A nucleophilic addition reaction is a type of chemical addition reaction in which a nucleophile forms a sigma bond with an electron-deficient species. In organic chemistry, this reaction occurs when a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, causing the double or triple bond to break. The carbonyl carbon is an excellent electrophile and will undergo reactions with a large variety of nucleophiles. The most important reactions involving carbonyl groups characteristically have a nucleophilic addition as part of their mechanism/19%3A_Aldehydes_and_Ketones-_Nucleophilic_Addition_Reactions/19.04%3A_Nucleophilic_Addition_Reactions_of_Aldehydes_and_Ketones).

Nucleophilic addition reactions of nucleophiles with electrophilic double or triple bonds (π bonds) create a new carbon center with two additional single, or σ, bonds. Addition of a nucleophile to carbon–heteroatom double or triple bonds such as >C=O or -C≡N show great variety. These types of bonds are polar, meaning their carbon atoms carry a partial positive charge.

Nucleophilic addition reactions are considered very important in organic chemistry since they enable the conversion of carbonyl groups into a variety of functional groups. Generally, nucleophilic addition reactions of carbonyl compounds can be broken down into the following three steps:

• The electrophilic carbonyl carbon forms a sigma bond with the nucleophile.
• The carbon-oxygen pi bond is now broken, forming an alkoxide intermediate (the bond pair of electrons are transferred to the oxygen atom).
• The alkoxide intermediate is protonated, forming the final product.

Examples of nucleophilic addition reactions include the hydration of aldehydes and ketones, where the carbonyl oxygen is protonated before the nucleophilic attack is carried out by the alcohol/04%3A_Organic_reactions/4.06%3A_Nucleophilic_Addition_Reactions). Aldehydes and ketones undergo nucleophilic addition reactions with monohydric alcohols to yield hemiacetals. Neutral nucleophiles can also undergo nucleophilic addition to carbonyls/19%3A_Aldehydes_and_Ketones-_Nucleophilic_Addition_Reactions/19.04%3A_Nucleophilic_Addition_Reactions_of_Aldehydes_and_Ketones).