Previous Year NEET Question- Hydrocarbons L-2

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

A) 2, 3-dimethylbutane

B) n-heptane

C) n-butane

D) n-hexane

Answer: n-heptane#

Sol:#

In Wurtz reaction, an alkyl halide $(R-X)$ give a symmetrical alkane $(R-R)$ of even number of carbon atom with $100 %$ yield. $$ 2 R-X \underset{-2 \mathrm{NaX}}{\stackrel{2 \mathrm{Na} / \text { ether }}{\longrightarrow}} R-R(100 %) $$

An unsymmetrical alkane $\left(R-R^{\prime}\right)$ with odd number of carbon atom can be obtained with $33 %$ yield only, if we take a mixture of two different alkyl halides, $R$ - X and $R^{\prime}-X$. $$ \begin{aligned} R-X+R^{\prime}-X \longrightarrow \underbrace{R-R+R^{\prime}-R^{\prime}}{67 %} \ +\underbrace{R-R^{\prime}}{33 %} \end{aligned} $$

So, here $n$-heptane $\left(\mathrm{C}7 \mathrm{H}{16}\right)$ cannot be obtained with good yield. Hydrocarbons in options (a), (c) and (d) have even number of carbon atom (symmetrical, alkanes).