proton is transferred from the nitrogen to the oxygen atom to create a carbinolamine. In the case of a primary amine, a water molecule can be eliminated from the carbinolamine intermediate to yield an imine or its trimer, a hexahydrotriazine This reaction is catalyzed by acid. Hydroxylamine () can also add to the carbonyl group. After the elimination of water, this results in an oxime. An ammonia derivative of the form such as hydrazine () or 2,4-dinitrophenylhydrazine can also be the nucleophile and after the elimination of water, resulting in the formation of a hydrazone, which are usually orange crystalline solids. This reaction forms the basis of a test for aldehydes and ketones. Carbon nucleophiles The cyano group in HCN can add to the carbonyl group to form cyanohydrins, . In this reaction the ion is the nucleophile that attacks the partially positive carbon atom of the carbonyl group. The mechanism involves a pair of electrons from the carbonyl-group double bond transferring to the oxygen atom, leaving it single-bonded to carbon and giving the oxygen atom a negative charge. This intermediate ion rapidly reacts with , such as from the HCN molecule, to form the alcohol group of the cyanohydrin. Organometallic compounds, such as organolithium reagents, Grignard reagents, or acetylides, undergo nucleophilic addition reactions, yielding a substituted alcohol group. Related reactions include organostannane additions, Barbier reactions, and the Nozaki–Hiyama–Kishi reaction. In the aldol reaction, the metal enolates of ketones, esters, amides, and carboxylic acids add to aldehydes to form β-hydroxycarbonyl compounds (aldols). Acid or base-catalyzed dehydration then leads to α,β-unsaturated carbonyl compounds. The combination of these two steps is known as the aldol condensation. The Prins reaction occurs when a nucleophilic alkene or alkyne reacts with an aldehyde as electrophile. The product of the Prins reaction varies with reaction conditions and substrates employed. Bisulfite reaction Aldehydes characteristically form "addition compounds" with bisulfites: RCHO + HSO3- -> RCH(OH)SO3- This reaction is used as a test for aldehydes and is useful for separation or purification of aldehydes. More complex reactions Dialdehydes A dialdehyde is an organic chemical compound with two aldehyde groups. The nomenclature of dialdehydes have the ending -dial or sometimes -dialdehyde. Short aliphatic dialdehydes are sometimes named after the diacid from which they can be derived. An example is butanedial, which is also called succinaldehyde (from succinic acid). Biochemistry Some aldehydes are substrates for aldehyde dehydrogenase enzymes which metabolize aldehydes