Describe the following:
$(i)$ Acetylation
$(ii)$ Cannizzaro reaction
$(iii)$ Cross aldol condensation
$(iv)$ Decarboxylation

(N/A) $(i)$ Acetylation:
The introduction of an acetyl functional group into an organic compound is known as acetylation. It is usually carried out in the presence of a base such as pyridine,dimethylaniline,etc. This process involves the substitution of an acetyl group for an active hydrogen atom. Acetyl chloride and acetic anhydride are commonly used as acetylating agents.
For example,acetylation of ethanol produces ethyl acetate.
$CH_3-CH_2-OH + CH_3COCl \xrightarrow{\text{Pyridine}} CH_3COOC_2H_5 + HCl$
$(ii)$ Cannizzaro reaction:
The self oxidation-reduction (disproportionation) reaction of aldehydes having no $\alpha$-hydrogens on treatment with concentrated alkalis is known as the Cannizzaro reaction. In this reaction,two molecules of aldehydes participate where one is reduced to alcohol and the other is oxidized to carboxylic acid.
For example,when methanal is treated with concentrated potassium hydroxide,methanol and potassium methanoate are produced.
$2HCHO + KOH \rightarrow CH_3OH + HCOOK$
$(iii)$ Cross-aldol condensation:
When aldol condensation is carried out between two different aldehydes,or two different ketones,or an aldehyde and a ketone,then the reaction is called a cross-aldol condensation. If both the reactants contain $\alpha$-hydrogens,four compounds are obtained as products.
For example,ethanal and propanal react to give four products.
$(iv)$ Decarboxylation:
Decarboxylation refers to the reaction in which carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with soda-lime.
$CH_3-COONa + NaOH \xrightarrow{\Delta, CaO} CH_4 + Na_2CO_3$
Decarboxylation also takes place when aqueous solutions of alkali metal salts of carboxylic acids are electrolyzed. This electrolytic process is known as Kolbe's electrolysis.