Advices For Studying Organic Chemistry

A Stereochemistry of the Reaction

Addition of Bromine to cis- and trans-2-Butene
cis-2-Butene reacts with bromine to yield the enantiom eric 2,3-dibromobutanes:

(a) Br −(b) CC

Br

HH

3 C

HCH 3

Br

CC

Br CH 3

(2S,3S)-2,3-D

(chi

H 3 CH

H

Br

(2R, 3 R-)2,3-Dibromobutane

ibromobutane

(a)

Bromonium ion (b)

c reacts with bromine to

yield an achiral bromonium ion and

a bromide ion. [Reaction at the

other face of the alkene (top) d

yield the same bromonium ion.]

The bromonium ion reacts with the

bromide ions at equal rates by paths (a)

d (b) to yield the two enantiomers in

equal amounts (i.e., as the racemic form).

CC

HCH

3

H H

3 C

Br

Br

Br

δ+

δ−

+

H H CCHCH 3

3 C

(achiral)

(chiral)

ral)

is-2-butene

woul

an

trans-2-Butene reacts with bromine to yield m eso-2,3-dibromobutane.:

(a) Br −(b) CC

Br

HH

3 C

CH 3

H

Br

CC

Br

H 3 CH

HCH^3

Br

(R,S-)2,3-Dibromobutane

(R,S)-2,3-Dibromobutane

(a)

Bromonium ion (b)

CC

H 3 CH CHH^3

Br

Br

Br

δ+

δ−

+

H H CCHCH^3

3 C

(chiral)

(meso)

(meso)

trans-2-Butene reacts with bromine

yo yield chiral bromonium ions and

bromide ions. [Reaction at the other

face (top) would yield the enantiomer

of the bromonium ion as shown here.]

When the bromonium ions react by

either path (a) or path (b), they yiled

the same achiral meso compound.

[Reaction of the enantiomer of the

intermediate bromonium ion would

produce the same result.]^