Advices For Studying Organic Chemistry

8.3 STEREOCHEMISTRY OF THE IONIC ADDITION TO AN ALKENE

1. The addition of HX to 1-butene leads to the formation of 2-halobutane:

CH 3 CH 2 CH CH 2 + HX CH 3 CH 2 CHCH 3

X

*

2)

achiral.

3) When the halide ion reacts with this achiral carbocation in the second step,

i

The Stereochemistry of the Reaction

1) The product has a stereocenter and can exist as a pair of enantiomers.

The carbocation intermediate formed in the first step of the addition is trigonal

plannar and is

reaction is equally likely at either face.

) The reactions leading to the two enantiomers occur at the same rate, and the

enantiomers are produced in equal amounts as a racemic form.

Ionic Addition to an Alkene

C 2 H 5 CH CH 2 C H

H

(a) C 2 H 5 C

X

CH

H

(a)

H X H^2 (b) (b) HCH

2 5 C

CH

+ +

3

(S)-2-Halobutane (50%)

C 2 H 5 C^3

1-Butene accepts a proton from (R)-2-Halobutane (50%)

HX to form an achiral carbocation. The carbocation reacts wion at equal rates by path (a)ith the hali or (b) to de

formtheenantiomers as a racemate.

X −

Achiral,

trigonal planar carbocation X

(^)

8.4 ADDITION OF SULFURIC ACID TO ALKENES