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Alkenes are unsaturated hydrocarbons containing a carbon–carbon doublebond—that is, two adjacent carbon atoms joined together with two bonds.

The general formula for an alkene is CnH2n , where n is an integer greater

than 1.

The first smallest! member of the alkene family is C2H" #$%&C name,

ethene' common name, ethylene!.



The alkenes are non(olar com(ounds with (hysical (ro(erties similar tothose of the alkanes.

They are insoluble in water and ha)e boiling (oints close to those of thealkanes.

There is more )ariation in melting (oints among the alkenes than amongthe alkanes.



The general formula for an alkene is the same as for a cycloalkane*

+ince the molecular formula for an alkene and a cycloalkane is the same,CnH2n, the formula alone is not enough to distinguish between them.

&dditional information, such as their (hysical (ro(erties, is reuired toclassify an unknown substance.



&lthough the two bonds in the structural formula of an alkene are drawn

ha)ing eual weight, their bond strengths are uite different.

-ne strong bond* // k0mol sigma σ! bond

-ne weaker bond* 2/ k0mol (i π! bond

The weaker π bond is res(onsible for the increased reacti)ity of alkenes

o)er the alkanes.

The σ bond in either class of molecules has about the same energy.



The three s(2 orbitals (oint to the corners ofan euilateral triangle and the remaining (orbital lies (er(endicular to the (lane of thetriangle*

The four orbitals about a carbon atom in an alkane or cycloalkane are all

s(

hybridi3ed orbitals.#n the alkenes a different ty(e of hybridi3ation is used, s(2. The 2s andtwo of the 2( orbitals about the carbon atom hybridi3e to form threeeui)alent s(2 orbitals, lea)ing a single 2( orbital in an unhybridi3edstate.



C2H", ethene, is formed from two s(2 hybridi3ed carbon atoms and four

hydrogen atoms*

4ateral o)erla( of the unhybridi3ed ( orbitals, abo)e and below thetriangular (lanes, results in the π bond of the double bond.



The two carbons and all four hydrogens all lie within the same (lane.

The bond angles are all 12o

.



&lkenes (ossess two (ossibilities for the formation of constitutional

isomers*5ifferent carbon skeletons connecti)ity of atoms, also (ossible for alkanesand cycloalkanes!

5ifferent (lacements of the double bond within a carbon skeleton.

There are two C"H1 alkanes, howe)er, there are three C"H6 alkenes*











&lkenes containing fewer than fi)e carbon atoms are often known bytheir common names which em(loy the suffi7 –ylene instead of –ene.

87am(les are (ro(ylene instead of (ro(ene #$%&C!, and ethyleneinstead of ethene #$%&C!.



& C9C double bond has restricted rotation.

&s a result of restricted rotation, cis:trans isomerism is (ossible forcom(ounds containing a double bond. The italici3ed (refi7es cis- andtrans- are used before the #$%&C name to distinguish them from oneanother.



The cis- isomer contains two similar substituents on the same side of thedouble bond while the trans- isomer has two similar substituents ono((osite sides of the double bond.



Cis and trans isomers are different chemical substances ha)ing differentchemical (ro(erties*

cis:2:butene* ;%. :1<o C

trans:2:butene* ;%. :1=o C

The π bond in an alkene causes a significant barrier to rotation about theC:C σ bond. >ithout this barrier, a cis isomer would be easily con)erted

into the more stable trans isomer.

+ince cis and trans isomers (ossess the same connecti)ity, the differencein the isomers resides in their configurations.

This im(lies that each of the carbon atoms in a double bond is astereocenter* an atom in a com(ound bearing substituents of such

identity that a hy(othetical e7change of the (ositions of any twosubstituents would con)ert one stereoisomer into another stereoisomer.

+tereocenters in alkenes are called trigonal stereocenters.



?one of the following ty(es of alkenes (ossesses geometrical isomerism*





The use of the cis:trans nomenclature system is restricted to 1,2:disubstituted alkenes—that is, alkenes in which each carbon of thedouble bond has one hydrogen and one substituent other than hydrogen.

This restriction is analogous to that of the cis:trans nomenclature systemfor cycloalkanes in which each of two carbons of the ring has onehydrogen and one substituent other than hydrogen.

The cis:trans designation is ambiguous for*

@or com(ounds of this ty(e, the 8,A:nomenclature is used, but this willnot be (resented in this course.



The π bond of an alkene is sufficiently weak to undergo reaction with*

Hydrogen H2!

Halogen @2, Cl2, Br2!

Hydrogen halide HCl, HBr, H#!

>ater

8ach of the abo)e reagents undergoes an addition reaction with analkene*



Hydrogen and halogen are symmetrical reagents &9B!.

Hydrogen adds to a double bond*

&ddition of +ymmetrical eagents &9B!

&t room tem(erature a catalyst is reuired for this reaction. This isindicated by writing the catalyst abo)e the arrow connecting thereactants and the (roducts.

& catalyst (ro)ides an alternate, lower acti)ation energy, (athway for a

chemical reaction.

Hydrogenation is an e7am(le of a reduction reaction.



Bromine adds to a double bond*

Bromination (roceeds readily at room tem(erature.

&lkenes are much more reacti)e than alkanes at ambient tem(eratures.

&lkanes do not react with hydrogen at all.

&lkanes react with halogens only in the (resence of $D radiation orat high tem(eratures 2o C!, and then the reaction is of a

substitution ty(e, not an addition ty(e as for the alkenes.



The ready addition of bromine to alkenes (ro)ides a simple chemicaldiagnostic test for alkenes.

Takes (lace ra(idly

Ei)es a (ositi)e result )isible to the naked eye.

Bromine has a dee( red color which disa((ears shortly after addition toan alkene*

&dd some bromine to an unknown com(oundand wait a few minutes*

#f the red color disa((ears, addition hastaken (lace and the unknown is an alkene.

#f the red color does not disa((ear, additionhas not taken (lace, and the unknown is analkane.



&ddition of &symmetrical eagents &FB!

Hydrogen halides H@, HCl, H#! and water H2-! are asymmetrical

reagents.

#n each case abo)e, &B re(resents two fragments, one of which is H, andthe other is either a halogen or –-H.

The addition of a hydrogen halide is called a hydrohalogenation*

The addition of water is called a hydration*

Hydration reuires the (resence of a strong acid catalyst such as H2+-".



&lkanes do not react with either hydrogen halides or water.

&lkenes can be se(arated into two classes*

+ymmetrical alkenes* CH29CH2, CH9CH, 2C9C2

$nsymmetrical alkenes* CH29CH, CH29C2, CH9C2

The addition of water or a hydrogen halide to a symmetrical alkeneresults in only one (ossible (roduct.

The addition of water or a hydrogen halide to an unsymmetrical alkenecan result in two different (ossible (roducts*

This is an e7am(le of a reaction that can (roceed by two com(eting(athways.



>hen two (roducts are (ossible*

+ome reactions are nonselective* Both (roducts are formed inmore or less eual amounts.

+ome reactions are selective* The two (roducts are formed inuneual amounts. Major and minor (roducts are formed.



&dditions of asymmetrical reagents to alkenes follows the Markovnikovrule*

The major (roduct formed (laces the incoming hydrogen atom on thecarbon atom of the double bond that carried the most hydrogenatoms (rior to the reaction. GThe rich get richer.









&lkenes, as well as some other families of organic com(ounds, can add

to one another to form a co)alently linked chain of molecules called apolymer.

The indi)idually linked molecules in the (olymer are called monomers.

87am(les of synthetic (olymers include (lastics, fibers, and rubbers.

?aturally occurring (olymers include carbohydrates, (roteins, and nucleicacids.



&ddition (olymeri3aton of ethylene (roduces a long (olymeric moleculecalled (olyethylene*

#n this (olymeri3ation (rocess π bonds within each monomer are brokenand remade as σ bonds between the monomers.

The (rocess can be abbre)iated as*

where n is usually in the 1Is.

This ty(e of (olymeri3ation reuires the (resence of a catalyst, and doesnot occur in the (resence of additional reagents which might react(referentially with the double bonds.





&lkenes are o7idi3ed in the (resence of sufficient o7ygen to carbon

dio7ide and water*

&lkenes can also be selecti)ely o7idi3ed to alcohols, aldehydes, andketones or carbo7ylic acids by a )ariety of laboratory o7idi3ing agents

such as (ermanganate ;n-":!, dichromate Cr2-J2:!, as well as witho7ygen and o3one -! from the atmos(here.

%ermanganate (ur(le solution brown (reci(itate! and dichromateorange solution green solution CrK!! also ser)e as sim(le chemicaltests for the (resence of alkenes.



&lkynes are hydrocarbons containing a carbon:carbon tri(le bond.

The general formula for an alkyne is CnH2n:2. The first member of the

alkyne series is ethyne acetylene!*

Because of the tri(le bond, alkynes are more reacti)e than alkenes.



&lkynes react with the same reagents as alkenes, but with twice as muchreagent, since the double bond left after the first addition is alsoreacti)e.





Aromatic compounds are unsaturated hydrocarbons that do not

beha)e like other unsaturated hydrocarbons. They ha)e e7ce(tionalstability, far beyond what would be e7(ected.

The sim(lest aromatic hydrocarbon is ben3ene, C=H=*



&ll carbon and hydrogen atoms in ben3ene lie in one (lane.

The unsaturated ring system of ben3ene is called the benzene ring or

benzene system.

-rganic com(ounds whose structures contain a ben3ene ring are called aromatic compounds.

&lthough the ben3ene ring system looks like a triene with alternatingsingle and double bonds, reactions ty(ical of double bonds do not take(lace.



LHalogens, hydrogen halides, water, and sulfuric acid do not add tothe double bonds of ben3ene under any conditions. These reagents

readily add to the double bonds of alkenes.LThe combustion of aromatic com(ounds is less e7othermic than thecombustion of ali(hatic com(ounds. ;ore:stable com(ounds gi)e offless heat in undergoing a reaction than do less:stable com(ounds.!

The resistance of the ben3ene structure to change is called aromaticity

or aromatic behavior.

+ince the three double bonds in ben3ene do not beha)e as e7(ected, theben3ene structure is often re(resented as*





+e)eral heterocyclic aromatic rings are of im(ortance biologically such asthe (urine and (yrimidine ring systems found in nucleic acids such as

5?& and ?&.

-ther common e7am(les of biological aromatic systems can be found inthe amino acids and the heme grou( of hemoglobin.



The #$%&C system for naming monosubstituted ben3enes (laces thesubstituent name in front of the (arent name –benzene*



+e)eral monosubstituted ben3enes ha)e common names that ha)e beenado(ted by #$%&C as the (referred names*



5isubstituted ben3enes ha)e three constitution isomers numbered 1,2ortho, or o!, 1, meta, or m!, and 1," (ara, or (!



Certain disubstituted ben3enes are named as deri)ati)es of themonosubstituted ben3ene, if it has a (referred common name*



Trisubstituted ben3enes are named similarly, howe)er, numbers must beused since the o:, m:, (: system is not a((licable*



>hen the ali(hatic (art of a molecule is more com(le7 than the aromaticcom(onent, the name of the com(ound may be deri)ed from theali(hatic (art using the (refi7, (henyl, to indicate a ben3ene substituent*

The (henyl substituent may be indicated by C=H/:, %h:, or φ:.

& (henyl or substituted (henyl grou( is called an aryl grou( &r!.



The grou(* C=H

/:CH

2:, is known as a ben3yl grou(. #t is different from a

(henyl grou(.

>hen numbering the substituents around a ben3ene ring, the correctnumbering results in the lowest set of numbers.







The C9C double bonds in ben3ene are highly resistant to addition,howe)er, the C:H bonds in ben3ene will undergo substitution reactions.

Halogenation, sulfonation, nitration, and alkylation reactions all result inthe re(lacement of a hydrogen atom e7tending from the ben3ene ringwith another atom or grou(.



Halogenation and alkylation both reuire a metal halide catalyst, such as@eCl or &lCl.



?itration, alkylation, and sulfonation all also occur with alkenes, but do

so as addition reactions, not substitution reactions.+imilar to other organic com(ounds, aromatic com(ounds undergoo7idation to carbon dio7ide and water.

+electi)e o7idations of alkylben3enes using moderately strong o7idi3ingconditions hot acidic M;n-" or M2Cr2-J!, howe)er, the aromatic ring

remains intact*







Cha(ter 12 +ummary



• $nsaturated hydrocarbons, consisting of the alkene, alkyne, andaromatic families, contain carbon–carbon multi(le bonds—that is, doubleor tri(le bonds between adjacent carbon atoms.

Cha(ter 12 +ummary



Alkenes

• &lkenes contain a double bond—that is, two adjacent carbons joinedtogether with two bonds.

• The general formula for an alkene is CnH2n, which contains two fewer

hydrogens than the general formula for an alkane.

• The carbon atoms of the double bond (ossess three sp2:hybrid orbitalswith trigonal 12N! bond angles, and one unhybridi3ed 2 p orbital.

• The orbitals o)erla( to form one strong bond a σ:bond! and one weakbond a π:bond!.

Cha(ter 12 +ummary



Constitutional Isomers of Alkenes

• Constitutional isomerism e7ists in alkenes not only because a gi)enformula allows different carbon skeletons, as in the alkane family, but

also because it allows different (lacements of the double bond within acarbon skeleton.

Cha(ter 12 +ummary



Naming Alkenes

• #n the #$%&C system, alkenes are named on the basis of the longestcontinuous chain containing the double bond.

• The base name is (receded by a number indicating the (osition of thedouble bond.

• The family ending of alkene names is :ene.

• %refi7es with numbers identify any substituents attached to the longestchain.

Cha(ter 12 +ummary



Cis!rans "tereoisomerism in Alkenes

• Cis:trans geometrical! stereoisomers e7ist in alkenes when eachcarbon atom of the double bond has two different substituents.

• The cis isomer has substituents on the same side of the double bond.The substituents are on o((osite sides of the double bond in the trans

isomer.

Cha(ter 12 +ummary



Addition #eactions of Alkenes

• The (resence of the weak bond in the double bond greatly increasesthe reacti)ity of alkenes relati)e to the low reacti)ity of saturatedhydrocarbons.

• Darious addition reactions are (ossible* a reagent breaks into twofragments, each of which bonds adds! to one of the carbons of the

double bond.

• Hydrogen, halogens, hydrogen halides, and water are all able to add tothe alkene double bond.

• The addition of asymmetrical reagents hydrogen halides, water! to

asymmetrical alkenes (roceeds in a selecti)e manner.

• The ;arko)niko) rule is followed, meaning that the hydrogen fragmentfrom the reagent adds to the carbon of the double bond that alreadyholds more hydrogens before reaction.

Cha(ter 12 +ummary



Addition $olymerization

• &ddition (olymeri3ation is a self:addition reaction in which largenumbers of alkene molecules add to one another to form high:molecular:

mass molecules called (olymers.

Cha(ter 12 +ummary



%&idation of Alkenes

• &lkenes, like other organic com(ounds, can undergo combustion.

• +electi)e o7idation is also (ossible.

Cha(ter 12 +ummary



Alkynes

• &lkynes contain a tri(le bond, three bonds between two adjacentcarbon atoms.

• Two of the three bonds are weak π:bonds similar to those in the alkenedouble bond.

• The tri(le bond undergoes the same reactions as the double bond doesbut with twice as much of the reagent.

Cha(ter 12 +ummary



Aromatic Compounds

• &romatic com(ounds contain the ben3ene ring, a si7:membered ringwith alternating double and single bonds.

• +ubstituted ben3enes are named by #$%&C as deri)ati)es of ben3ene,with (refi7es indicating the substituents attached to the ring.

• 8ither a numbering system or the ortho-, meta-, para-system is usedto indicate the (ositions of substituents for disubstituted ben3enes.

• -nly the numbering system is used for trisubstituted ben3enes.

• +ome monosubstituted ben3enes ha)e common names that areacce(ted by #$%&C and used as the basis for naming their di: andtrisubstituted relati)es.

Cha(ter 12 +ummary



#eactions of Aromatic Compounds

• The ben3ene ring has e7ce(tional stability.

• Ben3ene is inert to the addition of halogens, hydrogen halides, sulfuricacid, and water—reagents that add to alkene double bonds.

• Ben3ene and other aromatic com(ounds undergo substitution reactionswith halogens, nitric acid, sulfur trio7ide, and alkyl halides.

• &lkylben3enes undergo o7idation of the alkyl grou(, but the ben3enering itself remains intact.

Cha(ter 12 +ummary of eactions



Cha(ter 12 +ummary of eactions



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