Question

A hydrocarbon ${C_5}{H_{10}}$ does not react with chlorine in dark but gives a single monochloro compound ${C_5}{H_9}Cl$ in bright sunlight. Identify the hydrocarbon.

Answer 1

The organic compounds that are made up only of carbon and hydrogen atoms are known as hydrocarbons. They are classified into two main groups: the open-chain compounds known as aliphatic chain compounds, and closed chain compounds known as cyclic compounds. These open-chain compounds are further sub-divided into straight-chain and branched-chain compounds.

Complete answer:
Step 1
The general formula of an alkane is ${C_n}{H_{2n + 2}}$, an alkene is ${C_n}{H_{2n}}$ , and alkyne is ${C_n}{H_{2n - 2}}$. Cycloalkanes are also regarded as alkanes. They are cyclic alkanes and possess one or more rings of carbon atoms. The general formula of monocyclic cycloalkane containing only one ring is ${C_n}{H_{2n}}$.
Step 2
Alkanes are very simple saturated compounds having only $C - CandC - H$bonds. As they do not have any functional group or multiple bonds hence are least reactive. They undergo a substitution reaction specially halogenation with either chlorine or bromine in the presence of sunlight. This reaction is called chlorination. Alkanes undergo halogenation through the formation of free radicals as intermediates in presence of diffused sunlight or ultraviolet light.
Step 3
Alkenes have a double bond and due to this, they are very reactive. Alkenes undergo additional reactions like the addition of halogen where they react with chlorine and bromine in the presence of carbon tetrachloride to give halogen derivatives.
Step 4
The compound ${C_5}{H_{10}}$ follows the general formula ${C_n}{H_{2n}}$. So, it can be either an alkene or a cyclic hydrocarbon.
The reaction takes place in sunlight and forms a monosubstituted derivative so it must have all the same type of $C - C and C - H$ bonds where anyone hydrogen gets substituted by a chlorine atom. The process of halogenation of alkanes does not take place in the dark as the initiation of free radicals does not take place in the dark. Thus it must be a cyclo-alkane.
Hence the compound is cyclopentane.

Note:
Alkanes undergo halogenation through the formation of free radicals as intermediates. In the presence of diffused sunlight or ultraviolet light, the chlorine molecule breaks up into chlorine-free radicals which attack the hydrocarbon and forms a carbon-free radical and the formation of the monochloro derivative takes place.