Question

Which of the following alkane cannot be made in good yield by Wurtz reaction ?

• A. n-Hexane
• B. 2,3-Dimethylbutane
• C. n-Heptane
• D. n-Butane

Answer 1

Answer: (C)

n-Heptane

Answer 2

$n$-butane, $n$-hexane, $2,3$-dimethylbutane are symmetrical alkanes. Such alkanes are prepared in good yield by Wurtz reaction as these require single alkyl halide for their preparation.
$CH_3CH_2Cl {->[+2\,Na][\text{dry ether}]} \underset{\text{(n-butane)}}{CH_3CH_2CH_2CH_3}$
$CH_3CH_2CH_2Cl {->[+2\,Na][\text{dry ether}]}$
$\underset{\text{(n-hexane)}}{CH_3CH_2CH_2CH_2CH_2CH_3}$
$CH_3- \underset{\overset{|}{CH_3}}{CH}-Cl {->[+2\,Na][\text{dry ether}]}\underset{\text{(2,3 -Dimethyl butane)}}{CH_3-\underset{\overset{|}{CH_3}}{CH}-\underset{\overset{|}{CH_3}}{CH} - CH_3}$
$n$-heptane is an unsymmetrical alkane that requires two different alkyl halides for its preparation. This will form three different alkanes, thus, lowering the yield of required alkane.
$CH_3CH_2CH_2Cl + CH_3CH_2CH_2CH_2Cl {->[+2\,Na][\text{dry ether}]}$
$CH_3CH_2CH_2CH_2CH_2CH_3$
$+ CH_3CH_2CH_2CH_2CH_2CH_2CH_3$
$+CH_3CH_2CH_2CH_2CH_2CH_2CH_2CH_2CH_3$