Nucleophilic addition reactions

Carbonyl compounds react with ammonia derivatives such as hydrazine (in feebly acidic medium) by nucleophilic addition followed by elimination of water, giving hydrazones.

Reactivity towards nucleophilic addition depends on steric and electronic effects.
Order: p-nitrobenzaldehyde > benzaldehyde > p-tolualdehyde > acetophenone
The –M effect of $-NO_2$ increases the positive charge on the $sp^2$ carbonyl carbon.

Hydrazine has a lone pair on nitrogen and acts as a nucleophile.
Step 1 (nucleophilic addition): $NH_2NH_2$ attacks the carbonyl carbon of RCHO to give $RCH(OH)-NH-NH_2$.
Step 2 (elimination): a molecule of water is lost to give the hydrazone $RCH=N-NH_2$.
So it is a nucleophilic addition–elimination reaction.

In the presence of dry HCl, one molecule of ethanol adds to the carbonyl group of acetone to give a hemiketal, $(CH_3)_2C(OH)(OC_2H_5)$.
With excess ethanol the hemiketal reacts with a second molecule of ethanol, losing water, to give the ketal.
$(CH_3)_2C=O + 2C_2H_5OH \xrightarrow{HCl} (CH_3)_2C(OC_2H_5)_2 + H_2O$
So the product is 2,2-diethoxypropane, $(CH_3)_2C(OC_2H_5)_2$.