Question

Draw the alkene formed when 1-heptyne is treated with hbr in the presence of peroxide.

Answer 1

Heptene formed is -

$CH_3-CH_2-CH_2-CH_2-CH_2-CH=CHBr$

Explanation:

The two possibilities when the peroxide is not present

• $CH_{3}-CH_{2}-CH_{2}-CH_{2}-CH_{2}-C$≡$CH +HBr$ → $CH_3-CH_2-CH_2-CH_2-CH_2-CBr=CH_{2}$

• $CH_3-CH_2-CH_2-CH_2-CH_2-CBr=CH_2$ + HBr →$CH_3-CH_2-CH_2-CH_2-CH_2-CBr_2-CH_3$

In presence peroxide,

$CH_3-CH_2-CH_2-CH_2-CH_2-C$≡$CH$+ HBr →$CH_3-CH_2-CH_2-CH_2-CH_2-CH=CHBr$

• When peroxides are present in the reaction mixture, hydrogen bromide adds to the triple bond of heptane with regioselectivity.

• This reaction is opposite to that of Markovnikov's rule which says that when asymmetrical alkene reacts with a protic acid HX, then the hydrogen of an acid is attached to the carbon with more in number of hydrogen substituents, and the halide (X) group is attached to the carbon with more in number of substituents of alkyl.
• One mole of HBr adds to one mole of 1-heptane.
• The structure of heptene formed is -

$CH_3-CH_2-CH_2-CH_2-CH_2-CH=CHBr$