© e.v. blackburn, 2011 alkenes c n h 2n. © e.v. blackburn, 2011 alkenes called unsaturated...

© E.V. Blackburn, 2011

Alkenes

CnH2n


Alkenes

• called unsaturated hydrocarbons

• also known as olefins (oleum, latin, oil; facere, latin, make)

• CnH2n

• CnH2n + H2 CnH2n+2 - one degree of unsaturation

• contain carbon - carbon double bonds


Degree of unsaturationDegree of unsaturation = (2NC - NX + NN – NH + 2)/2

NC = number of carbons

NX = number of halogens

NN = number of nitrogens

NH = number of hydrogensNH2

ClH

O


Nomenclature – the E/Z system

1. To name alkenes, select the longest carbon chain which includes the carbons of the double bond. Remove the -ane suffix from the name of the alkane which corresponds to this chain. Add the suffix -ene.

a derivative of octene not nonane



2. Number this chain so that the first carbon of the double bond has the lowest number possible.

1

2 3

4

4-butyl-2-octene



C C C CH

H3C

CH3

H

H

H3C

H

CH3

diastereoisomersgeometric isomers

trans cis


C CH

H3C

Cl

Br

This molecule is a 1-bromo-1-chloropropene but is it cis or trans!

cis/trans problems


C CH

H3C

Cl

Br

• then compare the relative positions of the groups of higher priority on these two carbons.

• if the two groups are on the same side, the compound has the Z configuration (zusammen, German, together).

• if the two groups are on opposite sides, the compound has the E configuration (entgegen, German, across).

(Z)-1-bromo-1-chloropropene


• use the Cahn-Ingold-Prelog system to assign priorities to the two groups on each carbon of the double bond.


E-Z designations

CH2CH2CH3

CH3

H3CH2C

Br

CH2Br

OCH3

C(CH3)3

CH(CH3)2

H3CH2C

H3C


Relative stabilities of alkenes• Cis isomers are generally less stable than trans isomers due to strain caused by crowding of the two alkyl groups on the same side of the double bond• Stabilities can be compared by measuring heats of hydrogenation of alkenes.

H

CH3

H

H3C H2

catalystCH3CH2CH2CH3

H = -119.7 kJ/mol

H

CH3

H3C

H H2

catalystCH3CH2CH2CH3

H = -115.5 kJ/mol


Overall relative stabilities of alkenes

CH3

CH3

H3C

H3C>

H

C2H5

H3C

H3C>

H

C2H5

C2H5

H>

H

C2H5

H

C2H5

>H

CH2CH2CH2CH3

H

H


Synthesis of alkenes by elimination reactions

H

X base- HXdehydrohalogenation:

dehydration:H

OH H+/- H2O


Dehydrohalogenation of alkyl halides

a 1,2 elimination reaction

C CH

X+ KOH

C2H5OH+ KX + H2O

Reactivity: RX 3o > 2o > 1o


Dehydrohalogenation

C CH

X+ KOH

C2H5OH+ KX + H2O

Br C2H5ONa

C2H5OH, 55oC 79%+ NaBr + C2H5OH

Br C2H5ONa

C2H5OH, 55oC 91%+ NaBr + C2H5OH


Alkoxide ions – bases used in dehydrohalogenation

CH3CH2OHNa

CH3CH2O- Na+

sodium ethoxide

(CH3)2CHOHAl

((CH3)2CHO-)3 Al3+

aluminum isopropoxide

(CH3)3COHK

(CH3)3CO- K+

potassium t-butoxide


Dehydrohalogenation of alkyl halides

CH3CH2CH2ClKOH

C2H5OHCH3CH=CH2

CH3CH2CH2CH2ClKOH

C2H5OHCH3CH2CH=CH2

- no rearrangement

CH3CH2CHCH3

Cl

KOH

C2H5OHCH3CH=CHCH3

80%CH3CH2CH=CH2

20%

+


The mechanismIn the presence of a strong base, the reaction follows second order kinetics:

rate = k[RX][B-]

However, with weak bases at low concentrations and as we move from a primary halide to a secondary and a tertiary, the reaction becomes first order.

There are two mechanisms for this elimination: E1 and E2.


E2 mechanism

C CX

H:B

+ HB + X-C CX

HB-

-


E1 mechanism

slow

fast

C CH

XC C

H

++ X-

C CH

+

:B

+ HB


Evidence for the E1 mechanism

• Same structural effects on reactivity as for SN1 reactions - 3 > 2 > 1

• Rearrangements can occur indicative of the formation of carbocations

• Follows first order kinetics


Evidence for the E2 mechanism

• There are no rearrangements

• There is a large deuterium isotope effect

• There is an anti periplanar geometry requirement

• The reaction follows second order kinetics


Isotope effects

A difference in rate due to a difference in the isotope present in the reaction system is called an isotope effect.


Isotope effectsIf an atom is less strongly bonded in the transition state than in the starting material, the reaction involving the heavier isotope will proceed more slowly.

C H + ZkH

C H Z C + HZ

The isotopes of hydrogen have the greatest mass differences. Deuterium has twice and tritium three times the mass of protium. Therefore deuterium and tritium isotope effects are the largest and easiest to determine.


Primary isotope effectsThese effects are due to breaking the bond to the isotope.

C H + ZkH

C H Z C + HZ

C D + ZkD

C D Z C + DZ

kH

kD = 5 - 8

Thus the reaction with protium is 5 to 8 times faster than the reaction with deuterium.


CH3CHCH3

Br NaOEt

kH

CH3CH=CH2

CD3CHCD3

Br NaOEt

kD

CD3CH=CD2

kH/kD = 7

Evidence for the E2 mechanism - a large isotope

effect


RI > RBr > RCl > RF

Further evidence for the E2 mechanism

C CX

H:B

+ HB + X-C CX

HB-

-


Orientation and reactivity

CH3CH2CHCH3

Cl

KOH

C2H5OHCH3CH=CHCH3

80%

CH3CH2CH=CH2

20%

+

The ease of alkene formation follows the sequence:-

R2C=CR2 > R2C=CHR > R2C=CH2, RHC=CHR > RHC=CH2

This is also the order of alkene stability. Therefore the more stable the alkene formed, the faster it is formed.

Why?


Let’s look at the transition state for the reaction:

The double bond is partially formed in the transition state and therefore the transition state resembles an alkene. Thus the factors which stabilize alkenes will stabilize this nascent alkene.

A Zaitsev elimination.

Orientation and reactivity

C CH

X

:B

+ HB + X-C C

H

X

B

-

-


anti elimination

HX

:B

X

HB:


?

anti eliminationanti

anti

+

KOHCl

H

H3C

H

H

HH

CH(CH3)2

neomenthyl chloride


anti elimination

ClH

H3C

H

H

CH(CH3)2

HH

menthylchloride

KOH?


Formation of the less substituted alkene

Dehydrohalogenation using a bulky base favours the formation of the less substituted alkene:

H3CHC CCH3

CH3+

CH3

CH2H3CH2CC

27.5% 72.5%

(CH3)3CO- + CH3CH2-C-Br

CH3

CH375oC

(CH3)3COH


Substitution vs elimination

substitution elimination

X

HNu:

X

HNu:

SN2

X

H:Nu

E2

SN2 v E2


Substitution vs elimination

H

+Nu:

Nu

H

SN1

:NuE1

C CH

XC C

H

++ X-

SN1 v E1


Substitution vs eliminationRX + -OR' R-O-R' + 1o

X-

RX + 1o

'R C C-

'R C C R + X-

RX + 1o

-RCN + X-CN

RX = 1o 2o 3o

elimination

substitution

SN2


Dehydration of alcohols

C CH

OH

acid

+ H2O

H2SO4, Al2O3 or H3PO4

H3C CCH3

CH3

OHH

H

H3C

H3C+ H2O


Dehydration of alcohols - the mechanism

(CH3)3C-OH + H+ (CH3)3C-O-HH

+1.

2. (CH3)3C-O-HH

+(CH3)3C+ + H2O

3. CH3C

H3C +

H

HH

ROH

H

H

H3C

H3C+ ROH2

+


Dehydration of alcohols - orientation

CH3CH2CH2CH2OHH+

CH3CH=CHCH3

CH3CH2CHCH2OHCH3 CH3H+

CH3CH=CCH3


The Zaitsev product predominates

CH3CH2CH2CH2OHH+

CH3CH=CHCH3

CH3CH2CH2CH2OH CH3CH2CH2CH2OH2+

CH3CH2CH2CH2OH2+

CH3CH2CH2CH2+-H2O

CH3CH2CH2CH2+

CH3CH2CHCH3+

CH3CH2CHCH3+

?


The Zaitsev product predominates

The transition state explains the orientation:

CH3CH2CHCH3+

?

H3C CH

CH

CH3+HOR

H +

H3C CH

CH

C+

ORH +

H

HH

H


C CX X

Zn

+ ZnX2

Dehalogenation of vicinal dihalides


Hydrogenation of alkynes

Lindlar catalyst: Pd/CaCO3/Pb(OAc)2/quinoline

R R

H2

Lindlar's catalyst

Na or Li

R

H

R

H

H

R

R

H

NH3


Synthesis of alkynes by elimination reactions

C CH

X

H

X

KOH

alcohol

H

X

NaNH2C C

CH3CH=CH2

Br2CH3CHBrCH2Br

NaNH2C C HCH3-CH3CHBrCH2Br


CC-

R'

RH

C C'R

RX2H2/Pt

R

Bu3SnH

R-R'

X2/h

ROH

RI

ROR'

RCN

Br2/CCl4Br

Br

H2SO4/

Zn/

1.

HH

HH

1.H2, Pd/CaCO3

quinoline

Na, NH3, -78oC

© e.v. blackburn, 2011 alkenes c n h 2n. © e.v. blackburn, 2011 alkenes called unsaturated...

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