Question

Among the isomeric alkanes of molecular formula ${{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}$, identify the one that on photochemical chlorination yields:
i. A single monochloride
ii. Three isomeric monochlorides
iii. Four isomeric monochlorides
Given:
1. The class of the compound is: alkane
2. The molecular formula is: ${{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}$
3. Reaction is: photochemical chlorination

Answer 1

The phenomenon of isomerism in alkanes along with the understanding of photochemical chlorination reaction can be used to deduce the three isomers.

Complete step by step solution
We know that in photochemical chlorination; ${\rm{C}}{{\rm{l}}_{\rm{2}}}$ can be added to an alkane $\left( {{\rm{RH}}} \right)$ in the presence of sunlight by free radical mechanism to give a monochlorinated alkane. We can write a general chemical equation for the same as follows:
${\rm{RH}} + \left( {{\rm{C}}{{\rm{l}}_{\rm{2}}}{\rm{/h}}\nu } \right) \to {\rm{RCl}}$

Now, let’s talk about our given reactant. The compounds with the same molecular formula but different properties due to difference in their structure are called structural isomers. Here, we are given the class of the compounds to be alkanes and the molecular formula as ${{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}$ Let’s draw the different isomers as follows:

As we can see, in pentane, all the five carbons are in a straight chain; in isopentane, only four are in a straight chain and one is present as methyl group and in neopentane, only three are in a straight chain and two are present as methyl groups.

As we have seen that in mono-chlorination, one hydrogen atom will be replaced by one chlorine atom. Let’s consider the non-equivalent hydrogen atoms present in these isomers which upon replacement would give different isomers:

We can see that monochlorination of pentane would give us three isomeric products by replacing hydrogen atoms from first, second and third carbon atoms. Similarly, in case of isopentane, we will get four isomeric products upon replacing hydrogen atoms from first, second, third and fourth carbons. Now, for neopentane, there would be only one product as all the hydrogen atoms are equivalent in this case.

Hence, the isomer that would give a single monochloride is neopentane, one that would give three isomeric mono-chlorides is pentane and one that would give four isomeric mono-chlorides is isopentane.

Note:
Usually, photochemical halogenation results in mono- as well as poly-chlorination giving various isomers as now not just the position but number of chlorines would also differ giving many products.