ALKENES

Alkenes CnH2n “unsaturated” hydrocarbons C2H4 ethylene Functional group = carbon-carbon double bond

C3H6 propylene C4H8 butylene CH3CH=CHCH3 β-butylene isobutylene 2-butene 2-methylpropene

there are two 2-butenes: cis-2-butene trans-2-butene Diastereomers

C=C are called “vinyl” carbons If either vinyl carbon is bonded to two equivalent groups, then no geometric isomerism exists. CH3CH=CHCH3 CH3CH2CH=CH2 yes no CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 no yes

E/Z system is recommended by IUPAC for the designation of geometric isomerism. Use the sequence rules to assign the higher priority * to the two groups attached to each vinyl carbon. 2. * * * * (Z)- “zusammen” (E)- “entgegen” together opposite

* * (Z)- (E)- * *

Nomenclature, alkenes: Parent chain = longest continuous carbon chain that contains the C=C. alkane => change –ane to –ene prefix a locant for the carbon-carbon double bond using the principle of lower number. Similar to alkanes If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.

* * * * (Z)-3-methyl-2-pentene (3-methyl-cis-2-pentene) (E)-1-bromo-1-chloropropene *

CH3 CH3CH2 CHCH2CH3 \ / C = C 3-ethyl-5-methyl-3-heptene / \ CH3CH2 H (not a geometric isomer)

-ol has the priority when naming CH2=CHCH2-OH 2-propen-1-ol CH3CHCH=CH2 3-buten-2-ol OH

Physical properties: non-polar or weakly polar no hydrogen bonding low mp/bp similar to alkanes Insoluble in water Importance: common group in biological molecules

Preparation of alkenes: dehydrohalogenation of alkyl halides 2. dehydration of alcohols dehalogenation of vicinal dihalide

2. dehydration of alcohols

dehydrohalogenation of alkyl halides | | | | — C — C — + KOH(alc.)  — C = C — + KX + H2O | | H X

dehalogenation of vicinal dihalides | | | | — C — C — + Zn  — C = C — + ZnX2 | | X X eg. CH3CH2CHCH2 + Zn  CH3CH2CH=CH2 + ZnBr2 Br Br

R-OH H+ R-X KOH Alkene (alc.) Zn vicinal dihalide

Reduction of alkynes

Reactions of Alkenes Addition of hydrogen

Addition of halogens

Addition of hydrogen halides

Addition of hydrogen halides

Additon of water

Halohydrin formation

Dimerization

Alklyation

Oxymercuration-Demercuration

Hydroboration-Oxidation

Polymerization

Hydroxylation