Ncert Exercises & Solutions

10.21 Primary alkyl halide C4H9Br (a) reacted with alcoholic KOH to give compound (b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.

There are two primary alkyl halides having the formula, C4H9Br. They are n − bulyl bromide
and isobutyl bromide.

$C H_{3} — C H_{2} — C H_{2} — C H_{2} — B r C H_{3} — C H — C H_{2} — B r$

$|$

$C H_{3}$

$n - B u t y l b r o m i d e l s o b u t y l b r o m i d e$

Therefore, compound (a) is either n−butyl bromide or isobutyl bromide.
Now, compound (a) reacts with Na metal to give compound (b) of molecular formula,
C8H18, which is different from the compound formed when n−butyl bromide reacts with Na
metal. Hence, compound (a) must be isobutyl bromide.

$2 C H_{3} C H_{2} C H_{2} C H_{2} B r \to_{(W u r t z r e a c t i o n)}^{2 N a / d r y e t h e r} C H_{3} C H_{2} C H_{2} C H_{2} C H_{2} C H_{2} C H_{2} C H_{3} + 2 N a B r$

$n - B u t y l b r o m i d e n - O c \tan e$

$C H_{3} C H C H_{2} B r \to_{(W u r t z r e a c t i o n)}^{2 N a / d r y e t h e r} C H_{3} C H C H_{2} C H_{2} C H C H_{3} + 2 N a B r$

$| | |$

$l s o b u t y l b r o m i d e C H_{3} C H_{3}$

$(a) 2 - 5 - D i m e t h y l h e x a n e$

Thus, compound (d) is 2, 5−dimethylhexane.
It is given that compound (a) reacts with alcoholic KOH to give compound (b). Hence,
compound (b) is 2−methylpropene.

$C H_{3} — C H — C H_{2} — B r \to_{(D e h y d r o h a \log e n a t i o n)}^{K O H (a l c) ∆} C H_{3} — C = C H_{2} + H B r$

$| |$

$l s o b u t y l c h l o r i d e 2 - M e t h y l p r o p e n e$

$(a) (b)$

Also, compound (b) reacts with HBr to give compound (c) which is an isomer of (a).
Hence, compound (c) is 2−bromo−2−methylpropane.

$B r$

$|$

$C H_{3} — C H = C H_{2} \to_{(M a r k o v n i k o v a d d i t i o n)}^{H B r} C H_{3} — C H — C H_{3}$

$|$

$2 - M e t h l p r o p e n e 2 - B r o m o - 2 - m e t h y l p r o p a n e$

$(b) (c)$

$(a n i s o m e r o f (a))$

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