In the following reactions, P, Q, R and S are the major products....

The isobutyl chloride on reaction with magnesium metal in dry ether give respective Grignard reagent. Grignard reagents produce hydrocarbon on reaction with water.

Grignard reagent can react with carbon dioxide and acidic water to give respective carboxylic acid. Carboxylic acid gives carboxylate with sodium hydroxide.This salt of carboxylic acid undergo Kolbe's electrolysis to give hydrocarbon as shown below.

Kolbe electrolysis :

The two isobutyl radicals formed in the above reaction combines to give a hydrocarbon that contain eight carbons.

The Grignard reagent formed from isobutyl chloride, reacts with acetaldehyde give respective secondary alcohol, which undergo oxidation with ${CrO}_{3}$ gives ketone as shown below.

The above product undergoes aldol condensation reaction but not Cannizzaro's reaction.

The isobutyl chloride gives isobutyl cyanide on reaction with $ethanolicNaCN$. The cyanide undergo reduction with hydrogen in nickel gives primary amine. The primary amine formed undergo carbyl amine reaction gives isocyanide. The isocyanide gives secondary amine with lithium aluminium hydride.