Alkene Reaction

CARBONCARBONCHEMISTRY 2CHEMISTRY 2

AlkeneAlkeneReaction and MechanismsReaction and Mechanisms

6-2

66

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Characteristic ReactionsCharacteristic Reactions

+

Hydrochlorination (hydrohalogenation)

CC C CH C l

HC l

+

Hydration

C C C CH OH

H2 O

6-3

66



+

Bromination (halogenation)

C C C CB r B r

B r2

+

Hydroxylation (oxidation)

C C C CHO OH

OsO4

6-4

66



Polymerization

CC C Cinitiator

nn

+

Hydrogenation (reduction)

CC C CHH

H2

6-5

66


PE DiagramsPE Diagrams

Ea

ΔH

transitionstate

A + B

C + D

Reaction Coordinate

Pote

ntia

l ene

rgy

6-6

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Addition of HXAddition of HX• Addition is regioselective

•• MarkovnikovMarkovnikov’’s s rulerule: in the addition of HX or H2Oto an alkene, H adds to the carbon of the doublebond that has the greater number of hydrogensbonded to it

1-Chloropropane (not observed)

2-Chloropropane

Propene

+

+

ClC l

CH3 CH=CH2 HCl

CH3CH-CH2 CH3CH- CH2

H H

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HCl HCl + 2-Butene+ 2-Butene• A two-step mechanism

• Step 1: formation of sec-butyl cation, a 2° carbocationintermediate

-+

+δ-δ+

H -C lCH3CH=CHCH3

C lsec-Butyl cation

slow, rate-limiting step

+H

CH3CH-CHCH3

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HCl HCl + 2-Butene+ 2-Butene• Step 2: reaction of the sec-butyl cation with chloride

ion

- +

sec-Butyl cation (a Lewis acid)

+C l

C l CH3CHCH2CH3CH3CHCH2CH3 Chloride ion(a Lewis base)

fast

6-9

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CarbocationsCarbocations•• CarbocationCarbocation: a species containing a positively

charged carbon• Carbocations are

• classified as 1°, 2°, or 3° depending on the number ofcarbons bonded to the carbon bearing the positivecharge

• electrophiles; that is, they are “electron-lovers”

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Carbocation Carbocation StructureStructure• Bond angles about the

positively charged carbon are120°

• Carbon uses sp2 hybrid orbitalsto form sigma bonds to thethree attached groups

• The unhybridized 2p orbital liesperpendicular to the sigmabond framework and containsno electrons

R CRR+

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Carbocation Carbocation StabilityStability• Relative stability

Methyl cation

(methyl)

Ethyl cation

(1°)

Isopropyl cation

(2°)

tert-Butyl cation

(3°)

Increasing carbocation stability

+ + + +CH

H

CH3 CCH3

CH3

HC

CH3

CH3CH3CH

H

H

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Carbocation Carbocation StabilityStability• Carbocations are stabilized by the electron-

withdrawing inductive effect of the positively-charged carbon• according to molecular orbital calculations:

HH

CH

H3CCH3

CCH3

+0.83

+0.06

+0.60

+0.13

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Addition of HAddition of H22OO• Addition of water is called hydration• Acid-catalyzed hydration of an alkene is

regioselective - hydrogen adds to the lesssubstituted carbon of the double bond

Propene 2-Propanol+

OH

CH3CH=CH2 H2OH2 SO4 CH3CH-CH2

H

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Addition of HAddition of H22OO• Step 1: proton transfer from solvent to the alkene

+

++

intermediateA 2o carbocation

+HO

H

HOHH

CH3CH=CH2

CH3CHCH3

slow, rate-limiting step

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Addition of HAddition of H22OO• Step 2: reaction of the carbocation intermediate with

H2O to form an oxonium ion

• Step 3: proton transfer to solvent

+

++

An oxonium ion

H OHH

CH3CHCH3 O-H CH3CHCH 3fast

++

+OH HOH

H

HH O HCH3CHCH3 CH3CHCH3

OH

fast

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Addition of ClAddition of Cl22 and Br and Br22• Carried out with either the pure reagents or in an

inert solvent such as CCl4 or CH2Cl2

2,3-Dibromobutane2-Butene+

Br BrBr2CH3CH=CHCH3 CH3CH -CHCH3CC l4

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Addition of ClAddition of Cl22 and Br and Br22• Addition is stereoselective

•• Stereoselective Stereoselective reactionreaction: a reaction in whichone stereoisomer is formed or destroyed inpreference to all others than might be formedor destroyed

trans-1,2-Dibromo-cyclohexane

Cyclohexene

+ Br2 CCl4Br

Br

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Addition of ClAddition of Cl22 and Br and Br22• Addition involves a two-step mechanism

• Step 1: formation of a bridged bromonium ionintermediate

C CB r

C C

B rBr

Br -

A bridged bromonium ion intermediate

+

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Addition of ClAddition of Cl22 and Br and Br22• Step 2: Attack of halide ion from the opposite side of

the three-membered ring

Anti addition-

C C

B r

C C

B r

B r

B r

+

6-20

66


Addition of ClAddition of Cl22 and Br and Br22• For a cyclohexene, anti coplanar addition

corresponds to trans-diaxial addition

trans-Diaxial(less stable)

trans-Diequatorial(more stable)

B r

B r

B rB rB r2

6-21

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Oxidation with OsOOxidation with OsO44• Oxidation by OsO4 converts an alkene to a

glycol, a compound with -OH groups on twoadjacent carbons• oxidation is syn stereoselective

H

HO

H

OH

+ ROOHOsO4

cis-Cyclopentanediol(a cis glycol)

Cyclopentene

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Oxidation with OsOOxidation with OsO44• Intermediate is a cyclic osmic ester containing a

five-membered ringH

O

H

OOs

O OA cyclic osmic ester

O OOs

O O

H

HO

H

OH

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Reduction of AlkenesReduction of Alkenes• Most alkenes react with H2 in the presence of a

transition metal catalyst to give alkanes• commonly used catalysts are Pt, Pd, Ru, and Ni

• The process is called catalytic reduction or,alternatively, catalytic hydrogenation

+ H2P t

Cyclohexene Cyclohexane

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Reduction of AlkenesReduction of Alkenes• the most common pattern is syn stereoselectivity

70% to 85%cis-1,2-Dimethyl-

cyclohexane

1,2-Dimethyl-cyclohexene

+CH3

CH3

CH3

CH3

CH3

CH3

H2/P t

30% to 15%trans-1,2-Dimethyl-

cyclohexane

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Reaction StereochemistryReaction Stereochemistry• how many stereoisomers are formed in the addition of

bromine to cis-2-butene?

C C HHH3C CH3cis--2-Butene

C CH

HH3C

CH3Br

BrC C

H

HH3C

CH3Br

Br+

A pair of enantiomers(a racemic mixture)

Br2

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Reaction StereochemistryReaction Stereochemistry• Consider the oxidation of 2-butene by OsO4

• three stereoisomers are possible for 2,3-butanediol;one pair of enantiomers and one meso compound

• which are formed in this oxidation?

CH3CH=CH2CH3 CH3CH-CHCH3

2-Butene 2,3-Butanediol

OsO4OH OH

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Reaction StereochemistryReaction Stereochemistry• syn hydroxylation of cis-2-butene produces only the

meso compound

identical;a meso compound

(2S,3R)-2,3-Butanediol

(2R,3S)-2,3-Butanediol

2

2

3

3C

HO

HO

CH

C

OH

HCH3

H3C

C

OH

H3CH

CH3H

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Reaction StereochemistryReaction Stereochemistry• Enantiomerically pure products can never be

formed from achiral starting materials and achiralreagents

• An enantiomerically pure product can begenerated in a reaction if at least one of thereactants is enantiomerically pure, or if thereaction is carried out in an achiral environment

Alkene Reaction

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