Notes - Organic Basics

8/3/2019 Notes - Organic Basics

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Organ ic I (Basics)

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Formulae

Molecular formu la

This is a list of each typ e of atom in a molecule, and the nu mber of atoms present. Forexample, the formula C4H 10 indicates that there are four carbon atoms and ten hydrogen

atoms.

Structural formu la

This indicates which atom s are bond ed to w hich other atoms. For example, bothCH 3CH 2CH 2CH 3 and CH 3CH(CH3)CH 3 have the molecular formu la C4H 10, but they areclearly different comp oun ds, butane and m ethyl propane respectively.

Structural formula can be abbreviated as above, or show n w ith all bond s, which is calledthe full structural formula or displayed structural formu la.

Draw full structural formulae for CH3CH2CH2CH3 and CH3CH(CH3)CH3.

These molecules have the same m olecular formula (atoms) but d ifferent structuralformulae (bond s). They are structural isomers.

Molecules can have the same stru ctural formula and still be different. The structuralformula only tells us about the connections between atoms, not abou t the spatialarrangement of the atoms. A good analogy for this idea is that the connections betweenbones in right and left hands are identical, but r ight and left hand s are not interchangeableand have a d ifferent shape (as is obvious if you try to pu t a right hand into a left glove).

Similarly, the stru ctural formu la CH 3CH=CHCH 3 denotes but-2-ene, but d oes not specifyhow the H and CH3 groups are d istributed across the rigid C=C bond, which is rigid andflat.

Draw the different arrangement of CH3 and H groups in but-2-ene molecules.

So, these isomers have the sam e molecular formu la (atoms) and the same stru cturalformula (bond s) but differ by the spatial arrangement of their atoms. They arestereoisomers .


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Hom ologous series and general formulae

A homologous series is a series of compou nd s that have a general formula and wh osemolecular formulae differ by a CH2 unit.

Alkanes

Compound Name Molecular formula

CH 4 methane

CH 3CH 3 ethaneCH 3CH 2CH 3 propane

CH 3CH 2CH 2CH 3 butane

The general formu la of alkanes is CnH 2n+2.

Alkenes

Compound Name Molecular formulaCH 2CH 2 ethene

CH 3CH=CH 2 propeneCH 3CH 2CH=CH 2 but-1-ene

CH 3CH 2CH 2CH=CH 2 pent-1-ene

The general formu la of alkanes is CnH 2n.

Halogenoalkanes

Compound Name Molecular formulaCH 3Br bromomethaneCH 3CH 2Br bromoethane

CH 3CH 2CH 2Br 1-bromo propane

CH 3CH 2CH 2CH 2Br 1-bromo butane

The general formu la of halogenoalkanes is CnH 2n+1X.

Alcohols

Compound Name Molecular formula

CH 3OH methanolCH 3CH 2OH ethanol

CH 3CH 2CH 2OH propan-1-ol

CH 3CH 2CH 2CH 2OH butan-1-ol

The general formu la of alcohols is CnH 2n+1OH or CnH 2nO

[NB The range of compou nd s used here to illustrate isomerism is wider than that coveredby the unit, and also discussion of the imp act of isomerism on physical properties is madeto link the top ic to structure and bond ing]


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Isomerism

Isomerism is about how d ifferent molecules can be mad e from the sam e set of atoms. Theset of atoms is called the molecular formu la. For exam ple, C4H 8 means that there are 4carbon atoms and 8 Hydrogen atoms.

C atoms have four bond s

H atoms have one bond N atoms have three bonds O atoms have two bonds Halogen atoms have one bond (in organic chemistry)Broad ly speaking, there are two ways in wh ich m olecules can be different yet have thesame molecular formu la:

1. The atoms can be joined u p in a different ord er, for example but-1-ene and but-2-enehave the sam e molecular formu la C4H 8 but their structural formulae are d ifferent.

This type of isomerism is structural isomerism .

2. Alternatively, the structural formula can be kep t the same bu t the shape of themolecule can be varied. For example, but-2-ene can exist as two different molecules,cis-but-2-ene and trans-but-2-ene.

This type of isomerism is an example ofstereoisomerism, called geometrical isomerism.


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Structural Isomerism

There are three types of structural isomerism, Chain (or Branch), Positional and FunctionalGroup.

Chain IsomerismDraw all the molecules that have the molecular formula C5H12.

Simple chain isomers have very similar chemical properties has they are governed by thechemistry of C-H and C-C bond s.

How ever, you need to und erstand how the boiling point of the above molecules isaffected by bran ching.

When the above substances boil, the intermolecular forces are broken by the heat energybeing sup plied. The C-C and C-H bond s do not break a t all. The intermolecular force in

operation betw een these molecules is the indu ced dipole force, wh ich is stud ied in moredetail in Structure and Bond ing. Usually we think abou t the size of the electron cloud withthis type of force, but since the molecules are isomers they w ill have the sam e size electroncloud because they have exactly the same atom s. Instead , we look to for bran ching in themolecule because branching lowers the su rface area, which in turn lowers the points ofcontact between the molecules.

Show how pentane molecules have more points of contact than dimethylpropane molecules.


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Positional IsomerismIn positional isomerism, the position of an atom or group of atoms moves along a carbonchain that otherw ise stays the same.

Draw butan-1-ol and butan-2-ol.

Draw 1-bromo butane and 2-bromo butane.

Comp ounds tha t are met later in different Units can also und ergo positional isomerism.

Draw positional isomers of dichlorobenzene


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The chemical properties of positional isomers are broadly similar because they w illcontain the same fun ctional group, bu t there w ill be some variation because the reactionsof a functional group do depend on its environment. The ph ysical p roperties such asboiling point d epend on the intermolecular forces.

Discuss the order of boiling points between 1- and 2-bromo butanes (Hint , they depend upon theexposure of the C-Br dipole and surface area).

1-bromobutane has the higher boi l ing point because

i t has the larger surface area as it is not branched, so there are more points of contactbetween the molecul es as they pass by each other, whi ch enables the induced di pole f orces

of attraction to in teract more.

Also, the polar C-Br bond at the end of the molecule can in teract more f reely with otherpolar C-Br bonds than when i t is in the body of the molecul e, with bul ky CH3 groups on

ei ther side steri call y hin deri ng (gett ing in the way of ) i ts in teracti ons.

Discuss the order of boiling points between butan-1 and 2-ols (Hint , they depend upon the exposureof the OH group and surface area).

Butan-1-ol has the higher boi l ing point because

i t has the larger surface area etc i ts OH groups are exposed and can H-bond more ef f ecti vely to OH groups in another

molecul e.


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Fun ctional Grou p IsomerismIn functional group isomerism, a structural change completely alters the functional groupof the m olecule. Consequently, both chemical and physical properties change significantly.

Draw all the isomers of C2H6O, using what you know about how many bonds each atom may have.

Draw all the isomers of C3H6O that are aldehydes or ketones.

Explain why CH3CH2OH is a liquid whereas CH3OCH3 is a gas (Hint, it is something to do withintermolecular forces!)

Molecul es of ethanol have OH groups, so they have H-bondi ng between them. Molecul es of

ethoxy ethane do not, they have a mi xture of induced and permanent dipoles. The

intermolecular f orces in ethanol are stronger due to the presence of H-bonding, which

explains why i t is a l iqu id at room temperature, y et ethoxy ethane is a gas.


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Stereoisomerism

There are two typ es of stereoisomersim, geometrical an d optical (Optical isomerism iscovered in a later unit). In both types, the structural formula stays the same. The m oleculesd iffer because of the spatial arrangemen t of the atoms.

Geometrical

A restriction of rotation (such as C=C) allows the groups around it to be arranged in tw odifferent ways. Often the w ords cis and trans can be used to identify the isomers.

Draw out the geometrical isomers of CH3CH=CHCH3

Draw out the geometrical isomers of CH3CH=CHC2H5

Draw out the geometrical isomers of CH3CH=C(OH)Br

The fur ther condition is that there cannot be two identical group s on either C in the d oublebond.

To illustrate this, try drawing CH2

=CHCH3

as geometrical isomers.

It doesnt work!


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In add ition, you need to und erstand w hy there is no free rotation about a dou ble bond.

Draw out the p orbitals that make the pi bond in a C=C, and show that there is no free rotation dueto sideways overlap of p orbitals.

NB there are plenty of examples of geometrical isomers that d o not have a C=C, andequally they might not even be organic molecules.

Cyclic isomers of C3H4Cl2

Isomers of CH3CH=NC6H5

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Notes - Organic Basics