Carbonyl compounds do not undergo nucleophillc substitution reactions.why?

Nucleaphilic substitution reactions, at the minimum H-(hydride) ion from aldehydes and -CH₃ from ketones needs to be withdrawn. These are strong nucleophiles, and can not be displaced by other nucleophiles, therefore, carbonyl compounds can not undergo nucleophilic substitution reactions.

Only aldehydes, methyl ketones and cyclic ketones react wilh sodium bisulphite.explain why?

Due to the larger size at sulphite ion, it can attack only those carbonyl compounds. where, there is lesser sterie hinderance, that's  why, it reacts with only aldehydes, methyl ketones and cyclic ketones.

Why does benzene undergoes substitution reaction instead of addition reaction?

Benzene nucleus present in all the aromatic hydrocarbons is very stable; it gives a number of substitution reactions but resists addition reactions. Benzene also fails to give tests for unsaturation i.e. it does not decolourise alkaline KMnO4 and bromine water.    

Benzene gives electrophilic substitution due to It is due to the following reasons:    

(2) Due to the presence of π-electron cloud above and below the plane of carbon atoms of the benzene ring.

(2) Benzene ring acts as a source of electrons for electrophilic reagents.  

(3) In electrophilic substitution aromatic character of benzene ring is preserved due to resonance stabilisation. The formation of formation of addition product will destroyed the aromatic ring structure of benzene and therefore addition takes place with very difficulty. On the above explanation we can say that electrophilic substitution reactions are feature of benzene ring.