Alkenes CnH2n ―unsaturated‖ hydrocarbons

C2H4 ethylene

Functional group = carbon-carbon double bond

sp2 hybridization => flat, 120o bond angles

σ bond & π bond => H2C=CH2

No rotation about double bond!

C C

H

H H

H

C3H6 propylene CH3CH=CH2

C4H8 butylenes CH3CH2CH=CH2

α-butylene

1-butene

CH3

CH3CH=CHCH3 CH3C=CH2

β-butylene isobutylene

2-butene 2-methylpropene

there are two 2-butenes:

cis-2-butene trans-2-butene

―geometric isomers‖ (diastereomers)

C C C CH3C

H H

CH3 H3C

H CH3

H

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

Confusion about the use of cis- and trans-. According to

IUPAC rules it refers to the parent chain.

―cis-‖

????????

C C

H

H3C CH2CH3

CH3

C C

H3C

H Br

Cl

E/Z system is now recommended by IUPAC for the

designation of geometric isomerism.

1. 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

C C

H

H3C CH2CH3

CH3

C C

H3C

H Br

Cl

*

* *

*

(Z)-

(E)-

Nomenclature, alkenes:

1. 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.

2. Etc.

3. If a geometric isomer, use E/Z (or cis/trans) to indicate

which isomer it is.

C C

H

H3C CH2CH3

CH3

C C

H3C

H Br

Cl

*

* *

*

(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 takes precedence over –ene

CH2=CHCH2-OH 2-propen-1-ol

CH3CHCH=CH2 3-buten-2-ol

OH

Physical properties:

non-polar or weakly polar

no hydrogen bonding

relatively low mp/bp ~ alkanes

water insoluble

Importance:

common group in biological molecules

starting material for synthesis of many plastics

Syntheses, alkenes:

1. dehydrohalogenation of alkyl halides

2. dehydration of alcohols

3. dehalogenation of vicinal dihalide

4. (later)

3. dehalogenation of vicinal dihalides

| | | |

— C — C — + Zn — C = C — + ZnX2

| |

X X

eg. CH3CH2CHCH2 + Zn CH3CH2CH=CH2 + ZnBr2

Br Br

Not generally useful as vicinal dihalides are usually made

from alkenes. May be used to ―protect‖ a carbon-carbon

double bond.

1. dehydrohalogenation of alkyl halides

| | | |

— C — C — + KOH(alc.) — C = C — + KX + H2O

| |

H X

a) RX: 3o > 2o > 1o

b) no rearragement

c) may yield mixtures

d) Saytzeff orientation

e) element effect

f) isotope effect

g) rate = k [RX] [KOH]

h) Mechanism = E2

rate = k [RX] [KOH] => both RX & KOH in RDS

R-I > R-Br > R-Cl ―element effect‖

=> C—X broken in RDS

R-H > R-D ―isotope effect‖

=> C—H broken in RDS

Concerted reaction: both the C—X and C—H bonds are

broken in the rate determining step.

Mechanism = elimination, bimolecular E2

One step! ―Concerted‖ reaction.

base:

C

W

C

H

C C + H:base + :WRDS

CH3CHCH3 + KOH(alc) CH3CH=CH2

Br isopropyl bromide propylene

CH3CH2CH2CH2-Br + KOH(alc) CH3CH2CH=CH2 n-butyl bromide 1-butene

CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2

Br 1-butene 19%

sec-butyl bromide +

CH3CH=CHCH3

2-butene 81%

Problem 8.6. What akyl halide (if any) would yield each of the following

pure alkenes upon dehydrohalogenation by strong base?

CH3 CH3

isobutylene KOH(alc) + CH3CCH3 or CH3CHCH2-X

X

1-pentene KOH(alc) + CH3CH2CH2CH2CH2-X

note: CH3CH2CH2CHCH3 would yield a mixture!

X

2-pentene KOH(alc) + CH3CH2CHCH2CH3

X

2-methyl-2-butene KOH(alc) + NONE!

2-methyl-2-butene

H2C CHCHCH3

CH3

+ CH3CH CCH3

CH3

CH3CH CCH3

CH3KOH(alc)

????????? PURE!

KOH(alc)

CH3CHCHCH3

X

CH3

CH3CH2CCH3

CH3

X

KOH(alc)

CH3CH2C CH2

CH3

+ CH3CH CCH3

CH3

No alkyl halide will yield 2-methyl-2-butene as the only product of dehydrohalogenation

Saytzeff orientation:

Ease of formation of alkenes:

R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2

Stability of alkenes:

R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2

CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2 RCH=CH2

Br 1-butene 19%

sec-butyl bromide +

CH3CH=CHCH3 RCH=CHR

2-butene 81%

KOH (alc)

CH3CH2CH2CHBrCH3 CH3CH2CH=CHCH3 + CH3CH2CH2CH=CH2

71% 29%

CH3 CH3 CH3

CH3CH2CCH3 + KOH(alc) CH3CH=CCH3 + CH3CH2C=CH2

Br 71% 29%

CH3 CH3 CH3

CH3CHCHCH3 + KOH(alc) CH2=CHCHCH3 + CH3CH=CCH3

Br major product

Order of reactivity in E2: 3o > 2o > 1o

CH3CH2-X CH2=CH2 3 adj. H’s

CH3CHCH3 CH3CH=CH2 6 adj. H’s & more stable

X alkene

CH3 CH3

CH3CCH3 CH=CCH3 9 adj. H’s & most stable

X alkene

C C

H W

RDSC C

H

1) + :W

2) C C

H

- HC C

Elimination unimolecular E1

Elimination, unimolecular E1

a) RX: 3o > 2o > 1o

b) rearragement possible

c) may yield mixtures

d) Saytzeff orientation

e) element effect

f) no isotope effect

g) rate = k [RW]

E1:

Rate = k [RW] => only RW involved in RDS

R-I > R-Br > R-Cl ―element effect‖ =>

C—X is broken in RDS

R-H R-D no ―isotope effect‖ =>

C—H is not broken in the RDS

Elimination, unimolecular E1

a) RX: 3o > 2o > 1o carbocation

b) rearragement possible ―

c) may yield mixtures

d) Saytzeff orientation

e) element effect C—W broken in RDS

f) no isotope effect C—H not broken in RDS

g) rate = k [RW] only R-W in RDS

alkyl halide + base substitution or elimination?

C C

H

X

:Z

SN2

E2

R-X + base ????????

1) If strong, conc. base:

CH3 > 1o => SN2 R-Z

3o > 2o => E2 alkene(s)

2) If weak, dilute base:

3o > 2o > 1o => SN1 and E1 R-Z + alkene(s)

3) If KOH(alc.)

3o > 2o > 1o => E2 alkene(s)

SN2

CH3CH2CH2-Br + NaOCH3 CH3CH2CH2-O-CH3

1o

CH3 E2 CH3

CH3CCH3 + NaOCH3 CH3C=CH2 + HOCH3

Br

3o

E2

CH3CH2CH2-Br + KOH(alc) CH3CH=CH2

CH3 CH3

CH3CHCHCH3 + dilute OH- CH3CCH2CH3 SN1

Br OH

CH3

+ CH3C=CHCH2 E1

CH3

CH3CHCHCH3 CH3

+ CH2=CCH2CH3 E1

[1,2-H]

CH3

CH3CCH2CH3

2. dehydration of alcohols:

| | | |

— C — C — acid, heat — C = C — + H2O

| |

H OH

a) ROH: 3o > 2o > 1o

b) acid is a catalyst

c) rearrangements are possible

d) mixtures are possible

e) Saytzeff

f) mechanism is E1

note: reaction #3 for alcohols!

Mechanism for dehydration of an alcohol = E1

C C

H OH

+ H C C

H OH2

1)

2) C C

H OH2

RDSC C

H

+ H2O

C C

H

3) C C + H

CH3CH2-OH + 95% H2SO4, 170oC CH2=CH2

CH3 CH3

CH3CCH3 + 20% H2SO4, 85-90oC CH3C=CH2

OH

CH3CH2CHCH3 + 60% H2SO4, 100oC CH3CH=CHCH3

OH

+ CH3CH2CH=CH2

CH3CH2CH2CH2-OH + H+, 140oC CH3CH2CH=CH2

rearrangement! + CH3CH=CHCH3

Synthesis of 1-butene from 1-butanol:

CH3CH2CH2CH2-OH + HBr CH3CH2CH2CH2-Br

SN2 E2 KOH(alc)

CH3CH2CH=CH2

only!

To avoid the rearrangement in the dehydration of the alcohol

the alcohol is first converted into an alkyl halide.

Syntheses, alkenes:

1. dehydrohalogenation of alkyl halides

E2

2. dehydration of alcohols

E1

3. dehalogenation of vicinal dihalide

4. (later)

R-OH

R-X Alkene

vicinal

dihalide

H+

Zn

KOH

(alc.)