# Why do carbonyl compounds undergo nucleophilic and not electrophilic addition?

Why aldehydes and ketones undergo nucleophilic addition reactions and not electrophilic addition?

Alkenes show electrophilic addition because of pi bond, which is also present in aldehydes and ketones. But there is nucleophilic addition only. Why?

They sure do. Except that the oxygen has lone pairs, which are more nucleophilic than the $\pi$ bond. Once those electrons attack an electrophile, it further deactivates the $\pi$ bond as a nucleophile.

# General Explanation

According to Feltre and Setsuo (Organic Chemistry, vol.4), the π bond is attracted to the oxygen (through mesomeric effect), leaving a partial positive charge on the adjacent carbon. Then the nucleophile attacks the carbon, leaving for the electrophile the mission to bond to the (now negative) oxygen.

## To further out the question:

What I seem to miss is why the nucleophilic attack happens first (thus naming the reaction), since the exposed O- seems a lot more vulnerable to electrostatic interaction.

Aldehydes and ketones prefer nucleophilic addition reactions Instead of electrophilic addition reactions this is because in case of nucleophilic addition reaction a stable intermediate is formed as compared to the electrophilic addition reactions where the intermediate formed is carbocation which is highly unstable.

• stable intermediate? $\mathrm{S_N2}$ has no intermediates, only a transition state.. – William R. Ebenezer Jun 28 '19 at 2:31