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Nucleaphilic substitution reactions, at the minimum H-(hydride) ion from aldehydes and -CH₃ from ketones needs to be withdrawn.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.

Fructose contains a keto group, yet it reduces Fehling solution and Tollen's reagent. Why ?

Fructose although contains a ketone group yet it acts as a reducing sugar. This is explained by the formation of glucose from fructose by enolisation. Enolisation results in the formation of glucose from fructose and vice versa. (Lobry de Bruyn Van Ekenstein rearrangement).

There are two -NH2 groups in semicarbazide that might react with a ketone or aldehyde. Explain why the reaction occurs with only one -NH2.

The -NH₂ group closer to the carbonyl group is deactivated (resonance-stabilized) compared with the other end -NH₂ group:

Related Questions:

Penta-1,3-diene is more stable than penta-1,4 -diene. Why?

Phenoxide ion is more stable than an alkaoxide. why?

Why does aqueous sodium bicarbonate solution dissolve carboxylic acids but not phenol though they are also acidic ?

Why enol form of Beta Dicarbonyl compound more stable than those of monocarbonyl compounds?

Why enol form of phenol is more stable than its keto isomers?

Reactivity order of Pyrrole, Furan and Thiophene towards Electrophilic substitution :

Phenyl group is known to extract negative inductive effect, but each phenyl ring in biphenyl is more reactive than benzene towards Electrophilic substation. Why?

Chloroform is more acidic than fluoroform why?

Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ? 

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

Why is the Tropylium carbocation less stable than the tricyclopropylmethyl carbocation?