Question

Heating of alkanes with fuming sulphuric acid or oleum at high temperature, which forms sulphonic acid, is called:
(A) nitration
(B) halogenation
(C) sulphonation
(D) oxidation

Answer 1

Hint: A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. It is formed when an alkane group reacts with sulphur trioxide.

Complete step by step answer: A sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula $RS{O}_{2}OH$, where R is an organic alkyl or aryl group and the $S{O}_{2}OH$ group a sulphonyl hydroxide. As a substituent, it is known as a sulpho group. The parent compound (with the organic substituent replaced by hydrogen) is the parent sulphonic acid, $HS{O}_{2}OH$, a tautomer of sulphurous acid, $S(O){(OH)}_{2}$. Salts or esters of sulfonic acids are called sulphonates.
Now, when an alkane reacts with sulphuric acid $({H}_{2}S{O}_{4})$ or oleum $({H}_{2}{S}_{2}{O}_{7})$ at high temperature, the sulphur group is added, which leads to the formation of sulphonic acid. This process is called sulphonation. The reactions below explain the process.
When oleum, $({H}_{2}{S}_{2}{O}_{7})$ is heated at high temperature it breaks down and form sulphuric acid and sulphur trioxide.
${ H }_{ 2 }{ S }_{ 2 }{ O }_{ 7 }\quad \xrightarrow {High \quad temp}{ H }_{ 2 }S{ O }_{ 4 }\quad +\quad S{ O }_{ 3 }$
And then this sulphur trioxide reacts with an alkane to form sulphonic acid.
$R{ H }_{ 3 }\quad +\quad { SO }_{ 3 }\quad \longrightarrow \quad RS{ O }_{ 2 }OH$
Due to the addition of the sulphur group, this process is known as sulphonation.

Hence, option (c) is the correct option.

Note: This reaction only takes place at high temperatures. If the reaction does not take place at high temperatures, oleum will not break to form sulphuric acid and sulphur trioxide. And the formation of sulphuric acid and sulphur trioxide as the products is an important step for this reaction to take place.