Q.Draw structure of propanone and indicate its polarity.

The carbonyl group's polarity comes from oxygen's higher electronegativity plus resonance between a neutral C=O major contributor and a minor charge-separated (C(+)-O(-)) contributor, giving a resonance hybrid with a partially positive (electrophilic) carbonyl carbon -- which is why nucleophiles attack it. Aldehydes are more reactive than ketones toward nucleophiles for two combined reasons: electronically, a ketone's two electron-donating alkyl groups reduce its carbonyl carbon's electrophilicity more than an aldehyde's single alkyl group does; sterically, a ketone's two bulky alkyl groups hinder the appro …