Organic Chemistry

An alkyl peroxide has the same effect on the addition of HBr to an alkyne that it has

on the addition of HBr to an alkene (Section 4.10)—it reverses the order of addition

because the peroxide causes to become the electrophile.

The mechanism of the reaction is the same as that for the addition of HBr to an alkene

in the presence of a peroxide. That is, the peroxide is a radical initiator and creates a

bromine radical (an electrophile). If the alkyne is a terminal alkyne, the bromine radical

adds to the spcarbon bonded to the hydrogen; if it is an internal alkyne, the bromide

radical can add with equal ease to either of the spcarbons. The resulting vinylic radical

abstracts a hydrogen atom from HBr and regenerates the bromine radical. Any two

radicals can combine in a termination step.

The halogens and also add to alkynes. In the presence of excess halogen, a

second addition reaction occurs. Typically the solvent is

PROBLEM 8

Give the major product of each of the following reactions:

a. d.

b. e.

c. f. CH 3 C CCH 2 CH 3

excess

HBr

HC CCH 3

excess

HBr

CH 3 C CCH 3

excess

HBr

HC CCH 3

HBr

CH 3 C CCH 3

Br 2

HC CCH (^3) CH 2 Cl 2
HBr
peroxide
CH 3 CH 2 C CCH 3
Cl 2
CH 2 Cl 2
Cl 2
CH 2 Cl 2
Cl
Cl
Cl
Cl
Cl
Cl
CH 3 CH 2 C CCH 3 CH 3 CH 2 C CCH 3
CH 3 CCH
Br 2
CH 2 Cl 2
Br 2
CH 2 Cl 2
Br
Br
Br
Br
Br
Br
CH 3 CCH CH 3 CCH
CH 2 Cl 2.
Cl 2 Br 2
RO OR 2 RO
RO + H Br Br
Br
RO H +
CH 3 CH 2 CCH+ Br CH 3 CH 2 CCH
CH 3 CH 2 CCH++HBr Br
Br
CH 3 CH 2 C CHBr
mechanism for the addition of HBr in the presence of a peroxide
H
HBr CH 3 CH 2 CH CHBr
peroxide
CH +
1-butyne
CH 3 CH 2 C
1-bromo-1-butene
Br–
CH 3 CH 2 CCHCCH 2 CH 3 + HBr 3 CH 2 CH 2 CCH 2 CH 3
Br
Br
3,3-dibromohexane
3-hexyne excess
Section 6.5 Addition of Hydrogen Halides and Addition of Halogens 245
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