/wiki/Alcoholhttps://en.wikipedia.org/wiki/Etherhttps://en.wikipedia.org/wiki/Alkenehttps://en.wikipedia.org/wiki/Glycerolhttps://en.wikipedia.org/wiki/Acroleinhttps://en.wikipedia.org/wiki/Dehydration_reaction#cite_note-1https://en.wikipedia.org/wiki/Dehydration_reaction#cite_note-2https://en.wikipedia.org/wiki/Desiccation


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&onversion

of carboxylic

acidsto acid

anhydrides

#

)&OO

+

()&O!#

O $

#O

&onversion

of amides to

nitriles

)&O1

# +

)*&1

$ #O

Dienol

ben2ene

rearrangeme

nt

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5ome dehydration reactions can be mechanistically complex, for instance the reaction of

a sugar (sucrose! with concentrated sulfuric acid60 to form carbon as a graphitic foam involves

formation of carbon*carbon bonds.70 %he reaction is driven by the strongly exothermic reaction as

sulfuric acid reacts with water, which produces dangerous sulfuric*acid containing steam, therefore

the experiment should only be performed in a fume*hood or well ventilated area.

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Intramolecular Dehydration

A very important reaction by which alcohols can be converted to alkenesis intramolecular dehydration.

An equation for such a reaction is shown here (and in Example 10-a in yourworkbook.

H H

| |

H-C-C-H

| |

H OH

H2SO4

→ heat

H H

| |

H-C=C-H

+ H2O

!n this reaction the alcohol is heated inthe presence of sulfuric acid. "he -#$%roup on one carbon atom and thehydro%en atom on an adjacent carbonatom are split away from the moleculeand are essentially replaced by a doublebond. $ow concentrated the acid has tobe and how hot the mixture has to beheated depends on the particular alcoholthat is bein% dehydrated.

Another important thin% to note& somethin% that will become even more important

later& is that since this dehydration occurs within a molecule it iscalled intramolecular dehydration. "hat name is used to distin%uish this reaction froman intermolecular dehydration reaction in which the $- and the -#$ come from twodifferent molecules.

'otice that this is ust the opposite of thereaction in which an alkene is convertedto an alcohol (refer to Example )-a in yourworkbook. "he fact that this kind of a

reaction can %o in both directions usin%the same catalyst& tells you that we aredealin% with anequilibrium reaction.

*ecause of this& when you start with an alkene and chan%e it into an alcohol& youshould expect to end up with a mixture containin% some of the ori%inal alkene aswell as some of the alcohol bein% made. !f you start with an alcohol and chan%e itinto an alkene you would expect to have some of the ori%inal alcohol remainin% in


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solution.

+ou should also reali,e that the ratio of alkene to alcohol in the resultin% solution is%oin% to depend upon the conditions under which the solution is kept. onditionssuch as the concentration of acid& the temperature of the solution& and even theabundance of water will affect the equilibrium position and consequently the ratio of

alkene to alcohol in the reaction mixture.

As an aside& lets relate this to theequilibrium constant expressions that youworked with last term. !n reactions likethis the concentration of water will be lowenou%h to chan%e considerably as thereaction takes place. onsequently&you cannot simply ignore theconcentration of water as we did whenwe were dealin% with ionic reactions.

"hose earlier ionic reactions were takin% place in aqueous solution where theconcentration of water was so lar%e (about // moles per liter that the formation orreaction of water had very little effect upon the position of equilibrium. "hat is&the changes in the amount and concentration of water were so small compared towhat was there that they did not influence the equilibrium position very much."herefore& we didnt even include it in the equilibrium constant expressions. !f you

were to deal with the equilibrium expression of a reaction like this one above&you would have to take water into account.

As ! pointed out earlier& this kind ofdehydration reaction in which you form analkene from an alcohol is calledintramolecular dehydration because all ofthe atoms for the water molecule camefrom the same alcohol molecule. "he $

and #$ were on adacent carbon atomswithin the same molecule. "hisis intramolecular dehydration.


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Phosphodiester bond

Diagram of phosphodiester bonds (8O439! between nucleotides. :hich presents %hymine (T ! and two molecules

of ;denine ( A!.

with hydroxyl groups on other molecules

; phosphodiester bond occurs when exactly two of the hydroxyl groups in phosphoric acid react

with hydroxyl groups on other molecules to form two ester bonds./0

8hosphodiester bonds are central to all life on a near ?, they are negatively charged at p @citation needed 0. %his repulsion forces the

phosphates to tae opposite sides of the D1; strands and is neutrali2ed by proteins (histones!,metal ions such as magnesium, and polyamines.

In order for the phosphodiester bond to be formed and the nucleotides to be 'oined, the tri*phosphate

or di*phosphate forms of the nucleotide building blocs are broen apart to give off energy reAuired

to drive the en2yme*cataly2ed reaction. :hen a single phosphate or two phosphates nown

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as pyrophosphates brea away and cataly2e the reaction, the phosphodiester bond is formed. citation

needed 0

ydrolysis of phosphodiester bonds can be cataly2ed by the action of phosphodiesterases which

play an important role in repairing D1; seAuences.citation needed 0

In biological systems, the phosphodiester bond between two ribonucleotides can be broen

by alaline hydrolysis because of the free #=hydroxyl group.

Enzyme activity

; phosphodiesterase is an en2yme that cataly2es the hydrolysis of phosphodiester bonds, for

instance a bond in a molecule of cyclic ;8 or cyclic C8.

;n en2yme that plays an important role in the repair of oxidative D1; damage is the 3=*

phosphodiesterase.

During the replication of D1;, there is a hole between the phosphates in the bacbone left by D1;

polymerase I. D1; ligase is able to form a phosphodiester bond between the nucleotides.

Covalent bond

; covalent bond is a chemical bond that involves the sharing of electron pairs between atoms.( An

atom is the defining structure of anelement, which cannot be broken by any chemical

means. A typical atom consists of a nucleus

ofprotons andneutrons withelectrons orbiting this nucleus.)

%hese electron pairs are nown as shared pairs or bonding pairs, and the stable balance of

attractive and repulsive forces between atoms, when they share electrons, is nown as covalent

bonding./0better source needed 0 or many molecules, the sharing of electrons allows each atom to attain the

eAuivalent of a full outer shell, corresponding to a stable electronic configuration.

&ovalent bonding includes many inds of interactions, including E*bonding, F*bonding, metal*to*

metal bonding, agostic interactions, bent bonds, and three*center two*electron bonds.#030 %he

term covalent bond dates from /G3G.40 %he prefix co- means ointly, associated in action, !artnered to

a lesser degree, etc.H thus a co*valent bond, in essence, means that the atoms share valence,

such as is discussed in valence bond theory.

In the molecule

#, the hydrogen atoms share the two electrons via covalent bonding. 60 &ovalency is greatest

between atoms of similar electronegativities. %hus, covalent bonding does not necessarily reAuire

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that the two atoms be of the same elements, only that they be of comparable electronegativity.

&ovalent bonding that entails sharing of electrons over more than two atoms is said to

be delocali2ed.

History


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notation or electron dot notation or %ewis dot structure, in which valence electrons (those in the outer

shell! are represented as dots around the atomic symbols. 8airs of electrons located between atoms

represent covalent bonds. ultiple pairs represent multiple bonds, such as double bonds and triple

bonds. ;n alternative form of representation, not shown here, has bond*forming electron pairs

represented as solid lines.

Jewis proposed that an atom forms enough covalent bonds to form a full (or closed! outer electron

shell. In the methane diagram shown here, the carbon atom has a valence of four and is, therefore,

surrounded by eight electrons (the octet rule!, four from the carbon itself and four from the

hydrogens bonded to it.


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have strong bonds that hold the atoms together, but there are negligible forces of attraction between

molecules. 5uch covalent substances are usually gases, for example, &l, 5O #, &O#, and &4. In

molecular structures, there are wea forces of attraction. 5uch covalent substances are low*boiling*

temperature liAuids (such asethanol!, and low*melting*temperature solids (such as iodine and solid

&O#!. acromolecular structures have large numbers of atoms lined by covalent bonds in chains,including synthetic polymers such as polyethylene and nylon, and biopolymers such

as proteins and starch. 1etwor covalent structures (or giant covalent structures! contain large

numbers of atoms lined in sheets (such as graphite!, or 3*dimensional structures (such

as diamond and Auart2!. %hese substances have high melting and boiling points, are freAuently

brittle, and tend to have high electrical resistivity.


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olecules with odd*electron bonds are usually highly reactive. %hese types of bond are only stable

between atoms with similar electronegativities.//0

Resonance

&ain article' (esonance )chemistry*

%here are situations whereby a single Jewis structure is insufficient to explain the electron

configuration in a molecule, hence a superposition of structures are needed. %he same two atoms in

such molecules can be bonded differently in different structures (a single bond in one, a double bond

in another, or even none at all!, resulting in a non*integer bond order . %he nitrate ion is one such

example with three eAuivalent structures. %he bond between the nitrogen and each oxygen is a

double bond in one structure and a single bond in the other two, so that the average bond order for

each 1*O interaction is (# $ / $ /!L3 - 4L3.

Aromaticity

&ain article' Aromaticity

In organic chemistry, when a molecule with a planar ring obeys Mcel=s rule, where the number of F

electrons fit the formula 4n $ # (where n is an integer!, it attains extra stability and symmetry.

In ben2ene, the prototypical aromatic compound, there are 7 F bonding electrons (n - /, 4n $ # - 7!.

%hese occupy three delocali2ed F molecular orbitals (molecular orbital theory! or form con'ugate F

bonds in two resonance structures that linearly combine (valence bond theory!, creating a regular

hexagon exhibiting a greater stabili2ation than the hypothetical /,3,6*cyclohexatriene.

In the case of heterocyclic aromatics and substituted ben2enes, the electronegativity differences

between different parts of the ring may dominate the chemical behaviour of aromatic ring bonds,

which otherwise are eAuivalent.

Hypervalence

&ain article' +y!ervalent molecule

&ertain molecules such as xenon difluoride and sulfur hexafluoride have higher co*ordination

numbers than would be possible due to strictly covalent bonding according to theoctet rule. %his is

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explained by the three*center four*electron bond (3c–4e! model in molecular orbital theory and

ionic*covalent resonance in valence bond theory.

Electron-deficiency

&ain article' lectron deficiency

In three*center two*electron bonds (3c–#e! three atoms share two electrons in bonding. %his type

of bonding occurs in electron deficient compounds lie diborane.


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% Of two orbitals in an atom, the one that can overlap the most with an orbital from another

atom will form the strongest bond, and this bond will tend to lie in the direction of the

concentrated orbital.

Building on this article, Pauling's 1939 textbook: On the Nature of the Chemical

Bond would become what some have called the "Bible" of modern chemistr! his

book hel#ed ex#erimental chemists to understand the im#act of $uantum theor on

chemistr! %owever, the later edition in 19&9 failed to ade$uatel address the

#roblems that a##eared to be better understood b molecular orbital theor! he

im#act of valence theor declined during the 19(s and 19)(s as molecular orbital

theor grew in usefulness as it was im#lemented in large digital computer #rograms!

*ince the 19+(s, the more dicult #roblems, of im#lementing valence bond theor

into com#uter #rograms, have been solved largel, and valence bond theor has

seen a resurgence!

olecular orbital theory

Main article: &olecular orbital theory

-olecular orbitals were .rst introduced b riedrich und/#0/30 and )obert 5. ullien/40/60 in

19/) and 19/+!/70/@0 he linear combination of atomic orbitals or "02" a##roximation for

molecular orbitals was introduced in 19/9 b 5ir ohn Jennard*ones!/K0 0inear

combinations of atomic orbitals 4025 can be used to estimate the molecular

orbitals that are formed u#on bonding between the molecule's constituent atoms!

*imilar to an atomic orbital, a *chr6dinger e$uation, which describes the behavior

of an electron, can be constructed for a molecular orbital as well! 0inear

combinations of atomic orbitals, or the sums and di7erences of the atomic

wavefunctions, #rovide a##roximate solutions to the artree–oc eAuations whichcorres#ond to the inde#endent8#article a##roximation of the molecular 5chrPdinger

eAuation!

hen atomic orbitals interact, the resulting molecular orbital can be of three t#es:

bonding, antibonding, or nonbonding!

Bonding -s:

Bonding interactions between atomic orbitals are constructive 4in8#hase5

interactions!

Bonding -s are lower in energ than the atomic orbitals that combine to #roduce

them!

2ntibonding -s:

2ntibonding interactions between atomic orbitals are destructive 4out8of8#hase5

interactions, with a nodal plane where the wavefunction of the antibonding orbital is

ero between the two interacting atoms

https://en.wikipedia.org/wiki/Digital_computerhttps://en.wikipedia.org/wiki/Digital_computerhttps://en.wikipedia.org/wiki/Molecular_orbital_theoryhttps://en.wikipedia.org/wiki/Friedrich_Hundhttps://en.wikipedia.org/wiki/Friedrich_Hundhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-12https://en.wikipedia.org/wiki/Covalent_bond#cite_note-12https://en.wikipedia.org/wiki/Covalent_bond#cite_note-13https://en.wikipedia.org/wiki/Robert_S._Mullikenhttps://en.wikipedia.org/wiki/Robert_S._Mullikenhttps://en.wikipedia.org/wiki/Robert_S._Mullikenhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-14https://en.wikipedia.org/wiki/Covalent_bond#cite_note-15https://en.wikipedia.org/wiki/Covalent_bond#cite_note-16https://en.wikipedia.org/wiki/Covalent_bond#cite_note-17https://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitalshttps://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitalshttps://en.wikipedia.org/wiki/John_Lennard-Joneshttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-18https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_methodhttps://en.wikipedia.org/wiki/Schr%C3%B6dinger_equationhttps://en.wikipedia.org/wiki/Schr%C3%B6dinger_equationhttps://en.wikipedia.org/wiki/Node_(physics)https://en.wikipedia.org/wiki/Digital_computerhttps://en.wikipedia.org/wiki/Molecular_orbital_theoryhttps://en.wikipedia.org/wiki/Friedrich_Hundhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-12https://en.wikipedia.org/wiki/Covalent_bond#cite_note-13https://en.wikipedia.org/wiki/Robert_S._Mullikenhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-14https://en.wikipedia.org/wiki/Covalent_bond#cite_note-15https://en.wikipedia.org/wiki/Covalent_bond#cite_note-16https://en.wikipedia.org/wiki/Covalent_bond#cite_note-17https://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitalshttps://en.wikipedia.org/wiki/John_Lennard-Joneshttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-18https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_methodhttps://en.wikipedia.org/wiki/Schr%C3%B6dinger_equationhttps://en.wikipedia.org/wiki/Schr%C3%B6dinger_equationhttps://en.wikipedia.org/wiki/Node_(physics)


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2ntibonding -s are higher in energ than the atomic orbitals that combine to

#roduce them!

;onbonding -s:

;onbonding -s are the result of no interaction between atomic orbitals because of

lack of com#atible smmetries!

;onbonding -s will have the same energ as the atomic orbitals of one of the

atoms in the molecule!

&omparison

he two theories di7er in the order that the electron configuration of the molecule is

built u#!/G0 ither theor has its advantages and uses! 2s valence bond theor builds the

molecular wavefunction out of localied bonds, it is more suited for the calculation

of bond energiesand the understanding of reaction mechanisms! =n #articular, valence

bond theor correctl #redicts the dissociation of homonuclear diatomic molecules

into se#arate atoms, while sim#le molecular orbital theor #redicts dissociation into

a mixture of atoms and ions! -olecular orbital theor, with delocalied orbitals that

obe its smmetr, is more suited for the calculation of ioni2ation energies and the

understanding of s#ectral absorption bands! -olecular orbitals are orthogonal, which

signi.cantl increases feasibilit and s#eed of com#uter calculations com#ared to

nonorthogonal valence bond orbitals!

2lthough the wavefunctions generated b both theories do not agree and do not

match the stabiliation energ b ex#eriment, the can be corrected b

con.guration interaction!/G0 his is done b combining the valence bond covalent

function with the functions describing all #ossible ionic con.gurations or b

combining the molecular orbital ground state function with the functions describing

all #ossible excited states using unoccu#ied orbitals! =t can then be seen that the

sim#le molecular orbital a##roach gives too much weight to the ionic structureswhile the sim#le valence bond a##roach gives too little! his can also be described

as saing that the molecular orbital a##roach neglects electron correlation while the

valence bond a##roach overestimates it! /G0

he two a##roaches are now regarded as com#lementar, each #roviding its own

insights into the #roblem of chemical bonding! -odern calculations in Auantum

chemistryusuall start from 4but ultimatel go far beond5 a molecular orbital rather

https://en.wikipedia.org/wiki/Electron_configurationhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Orbital_hybridisationhttps://en.wikipedia.org/wiki/Orbital_hybridisationhttps://en.wikipedia.org/wiki/Resonance_(chemistry)https://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitalshttps://en.wikipedia.org/wiki/Aufbau_principlehttps://en.wikipedia.org/wiki/Bond_energyhttps://en.wikipedia.org/wiki/Bond_energyhttps://en.wikipedia.org/wiki/Reaction_mechanismhttps://en.wikipedia.org/wiki/Ionization_energyhttps://en.wikipedia.org/wiki/Ionization_energyhttps://en.wikipedia.org/wiki/Absorption_bandhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Quantum_chemistryhttps://en.wikipedia.org/wiki/Quantum_chemistryhttps://en.wikipedia.org/wiki/Electron_configurationhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Orbital_hybridisationhttps://en.wikipedia.org/wiki/Resonance_(chemistry)https://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitalshttps://en.wikipedia.org/wiki/Aufbau_principlehttps://en.wikipedia.org/wiki/Bond_energyhttps://en.wikipedia.org/wiki/Reaction_mechanismhttps://en.wikipedia.org/wiki/Ionization_energyhttps://en.wikipedia.org/wiki/Absorption_bandhttps://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Covalent_bond#cite_note-Quanta-19https://en.wikipedia.org/wiki/Quantum_chemistryhttps://en.wikipedia.org/wiki/Quantum_chemistry


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than a valence bond a##roach, not because of an intrinsic su#eriorit in the former

but rather because the - a##roach is more readil ada#ted to numerical

com#utations! %owever, better valence bond #rograms are now available!

2 molecule is the smallest #article in achemical element or com#ound that has the chemical #ro#erties of thatelement or com#ound! -olecules are made u# of atom s that are heldtogether b chemical bonds! hese bonds form as a result of the sharing orexchange of electron s among atoms!

he atoms of certain elements readil bond with other atoms to formmolecules! >xam#les of such elements are oxgen and chlorine! he atomsof some elements do not easil bond with other atoms! >xam#les are neonand argon!

-olecules can var greatl in sie and com#lexit! he element helium is aone8atom molecule! *ome molecules consist of two atoms of the sameelement!


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&ugar is the generali2ed name for sweet, short*chain, soluble carbohydrates, many of which are used in

food. %hey arecarbohydrates, composed of carbon, hydrogen, and oxygen. %here are various types

of sugar derived from different sources. 5imple sugars are called monosaccharides and

include glucose (also nown as dextrose!, fructose andgalactose. %he table or granulated sugar

most customarily used as food is sucrose, a disaccharide. (In the body, sucrose hydrolyses intofructose and glucose.! Other disaccharides include maltose and lactose. Jonger chains of sugars

are called oligosaccharides. &hemically*different substances may also have a sweet taste, but are

not classified as sugars. 5ome are used as lower*calorie food substitutes for sugar described

as artificial sweeteners.

5ugars are found in the tissues of most plants, but are present in sufficient concentrations for

efficient extraction only insugarcane and sugar beet.citation needed 0 5ugarcane refers to any of several

species of giant grass in the genus$accharum that have been cultivated in tropical climates in 5outh

;sia and 5outheast ;sia since ancient times. ; great expansion in its production too place in the

/Kth century with the establishment of sugar plantations in the :est Indies and ;mericas. %his was

the first time that sugar became available to the common people, who had previously had to rely on

honey to sweeten foods. 5ugar beet, a cultivated variety of eta vulgaris, is grown as a root crop in

cooler climates and became a ma'or source of sugar in the /Gth century when methods for

extracting the sugar became available. 5ugar production and trade have changed the course of

human history in many ways, influencing the formation of colonies, the perpetuation of slavery, the

transition to indentured labour, the migration of peoples, wars between sugar*trade–controlling

nations in the /Gth century, and the ethnic composition and political structure of the 1ew :orld.

5ucrose a disaccharide of glucose (left! and fructose(right!, important molecules in the body.

RiboseD-Ribose

https://en.wikipedia.org/wiki/Carbohydratehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Galactosehttps://en.wikipedia.org/wiki/Sucrosehttps://en.wikipedia.org/wiki/Disaccharidehttps://en.wikipedia.org/wiki/Maltosehttps://en.wikipedia.org/wiki/Lactosehttps://en.wikipedia.org/wiki/Lactosehttps://en.wikipedia.org/wiki/Oligosaccharideshttps://en.wikipedia.org/wiki/Oligosaccharideshttps://en.wikipedia.org/wiki/Low-caloriehttps://en.wikipedia.org/wiki/Artificial_sweetenerhttps://en.wikipedia.org/wiki/Artificial_sweetenerhttps://en.wikipedia.org/wiki/Sugarcanehttps://en.wikipedia.org/wiki/Sugar_beethttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Saccharumhttps://en.wikipedia.org/wiki/South_Asiahttps://en.wikipedia.org/wiki/South_Asiahttps://en.wikipedia.org/wiki/Southeast_Asiahttps://en.wikipedia.org/wiki/Cultivarhttps://en.wikipedia.org/wiki/Beta_vulgarishttps://en.wikipedia.org/wiki/Beta_vulgarishttps://en.wikipedia.org/wiki/Beta_vulgarishttps://en.wikipedia.org/wiki/Slaveryhttps://en.wikipedia.org/wiki/New_Worldhttps://en.wikipedia.org/wiki/New_Worldhttps://en.wikipedia.org/wiki/Sucrosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Carbohydratehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructosehttps://en.wikipedia.org/wiki/Galactosehttps://en.wikipedia.org/wiki/Sucrosehttps://en.wikipedia.org/wiki/Disaccharidehttps://en.wikipedia.org/wiki/Maltosehttps://en.wikipedia.org/wiki/Lactosehttps://en.wikipedia.org/wiki/Oligosaccharideshttps://en.wikipedia.org/wiki/Low-caloriehttps://en.wikipedia.org/wiki/Artificial_sweetenerhttps://en.wikipedia.org/wiki/Sugarcanehttps://en.wikipedia.org/wiki/Sugar_beethttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Saccharumhttps://en.wikipedia.org/wiki/South_Asiahttps://en.wikipedia.org/wiki/South_Asiahttps://en.wikipedia.org/wiki/Southeast_Asiahttps://en.wikipedia.org/wiki/Cultivarhttps://en.wikipedia.org/wiki/Beta_vulgarishttps://en.wikipedia.org/wiki/Slaveryhttps://en.wikipedia.org/wiki/New_Worldhttps://en.wikipedia.org/wiki/Sucrosehttps://en.wikipedia.org/wiki/Glucosehttps://en.wikipedia.org/wiki/Fructose


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Chemical formula C5H10O5

Ribose is a carbohydrate with the formula &6/?O6H specifically, it isa pentose monosaccharide (simple sugar ! with linear form 9(&-O!9(&O!49, which has allthe hydroxyl groups on the same side in the ischer pro'ection.

%he term may refer to either of two enantiomers. %he term usually indicates D-ribose, which occurswidely in nature and is discussed here. Its synthetic mirror image, '-ribose, is not found in nature.

D*)ibose was first reported in /KG/ by


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%he SD*T in the name D*ribose refers to the stereochemistry of the chiral carbon atom farthest away

from the aldehyde group (&4=!. InD*ribose, as in all D*sugars, this carbon atom has the same

configuration as in D *glyceraldehyde.

•

U*D*)ibopyranose

•

Q*D*)ibopyranose

•

U*D*)ibofuranose

•

Q*D*)ibofuranose

https://en.wikipedia.org/wiki/Stereochemistryhttps://en.wikipedia.org/wiki/Chirality_(chemistry)https://en.wikipedia.org/wiki/Chirality_(chemistry)https://en.wikipedia.org/wiki/D-glyceraldehydehttps://en.wikipedia.org/wiki/D-glyceraldehydehttps://en.wikipedia.org/wiki/D-glyceraldehydehttps://en.wikipedia.org/wiki/Stereochemistryhttps://en.wikipedia.org/wiki/Chirality_(chemistry)https://en.wikipedia.org/wiki/D-glyceraldehyde


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)elative abundance of different forms of ribose in solution Q*D*ribopyranose (6GV!, U*D*

ribopyranose (#?V!, Q*D*ribofuranose (/3V!, U*D*ribofuranose (@V! and open chain (?./V!. 60

Deoyribose

This article is about the naturally-occurring D-form of deoxyribose. For the %-form, see %-deoxyribose.

D-deo(yribose

)ames

IW8;& name

#*deoxy*D*ribose

Deo(yribose, or more precisely !-deo(yribose, is a monosaccharide with ideali2ed formula 9

(&-O!9(&#!9(&O!39. Its name indicates that it is a deoxy sugar , meaning that it is derived from

the sugar ribose by loss of an oxygen atom. 5ince the pentose sugars arabinose and ribose only

differ by the stereochemistry at , #*deoxyribose and #*deoxyarabinose are eAuivalent, although

the latter term is rarely used because ribose, not arabinose, is the precursor to deoxyribose.

Structureedit0

5everal isomers exist with the formula 9(&-O!9(&#!9(&O!39, but in deoxyribose all

the hydroxyl groups are on the same side in the ischer pro'ection. %he term #*deoxyribose may

refer to either of two enantiomers the biologically important D*#*deoxyribose and to the rarely

encountered mirror image J *#*deoxyribose.#0 D*#*deoxyribose is a precursor to the nucleic acid D1;.

#*deoxyribose is an aldopentose, that is, a monosaccharide with five carbon atoms and having

an aldehyde functional group.

https://en.wikipedia.org/wiki/Ribose#cite_note-5https://en.wikipedia.org/wiki/L-deoxyribosehttps://en.wikipedia.org/wiki/Chemical_nomenclaturehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Deoxy_sugarhttps://en.wikipedia.org/wiki/Sugarhttps://en.wikipedia.org/wiki/Sugarhttps://en.wikipedia.org/wiki/Ribosehttps://en.wikipedia.org/wiki/Ribosehttps://en.wikipedia.org/wiki/Oxygenhttps://en.wikipedia.org/wiki/Arabinosehttps://en.wikipedia.org/wiki/Arabinosehttps://en.wikipedia.org/w/index.php?title=Deoxyribose&action=edit&section=1https://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Fischer_projectionhttps://en.wikipedia.org/wiki/Enantiomerhttps://en.wikipedia.org/wiki/L-Deoxyribosehttps://en.wikipedia.org/wiki/L-Deoxyribosehttps://en.wikipedia.org/wiki/L-Deoxyribosehttps://en.wikipedia.org/wiki/Deoxyribose#cite_note-moens-2https://en.wikipedia.org/wiki/Deoxyribose#cite_note-moens-2https://en.wikipedia.org/wiki/Nucleic_acidhttps://en.wikipedia.org/wiki/Nucleic_acidhttps://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/Aldopentosehttps://en.wikipedia.org/wiki/Aldopentosehttps://en.wikipedia.org/wiki/Carbonhttps://en.wikipedia.org/wiki/Carbonhttps://en.wikipedia.org/wiki/Atomhttps://en.wikipedia.org/wiki/Atomhttps://en.wikipedia.org/wiki/Aldehydehttps://en.wikipedia.org/wiki/Aldehydehttps://en.wikipedia.org/wiki/Aldehydehttps://en.wikipedia.org/wiki/Functional_grouphttps://en.wikipedia.org/wiki/Ribose#cite_note-5https://en.wikipedia.org/wiki/L-deoxyribosehttps://en.wikipedia.org/wiki/Chemical_nomenclaturehttps://en.wikipedia.org/wiki/Monosaccharidehttps://en.wikipedia.org/wiki/Deoxy_sugarhttps://en.wikipedia.org/wiki/Sugarhttps://en.wikipedia.org/wiki/Ribosehttps://en.wikipedia.org/wiki/Oxygenhttps://en.wikipedia.org/wiki/Arabinosehttps://en.wikipedia.org/w/index.php?title=Deoxyribose&action=edit&section=1https://en.wikipedia.org/wiki/Hydroxylhttps://en.wikipedia.org/wiki/Fischer_projectionhttps://en.wikipedia.org/wiki/Enantiomerhttps://en.wikipedia.org/wiki/L-Deoxyribosehttps://en.wikipedia.org/wiki/Deoxyribose#cite_note-moens-2https://en.wikipedia.org/wiki/Nucleic_acidhttps://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/Aldopentosehttps://en.wikipedia.org/wiki/Carbonhttps://en.wikipedia.org/wiki/Atomhttps://en.wikipedia.org/wiki/Aldehydehttps://en.wikipedia.org/wiki/Functional_group


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In aAueous solution, deoxyribose primarily exists as a mixture of three structures the linear form 9

(&-O!9(&#!9(&O!39 and two ring forms, deoxyribofuranose (&3X*endo!, with a five*membered

ring, and deoxyribopyranose (*endo!, with a six*membered ring. %he latter form is predominant

(whereas the &3X*endo form is favored for ribose!.

&hemical eAuilibrium of deoxyribose in solution

!iolo"ical importanceedit0

;s a component of D1;, #*deoxyribose derivatives have an important role in biology.30 %he D1; (deoxyribonucleic acid! molecule, which is the main repository of genetic information in

life, consists of a long chain of deoxyribose*containing units called nucleotides, lined

via phosphate groups. In the standard nucleic acid nomenclature, a D1; nucleotide consists of a

deoxyribose molecule with an organic base (usually adenine, thymine, guanine or cytosine! attached

to the /X ribose carbon. %he 6X hydroxyl of each deoxyribose unit is replaced by a phosphate (forming

a nucleotide! that is attached to the 3X carbon of the deoxyribose in the preceding unit.

%he absence of the #X hydroxyl group in deoxyribose is apparently responsible for the increased

mechanical flexibility of D1; compared to )1;, which allows it to assume the double*helix

conformation, and also (in the euaryotes! to be compactly coiled within the small cell nucleus. %he

double*stranded D1; molecules are also typically much longer than )1; molecules. %he bacbone

of )1; and D1; are structurally similar, but )1; is single stranded, and made from ribose as

opposed to deoxyribose.

Other biologically important derivatives of deoxyribose include mono*, di*, and triphosphates, as well

as 3X*6X cyclic monophosphates.

Cyclic compound

; cyclic compound (ring com!ound ! is a term for a compound in the field of chemistry in which one

or more series of atoms in the compound is connected to form a ring. )ings may vary in si2e from

three to many atoms, and include examples where all the atoms are carbon (i.e., are carbocycles!,

none of the atoms are carbon (inorganic cyclic compounds!, or where both carbon and non*carbon

atoms are present (heterocyclic compounds!. Depending on the ring si2e, the bond order of the

individual lins between ring atoms, and their arrangements within the rings, carbocyclic and

https://en.wikipedia.org/w/index.php?title=Deoxyribose&action=edit&section=2https://en.wikipedia.org/wiki/Deoxyribose#cite_note-3https://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/Geneticshttps://en.wikipedia.org/wiki/Nucleotideshttps://en.wikipedia.org/wiki/Nucleotideshttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Nucleic_acid_nomenclaturehttps://en.wikipedia.org/wiki/Base_(chemistry)https://en.wikipedia.org/wiki/Adeninehttps://en.wikipedia.org/wiki/Adeninehttps://en.wikipedia.org/wiki/Thyminehttps://en.wikipedia.org/wiki/Guaninehttps://en.wikipedia.org/wiki/Cytosinehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Nucleotidehttps://en.wikipedia.org/wiki/Eukaryotehttps://en.wikipedia.org/wiki/Cell_nucleushttps://en.wikipedia.org/wiki/Chemical_compoundhttps://en.wikipedia.org/wiki/Chemical_compoundhttps://en.wikipedia.org/wiki/Chemistryhttps://en.wikipedia.org/wiki/Chemistryhttps://en.wikipedia.org/wiki/Ring_(chemistry)https://en.wikipedia.org/wiki/Carbocyclehttps://en.wikipedia.org/wiki/Heterocyclichttps://en.wikipedia.org/wiki/Bond_orderhttps://en.wikipedia.org/wiki/Bond_orderhttps://en.wikipedia.org/wiki/Bond_orderhttps://en.wikipedia.org/w/index.php?title=Deoxyribose&action=edit&section=2https://en.wikipedia.org/wiki/Deoxyribose#cite_note-3https://en.wikipedia.org/wiki/DNAhttps://en.wikipedia.org/wiki/Geneticshttps://en.wikipedia.org/wiki/Nucleotideshttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Nucleic_acid_nomenclaturehttps://en.wikipedia.org/wiki/Base_(chemistry)https://en.wikipedia.org/wiki/Adeninehttps://en.wikipedia.org/wiki/Thyminehttps://en.wikipedia.org/wiki/Guaninehttps://en.wikipedia.org/wiki/Cytosinehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Nucleotidehttps://en.wikipedia.org/wiki/Eukaryotehttps://en.wikipedia.org/wiki/Cell_nucleushttps://en.wikipedia.org/wiki/Chemical_compoundhttps://en.wikipedia.org/wiki/Chemistryhttps://en.wikipedia.org/wiki/Ring_(chemistry)https://en.wikipedia.org/wiki/Carbocyclehttps://en.wikipedia.org/wiki/Heterocyclichttps://en.wikipedia.org/wiki/Bond_order


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heterocyclic compounds may be aromatic or non*aromatic, in the latter case, they may vary from

being fullysaturated to having varying numbers of multiple bonds between the ring atoms. Because

of the tremendous diversity allowed, in combination, by the valences of common atoms and their

ability to form rings, the number of possible cyclic structures, even of small si2e (e.g., Y/@ total

atoms! numbers in the many billions.

*yclic compound e(amples+ All-carbon ,carbocyclic and more comple( natural cyclic

compounds

•

Ingenol, a complex, terpenoid natural product, related to but simpler than the paclitaxel that follows,

which displays a complex ring structure including 3*, 6*, and @*membered non*aromatic, carbocyclic

rings.

•

&ycloalanes, the simplest carbocycles, including cyclopropane, cyclobutane,cyclopentane,

and cyclohexane. 1ote, elsewhere an organic chemistry shorthand is used where hydrogen atoms are

inferred as present to fill the carbon=s valence of 4 (rather than their being shown explicitly!.

•

8aclitaxel, another complex, plant*derivedterpenoid, also a natural product, displaying a complex multi*

ring structure including 4*, 7*, and K*membered rings (carbocyclic andheterocyclic, aromatic and non*

aromatic!.

https://en.wikipedia.org/wiki/Aromatic_compoundhttps://en.wikipedia.org/wiki/Aromatic_compoundhttps://en.wikipedia.org/wiki/Saturated_compoundhttps://en.wikipedia.org/wiki/Valence_(chemistry)https://en.wikipedia.org/wiki/Valence_(chemistry)https://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Ingenolhttps://en.wikipedia.org/wiki/Ingenolhttps://en.wikipedia.org/wiki/Terpenehttps://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Aromatichttps://en.wikipedia.org/wiki/Cycloalkanehttps://en.wikipedia.org/wiki/Carbocyclehttps://en.wikipedia.org/wiki/Carbocyclehttps://en.wikipedia.org/wiki/Cyclopropanehttps://en.wikipedia.org/wiki/Cyclobutanehttps://en.wikipedia.org/wiki/Cyclopentanehttps://en.wikipedia.org/wiki/Cyclopentanehttps://en.wikipedia.org/wiki/Cyclopentanehttps://en.wikipedia.org/wiki/Cyclohexanehttps://en.wikipedia.org/wiki/Organic_chemistryhttps://en.wikipedia.org/wiki/Organic_chemistryhttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Terpenehttps://en.wikipedia.org/wiki/Heterocyclichttps://en.wikipedia.org/wiki/Heterocyclichttps://en.wikipedia.org/wiki/Aromatichttps://en.wikipedia.org/wiki/Aromatic_compoundhttps://en.wikipedia.org/wiki/Saturated_compoundhttps://en.wikipedia.org/wiki/Valence_(chemistry)https://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Ingenolhttps://en.wikipedia.org/wiki/Terpenehttps://en.wikipedia.org/wiki/Natural_producthttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Aromatichttps://en.wikipedia.org/wiki/Cycloalkanehttps://en.wikipedia.org/wiki/Carbocyclehttps://en.wikipedia.org/wiki/Cyclopropanehttps://en.wikipedia.org/wiki/Cyclobutanehttps://en.wikipedia.org/wiki/Cyclopentanehttps://en.wikipedia.org/wiki/Cyclohexanehttps://en.wikipedia.org/wiki/Organic_chemistryhttps://en.wikipedia.org/wiki/Paclitaxelhttps://en.wikipedia.org/wiki/Terpenehttps://en.wikipedia.org/wiki/Heterocyclichttps://en.wikipedia.org/wiki/Aromatic


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;dding to their complexity and number, closing of atoms into rings may loc particular atoms with

distinct substitution (by functional groups! such that stereochemistry andchirality of the compound

results, including some manifestations that are uniAue to rings (e.g., configurational isomers!. ;s

well, depending on ring si2e, the three*dimensional shapes of particular cyclic structuresRtypically

rings of 6*atoms and largerRcan vary and interconvert such that conformational isomerism isdisplayed. Indeed, the development of this important chemical concept arose, historically, in

reference to cyclic compounds. inally, cyclic compounds, because of the uniAue shapes,

reactivities, properties, andbioactivities that they engender, are the largest ma'ority of all molecules

involved in the biochemistry, structure, and function of living organisms, and in the man*made

molecules (e.g., drugs, herbicides, etc.!.

eterocyclic &ompounds

&ompounds classified as heterocyclic probably constitute the largest and most varied family of

organic compounds. ;fter all, every carbocyclic compound, regardless of structure andfunctionality, may in principle be converted into a collection of heterocyclic analogs by replacing

one or more of the ring carbon atoms with a different element.


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;n easy to remember, but limited, nomenclature system maes use of an elemental prefix for

the heteroatom followed by the appropriate carbocyclic name. ; short list of some common

prefixes is given in the following table, priority order increasing from right to left.


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highest priority atom is \/ and continues in the direction that gives the next priority atom the

lowest number.

;ll the previous examples have been monocyclic compounds. 8olycyclic compounds

incorporating one or more heterocyclic rings are well nown. ; few of these are shown in the

following diagram. ;s before, common names are in blac and systematic names in blue. %he

two Auinolines illustrate another nuance of heterocyclic nomenclature. %hus, the location of a

fused ring may be indicated by a lowercase letter which designates the edge of the heterocyclic

ring involved in the fusion, as shown by the pyridine ring in the green shaded box.

eterocyclic rings are found in many naturally occurring compounds. ost notably, they

compose the core structures of mono and polysaccharides, and the four D1; bases that

establish the genetic code. By clicing on the above diagram some other examples of

heterocyclic natural products will be displayed.

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0reparation and Reactions

Three-Membered RingsOxiranes (epoxides! are the most commonly encountered three*membered heterocycles.


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base*cataly2ed reactions normally proceed by 6*exo*substitution (reaction /!, yielding a

tetrahydrofuran product. owever, if the oxirane has an unsaturated substituent (vinyl or

phenyl!, the acid*cataly2ed opening occurs at the allylic (or ben2ylic! carbon (reaction #! in a 7*

endo fashion. %he F*electron system of the substituent assists development of positive charge

at the ad'acent oxirane carbon, directing nucleophilic attac to that site.

Four-Membered Rings

0reparation5everal methods of preparing four*membered heterocyclic compounds are shown in the

following diagram. %he simple procedure of treating a 3*halo alcohol, thiol or amine with base is

generally effective, but the yields are often mediocre. Dimeri2ation and elimination are common

side reactions, and other functions may compete in the reaction. In the case of example /,

cycli2ation to an oxirane competes with thietane formation, but the greater nucleophilicity of

sulfur dominates, especially if a wea base is used. In example # both a2iridine and a2etidine

formation are possible, but only the former is observed. %his is a good example of the inetic

advantage of three*membered ring formation.


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Reactions

)eactions of four*membered heterocycles also show the influence of ring strain. 5ome

examples are given in the following diagram. ;cid*catalysis is a common feature of many ring*

opening reactions, as shown by examples /, # [ 3a. In the thietane reaction (#!, the sulfur

undergoes electrophilic chlorination to form a chlorosulfonium intermediate followed by a ring*

opening chloride ion substitution. 5trong nucleophiles will also open the strained ether, as

shown by reaction 3b. &leavage reactions of Q*lactones may tae place either by acid*cataly2ed

acyl exchange, as in 4a, or by alyl*O rupture by nucleophiles, as in 4b.


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Five-Membered Rings 0reparation

&ommercial preparation of furan proceeds by way of the aldehyde, furfural, which in turn isgenerated from pentose containing raw materials lie corncobs, as shown in the uppermost

eAuation below. 5imilar preparations of pyrrole and thiophene are depicted in the second row

eAuations. norr synthesis.

any other procedures leading to substituted heterocycles of this ind have been devised. %wo

of these are shown in reactions # and 3. uran is reduced to tetrahydrofuran by palladium*

cataly2ed hydrogenation. %his cyclic ether is not only a valuable solvent, but it is readily

converted to /,4*dihalobutanes or 4*haloalylsulfonates, which may be used to prepare

pyrrolidine and thiolane.

Dipolar cycloaddition reactions often lead to more complex five*membered heterocycles.

Indole is probably the most important fused ring heterocycle in this class. By clicing on the

above diagram three examples of indole synthesis will be displayed. %he first proceeds by an

electrophilic substitution of a nitrogen*activated ben2ene ring. %he second presumably taes

place by formation of a dianionic species in which the ;r& #(–! unit bonds to the deactivated

carbonyl group. inally, the ischer indole synthesis is a remarable seAuence of

tautomerism, sigmatropic rearrangement, nucleophilic addition, and elimination reactions

occurring subseAuent to phenylhydra2one formation. %his interesting transformation involves

the oxidation of two carbon atoms and the reduction of one carbon and both nitrogen atoms.

Reactions

%he chemical reactivity of the saturated members of this class of heterocycles tetrahydrofuran,

thiolane and pyrrolidine, resemble that of acyclic ethers, sulfides, and #^*amines, and will not be

described here. /,3*Dioxolanes and dithiolanes are cyclic acetals and thioacetals. %hese units

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are commonly used as protective groups for aldehydes and etones, and may be hydroly2ed by

the action of aAueous acid.

It is the aromatic unsaturated compounds, furan, thiophene and pyrrole that reAuire our

attention. In each case the heteroatom has at least one pair of non*bonding electrons that may

combine with the four F*electrons of the double bonds to produce an annulene having

an aromatic sextet of electrons. %his is illustrated by the resonance description at the top of thefollowing diagram. %he heteroatom 2 becomes sp#*hybridi2ed and acAuires a positive charge as

its electron pair is delocali2ed around the ring. ;n easily observed conseAuence of this

delocali2ation is a change in dipole moment compared with the analogous saturated

heterocycles, which all have strong dipoles with the heteroatom at the negative end. ;s

expected, the aromatic heterocycles have much smaller dipole moments, or in the case of

pyrrole a large dipole in the opposite direction. ;n important characteristic of aromaticity is

enhanced thermodynamic stability, and this is usually demonstrated by relative heats of

hydrogenation or heats of combustionmeasurements. By this standard, the three aromatic

heterocycles under examination are stabili2ed, but to a lesser degree than ben2ene.

;dditional evidence for the aromatic character of pyrrole is found in its exceptionally wea

basicity (p>a ca. ?! and strong acidity (p>a - /6! for a #^*amine. %he corresponding values for

the saturated amine pyrrolidine are basicity //.# and acidity 3#.

;nother characteristic of aromatic systems, of particular importance to chemists, is their pattern

of reactivity with electrophilic reagents. :hereas simple cycloalenes generally give addition

reactions, aromatic compounds tend to react by substitution. ;s noted for ben2ene and its

derivatives, these substitutions tae place by an initial electrophile addition, followed by a proton

loss from the onium intermediate to regenerate the aromatic ring. %he aromatic five*membered

heterocycles all undergo electrophilic substitution, with a general reactivity order pyrrole __

furan _ thiophene _ ben2ene. 5ome examples are given in the following diagram. %he reaction

conditions show clearly the greater reactivity of furan compared with thiophene. ;ll these

aromatic heterocycles react vigorously with chlorine and bromine, often forming

polyhalogenated products together with polymers. %he exceptional reactivity of pyrrole is

evidenced by its reaction with iodine (bottom left eAuation!, and formation of #*acetylpyrrole by

simply warming it with acetic anhydride (no catalyst!.

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%here is a clear preference for substitution at the #*position (U! of the ring, especially for furan

and thiophene. )eactions of pyrrole reAuire careful evaluation, since 1*protonation destroys its

aromatic character. Indeed, 1*substitution of this #^*amine is often carried out prior to

subseAuent reactions. or example, pyrrole reacts with acetic anhydride or acetyl chloride and

triethyl amine to give 1*acetylpyrrole. &onseAuently, the regioselectivity of pyrrole substitution is

variable, as noted by the bottom right eAuation.

;n explanation for the general U*selectivity of these substitution reactions is apparent from the

mechanism outlined below. %he intermediate formed by electrophile attac at &*# is stabili2ed

by charge delocali2ation to a greater degree than the intermediate from &*3 attac. rom

the ammond postulate we may then infer that the activation energy for substitution at the

former position is less than the latter substitution.

unctional substituents influence the substitution reactions of these heterocycles in much the

same fashion as they do for ben2ene. Indeed, once one understands the ortho*para and meta*

directing character of these substituents, their directing influence on heterocyclic ring

substitution is not difficult to predict. %he following diagram shows seven such reactions.

)eactions / [ # are 3*substituted thiophenes, the first by an electron donating substituent and

the second by an electron withdrawing group. %he third reaction has two substituents of different

types in the # and 6*positions. inally, examples 4 through @ illustrate reactions of /,#* and /,3*

oxa2ole, thia2ole and dia2ole. 1ote that the basicity of the sp#*hybridi2ed nitrogen in the

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dia2oles is over a million times greater than that of the apparent sp3*hybridi2ed nitrogen, the

electron pair of which is part of the aromatic electron sextet.

Other possible reactions are suggested by the structural features of these heterocycles. or

example, furan could be considered an enol ether and pyrrole an enamine. 5uch functions are

nown to undergoacid*cataly2ed hydrolysis to carbonyl compounds and alcohols or amines.

5ince these compounds are also heteroatom substituted dienes, we might anticipate Diels*;lder

cycloaddition reactions with appropriate dienophiles. %hese possibilities will be illustrated

above by clicing on the diagram. ;s noted in the upper example, furans may indeed be

hydroly2ed to /,4*dicarbonyl compounds, but pyrroles and thiophenes behave differently. %he

second two examples, shown in the middle, demonstrate typical reactions of furan and pyrrole

with the strong dienophile maleic anhydride. %he former participates in a cycloaddition reactionH

however, the pyrrole simply undergoes electrophilic substitution at &*#. %hiophene does not

easily react with this dienophile.

%he bottom line of the new diagram illustrates the remarable influence that additional nitrogen

units have on the hydrolysis of a series of 1*acetyla2oles in water at #6 ^& and p-@. %he

pyrrole compound on the left is essentially unreactive, as expected for an amide, but additional

nitrogens maredly increase the rate of hydrolysis. %his effect has been put to practical use in

applications of the acylation reagent /,/=*carbonyldiimida2ole (5taab=s reagent!.

;nother facet of heterocyclic chemistry was disclosed in the course of investigations concerning

the action of thiamine (following diagram!. ;s its pyrophosphate derivative, thiamine is a

coen2yme for several biochemical reactions, notably decarboxylations of pyruvic acid toacetaldehyde and acetoin. a ca. /3!, forming a relatively stable

ylide con'ugate base. ;s shown, this rationali2es the facile decarboxylation of thia2olium*#*

carboxylic acids and deuterium exchange at &*# in neutral heavy water.

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;ppropriate thia2olium salts cataly2e the conversion of aldehydes to acyloins in much the same

way that cyanide ion cataly2es the formation of ben2oin from ben2aldehyde, the ben.oin

condensation. By clicing on the diagram, a new display will show mechanisms for these two

reactions. 1ote that in both cases an acyl anion equivalent is formed and then adds to a

carbonyl function in the expected manner. %he ben2oin condensation is limited to aromatic

aldehydes, but the use of thia2olium catalysts has proven broadly effective for aliphatic andaromatic aldehydes. %his approach to acyloins employs milder conditions than the reduction of

esters to enediol intermediates by the action of metallic sodium .

%he most important condensed ring system related to these heterocycles is indole. 5ome

electrophilic substitution reactions of indole are shown in the following diagram. :hether the

indole nitrogen is substituted or not, the favored site of attac is &*3 of the heterocyclic ring.

Bonding of the electrophile at that position permits stabili2ation of the onium*intermediate by the

nitrogen without disruption of the ben2ene aromaticity.

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Six-Membered Rings 0roperties

%he chemical reactivity of the saturated members of this class of heterocycles tetrahydropyran,

thiane and piperidine, resemble that of acyclic ethers, sulfides, and #^*amines, and will not be

described here. /,3*Dioxanes and dithianes are cyclic acetals and thioacetals. %hese units are

commonly used as protective groups for aldehydes and etones, as well as synthetic

intermediates, and may be hydroly2ed by the action of aAueous acid. %he reactivity of partially

unsaturated compounds depends on the relationship of the double bond and the heteroatom

(e.g . 3,4*dihydro*#*pyran is an enol ether!.

ully unsaturated six*membered nitrogen heterocycles, such as pyridine, pyra2ine, pyrimidine

and pyrida2ine, have stable aromatic rings. Oxygen and sulfur analogs are necessarily

positively charged, as in the case of #,4,7*triphenylpyrylium tetrafluoroborate.

rom heat of combustion measurements, the aromatic stabili2ation energy of pyridine is #/

calLmole. %he resonance description drawn at the top of the following diagram includes charge

separated structures not normally considered for ben2ene. %he greater electronegativity of

nitrogen (relative to carbon! suggests that such canonical forms may contribute to a significant

degree. Indeed, the larger dipole moment of pyridine compared with piperidine supports thisview. 8yridine and its derivatives are wea bases, reflecting the sp# hybridi2ation of the nitrogen.

rom the polar canonical forms shown here, it should be apparent that electron donating

substituents will increase the basicity of a pyridine, and that substituents on the # and 4*

positions will influence this basicity more than an eAuivalent 3*substituent. %he p>a values given

in the table illustrate a few of these substituent effects. ethyl substituted derivatives have the

common names picoline (methyl pyridines!, lutidine (dimethyl pyridines! and collidine (trimethyl

pyridines!. %he influence of #*substituents is complex, consisting of steric hindrance and

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electrostatic components. 4*Dimethylaminopyridine is a useful catalyst for acylation reactions

carried out in pyridine as a solvent. ;t first glance, the sp3 hybridi2ed nitrogen might appear to

be the stronger base, but it should be remembered that 1,1*dimethylaniline has a p>a slightly

lower than that of pyridine itself. &onseAuently, the sp# ring nitrogen is the site at which

protonation occurs.

%he dia2ines pyra2ine, pyrimidine and pyrida2ine are all weaer bases than pyridine due to the

inductive effect of the second nitrogen. owever, the order of base strength is unexpected. ;

consideration of the polar contributors helps to explain the difference between pyra2ine and

pyrimidine, but the basicity of pyrida2ine seems anomalous. It has been suggested that electron

pair repulsion involving the vicinal nitrogens destabili2es the neutral base relative to its

con'ugate acid.

Electrophilic &ubstitution of 0yridine

8yridine is a modest base (p>a-6.#!. 5ince the basic unshared electron pair is not part of the

aromatic sextet, as in pyrrole, pyridinium species produced by 1*substitution retain the

aromaticity of pyridine. ;s shown below, 1*alylation and 1*acylation products may be prepared

as stable crystalline solids in the absence of water or other reactive nucleophiles. %he 1*acyl

salts may serve as acyl transfer agents for the preparation of esters and amides. Because of the

stability of the pyridinium cation, it has been used as a moderating component in complexes

with a number of reactive inorganic compounds. 5everal examples of these stable and easily

handled reagents are shown at the bottom of the diagram. %he poly(hydrogen fluoride! salt is a

convenient source of for addition to alenes and conversion of alcohols to alylfluorides, pyridinium chlorochromate (8&&! and its related dichromate analog are versatile

oxidation agents and the tribromide salt is a convenient source of bromine. 5imilarly, the

reactive compounds sulfur trioxide and diborane are conveniently and safely handled as

pyridine complexes.

;mine oxide derivatives of 3^*amines and pyridine are readily prepared by oxidation with

peracids or peroxides, as shown by the upper right eAuation. )eduction bac to the amine can

usually be achieved by treatment with 2inc (or other reactive metals! in dilute acid.

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rom the previous resonance description of pyridine, we expect this aromatic amine to undergo

electrophilic substitution reactions far less easily than does ben2ene. urthermore, as depictedabove by clicing on the diagram, the electrophilic reagents and catalysts employed in these

reactions coordinate with the nitrogen electron pair, exacerbating the positive charge at

positions #,4 [ 7 of the pyridine ring. %hree examples of the extreme conditions reAuired for

electrophilic substitution are shown on the left. 5ubstituents that bloc electrophile coordination

with nitrogen or reduce the basicity of the nitrogen facilitate substitution, as demonstrated by the

examples in the blue*shaded box at the lower right, but substitution at &*3 remains dominant.

;ctivating substituents at other locations also influence the ease and regioselectivity of

substitution. By clicing on the diagram a second time, three examples will shown on the left.

%he amine substituent in the upper case directs the substitution to &*#, but the weaer electron

donating methyl substituent in the middle example cannot overcome the tendency for 3*

substitution. ydroxyl substituents at &*# and &*4 tautomeri2e to pyridones, as shown for the #*

isomer at the bottom left.

8yridine 1*oxide undergoes some electrophilic substitutions at &*4 and others at &*3. %he

coordinate covalent 1–O bond may exert a push*pull influence, as illustrated by the two

examples on the right. ;lthough the positively charged nitrogen alone would have a strong

deactivating influence, the negatively charged oxygen can introduce electron density at &*#, &*4

[ &*7 by F*bonding to the ring nitrogen. %his is a controlling factor in the relatively facile

nitration at &*4. owever, if the oxygen is bonded to an electrophile such as 5O 3, the resulting

pyridinium ion will react sluggishly and preferentially at &*3.

%he fused ring heterocycles Auinoline and isoAuinoline provide additional evidence for thestability of the pyridine ring. Nigorous permanganate oxidation of Auinoline results in

predominant attac on the ben2ene ringH isoAuinoline yields products from cleavage of both

rings. 1ote that naphthalene is oxidi2ed to phthalic acid in a similar manner. By contrast, the

heterocyclic ring in both compounds undergoes preferential catalytic hydrogenation to yield

tetrahydroproducts.


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3ther Reactions of 0yridine

%hans to the nitrogen in the ring, pyridine compounds undergo nucleophilic substitution

reactions more easily than eAuivalent ben2ene derivatives. In the following diagram, reaction /

illustrates displacement of a #*chloro substituent by ethoxide anion. %he addition*elimination

mechanism shown for this reaction is helped by nitrogen=s ability to support a negative charge. ;

similar intermediate may be written for substitution of a 4*halopyridine, but substitution at the 3*

position is prohibited by the the failure to create an intermediate of this ind. %he two

&hichibabin aminations in reactions # and 3 are remarable in that the leaving anion is hydride

(or an eAuivalent!. ydrogen is often evolved in the course of these reactions. In accord with

this mechanism, Auinoline is aminated at both &*# and &*4.

;ddition of strong nucleophiles to 1*oxide derivatives of pyridine proceed more rapidly than topyridine itself, as demonstrated by reactions 4 and 6. %he dihydro*pyridine intermediate easily

loses water or its eAuivalent by elimination of the –O substituent on nitrogen.


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By clicing on the above diagram, five additional examples of base or nucleophile reactions with

substituted pyridine will be displayed. Because the pyridine ring (and to a greater degree the 1*

oxide ring! can support a negative charge, alyl substituents in the #* and 4*locations are

activated in the same fashion as by a carbonyl group. )eactions 7 and @ show alylation and

condensation reactions resulting from this activation. )eaction K is an example of 1*

alylpyridone formation by hydroxide addition to an 1*alyl pyridinium cation, followed by mildoxidation. Birch reduction converts pyridines to dihydropyridines that are bis*enamines and may

be hydroly2ed to /,6*dicarbonyl compounds. 8yridinium salts undergo a one electron transfer to

generate remarably stable free radicals. %he example shown in reaction G is a stable (in the

absence of oxygen!, distillable green liAuid. ;lthough 3*halopyridines do not undergo addition*

elimination substitution reactions as do their #* and 4*isomers, the strong base sodium amide

effects amination by way of a pyridyne intermediate. %his is illustrated by reaction /?. It is

interesting that 3*pyridyne is formed in preference to #*pyridyne. %he latter is formed if &*4 is

occupied by an alyl substituent. %he pyridyne intermediate is similar to ben2yne.

&ome 0olycyclic Heterocycleseterocyclic structures are found in many natural products.


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8orphyrin is an important cyclic tertrapyrrole that is the core structure of heme and chlorophyll.

%hese structures will be drawn above by clicing on the diagram.

Derivatives of the simple fused ring heterocycle purine constitute an especially important and

abundant family of natural products. %he amino compounds adenine and guanine are two of the

complementary bases that are essential components of D1;. 5tructures for these compounds

are shown in the following diagram. `anthine and uric acid are products of the metabolic

oxidation of purines. Wric acid is normally excreted in the urineH an excess serum accumulation

of uric acid may lead to an arthritic condition nown as gout.

https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/nucacids.htmhttps://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/nucacids.htm


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by tetrathiofulvalene and tetracyanoAuinodimethane has one of the highest electrical

conductivities reported for an organic solid.

#eterocyclic compound

; heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two

different elements as members of its ring(s!./0Heterocyclic chemistry is the branch of chemistry

dealing with the synthesis, properties and applications of these heterocycles. In contrast, the rings

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Dehydration Reaction