Organic Chemistry

A trans alkene can be prepared from an internal alkyne. The alkyne needed to synthe-

size the desired alkene can be prepared from 1-butyne and a methyl halide. 1-Butyne

can be prepared from ethyne and an ethyl halide.

Example 6.How could you prepare 3,3-dibromohexane from reagents that contain no

more than two carbon atoms?

A geminal dibromide can be prepared from an alkyne. 3-Hexyne is the desired alkyne

because it will form one geminal bromide, whereas 2-hexyne would form two differ-

ent geminal dibromides. 3-Hexyne can be prepared from 1-butyne and ethyl bromide,

and 1-butyne can be prepared from ethyne and ethyl bromide.

PROBLEM 21

Starting with acetylene, how could the following compounds be synthesized?

a. d.

b. e.

c. f.

Cl

Cl

CC CH 3 CCH 3

CH 3

HH

CH 3

Br

CH 3 CH“CH 2 CH 3 CHCH 3

O

CH 3 CH 2 CH 2 C‚CH CH 3 CH 2 CH 2 CH 2 CH

?

reagents with no more than 2 carbon atoms CH 3 CH 2 CCH 2 CH 2 CH 3

Br

Br

3,3-dibromohexane

Section 6.11 Designing a Synthesis I: An Introduction to Multistep Synthesis 257

CC CH 3 CH 2 C

CH 3 CH 2

CH 3 CH 2

HCH 3

H

CCH 3 CH 3 CH 2 CCH HCCH

CC

HCH 3

H

1. NaNH 2


2. CH 3 CH 2 Br
   
   
   1. NaNH 2
      2. CH 3 Br
   
   
   
   

NH 3 (liq)

HC CH CH 3 CH 2 CCH CH 3 CH 2 C CCH (^3) Na
retrosynthetic analysis
synthesis
CH 3 CH 2 CCH 2 CH 2 CH 3 CH 3 CH 2 C CCH 2 CH 3 CH 3 CH 2 CCH HCCH
Br
Br
retrosynthetic analysis

1. NaNH 2
   2. CH 3 CH 2 Br
   
   
   1. NaNH 2
   
   
   2. CH 3 CH 2 Br
   
   
   
   

excess HBr

HC CH CH 3 CH 2 CCH CH 3 CH 2 C CCH 2 CH 3 CH 3 CH 2 CCH 2 CH 2 CH 3

synthesis

Br

Br

Tutorial:

Retrosynthesis using alkynes

as reagents

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