sign in sign up
<<
Home , Regioselectivity

5.8 Preparation of : Elimination Reactions b-Elimination Reactions Overview

dehydrogenation of alkanes: X = Y = H

dehydration of alcohols: X = H; Y = OH

of alkyl halides: X = H; Y = Br, etc.

X C C Y C C + X Y b a Dehydrogenation

limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene

important economically, but rarely used in laboratory-scale syntheses 750°C CH3CH3 H2C CH2 + H2

750°C CH3CH2CH3 H2C CHCH3 + H2 5.9 Dehydration of Alcohols Dehydration of Alcohols

H2SO4 + CH3CH2OH H2C CH2 + H2O 3 2 160°C

H2SO4 OH + H2O 140°C (79-87%)

CH 3 H3C H SO 2 4 + H C C OH C CH2 + H2O 3 heat H3C CH3 (82%) R' Relative tertiary: R C OH Reactivity most reactive R" R'

R C OH

H

H

R C OH primary: least reactive H 5.10 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule Regioselectivity

H SO 2 4 + HO 80°C 10 % 90 %

A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. Regioselectivity

CH 3 CH3 CH3

H3PO4 OH + heat 16 % 84 %

A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. The Zaitsev Rule

When elimination can occur in more than one direction, the principal is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH

R C C CH2R

H CH3

three protons on this b carbon The Zaitsev Rule

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH

R C C CH2R

H CH3

two protons on this b carbon The Zaitsev Rule

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH

R C C CH2R

H CH3

only one proton on this b carbon The Zaitsev Rule

When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens.

R OH R CH2R C R C C CH2R C C R CH H CH3 3

only one proton on this b carbon 5.11 in Alcohol Dehydration Stereoselectivity

H2SO4 + heat OH (25%) (75%)

A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. 5.12 The Mechanism of -Catalyzed Dehydration of Alcohols A connecting point...

The dehydration of alcohols and the reaction of alcohols with hydrogen halides share the following common features:

1) Both reactions are promoted by

2) The relative reactivity decreases in the order tertiary > secondary > primary

These similarities suggest that carbocations are intermediates in the acid-catalyzed dehydration of alcohols, just as they are in the reaction of alcohols with hydrogen halides. Dehydration of tert-Butyl Alcohol

CH 3 H3C H SO 2 4 + H C C OH C CH2 H2O 3 heat H3C CH3

first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides Mechanism

Step 1: Proton transfer to tert-butyl alcohol H .. (CH ) C O: + H + 3 3 H O.. H H fast, bimolecular

H H + (CH3)3C O: + :O:

H H tert-Butyloxonium ion Mechanism

Step 2: Dissociation of tert-butyloxonium ion to carbocation H +

(CH3)3C O: H slow, unimolecular

H

+ + (CH3)3C :O: tert-Butyl cation H Mechanism

Step 3: Deprotonation of tert-butyl cation. H

H + :O: H3C

+ C CH2 H

H3C fast, bimolecular H H3C + C CH2 + H O:

H3C H Carbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols carbocations can:

react with nucleophiles lose a b-proton to form an alkene Dehydration of Primary Alcohols

H2SO4 + CH CH OH H2C CH2 + H2O 3 2 160°C avoids carbocation because primary carbocations are too unstable oxonium ion loses water and a proton in a bimolecular step Mechanism

Step 1: Proton transfer from acid to ethanol H .. CH CH O: + H 3 2 H O.. H H fast, bimolecular

H H + CH3CH2 O: + :O: H H Ethyloxonium ion Mechanism

Step 2: Oxonium ion loses both a proton and a water molecule in the same step.

H H + :O: + H CH2 CH2 O:

H H slow, bimolecular H H + : O H + H2C CH2 + :O:

H H 5.13 Rearrangements in Alcohol Dehydration

Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol. Example OH

H3PO4, heat

+ +

3% 64% 33% Rearrangement involves alkyl group migration

CH3 carbocation can lose a proton as shown CH C CHCH 3 + 3 or it can undergo a CH 3 methyl migration 3% CH3 group migrates with its pair of electrons to adjacent positively charged carbon Rearrangement involves alkyl group migration

CH3 CH3 97% + CH C CHCH CH C CHCH 3 + 3 3 3

CH3 CH3

3% tertiary carbocation; more stable

Rearrangement involves alkyl group migration

CH3 CH3 97% + CH C CHCH CH C CHCH 3 + 3 3 3

CH3 CH3

3%

Another rearrangement

CH3CH2CH2CH2OH

H3PO4, heat

CH3CH2CH CH2 + CH3CH CHCH3 12% mixture of cis (32%) and trans-2-butene (56%) Rearrangement involves hydride shift

H + oxonium ion can lose : CH3CH2CH2CH2 O water and a proton H (from C-2) to give 1-butene

doesn't give a carbocation directly because primary carbocations are too CH3CH2CH CH2 unstable Rearrangement involves hydride shift

H + CH CH CH CH O : CH3CH2CHCH3 3 2 2 2 3 2+ 3 H hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost

carbocation formed by CH3CH2CH CH2 carbocation formed by hydride shift is secondary Rearrangement involves hydride shift

H + CH CH CH CH O : CH3CH2CHCH3 3 2 2 2 3 2+ 3 H

CH3CH2CH CH2 CH3CH CHCH3 mixture of cis and trans-2-butene Hydride Shift

H +

CH3CH2CHCH2 O : H H

H + CH3CH2CHCH2 + : O : H H Carbocations can...

•react with nucleophiles

•lose a proton from the b-carbon to form an alkene

•rearrange (less stable to more stable)