Editing Aldehyde (section)

===Nucleophilic addition reactions===
[[Nucleophile]]s add readily to the carbonyl group. In the product, the carbonyl carbon becomes sp<sup>3</sup>-hybridized, being bonded to the nucleophile, and the oxygen center becomes protonated:
:{{chem2|RCHO + Nu- → RCH(Nu)O-}}
:{{chem2|RCH(Nu)O- + H+ → RCH(Nu)OH}}

In many cases, a water molecule is removed after the addition takes place; in this case, the reaction is classed as an [[addition reaction|addition]]–[[elimination reaction|elimination]] or [[addition reaction|addition]]–[[condensation reaction]]. There are many variations of nucleophilic addition reactions.

====Oxygen nucleophiles====
In the [[acetalisation]] reaction, under [[acid]]ic or [[base (chemistry)|basic]] conditions, an [[Alcohol (chemistry)|alcohol]] adds to the carbonyl group and a proton is transferred to form a [[hemiacetal]]. Under [[acid]]ic conditions, the hemiacetal and the alcohol can further react to form an [[acetal]] and water. Simple hemiacetals are usually unstable, although cyclic ones such as [[glucose]] can be stable. Acetals are stable, but revert to the aldehyde in the presence of acid. Aldehydes can react with water to form [[hydrate]]s, {{chem2|R\sCH(OH)2}}. These diols are stable when strong [[electron withdrawing group]]s are present, as in [[chloral hydrate]]. The mechanism of formation is identical to hemiacetal formation.

Another aldehyde molecule can also act as the nucleophile to give polymeric or oligomeric acetals called paraldehydes.

====Nitrogen nucleophiles====
In [[alkylimino-de-oxo-bisubstitution]], a primary or secondary amine adds to the carbonyl group and a 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]] ({{chem2|NH2OH}}) can also add to the carbonyl group. After the elimination of water, this results in an [[oxime]]. An [[ammonia]] derivative of the form {{chem2|H2NNR2}} such as [[hydrazine]] ({{chem2|H2NNH2}}) 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]].<ref name=Fuson/>

====Carbon nucleophiles====
The [[Cyanide|cyano]] group in [[Hydrogen cyanide|HCN]] can add to the carbonyl group to form [[cyanohydrin]]s, {{chem2|R\sCH(OH)CN}}. In this reaction the {{chem2|CN-}} ion is the [[nucleophile]] that attacks the partially positive carbon atom of the [[Carboxyl group|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 {{chem2|H+}}, such as from the HCN molecule, to form the alcohol group of the cyanohydrin.

[[Organometallic chemistry|Organometallic compounds]], such as [[organolithium reagent]]s, [[Grignard reagent]]s, or [[acetylide]]s, undergo [[nucleophilic addition]] reactions, yielding a substituted alcohol group. Related reactions include [[organostannane addition]]s, [[Barbier reaction]]s, and the [[Nozaki–Hiyama–Kishi reaction]].

In the [[aldol reaction]], the metal [[enolates]] of [[ketone]]s, [[ester]]s, [[amide]]s, and [[carboxylic acids]] add to aldehydes to form β-hydroxycarbonyl compounds ([[aldol]]s). 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 [[bisulfite]]s:
:{{chem2|RCHO + HSO3- → RCH(OH)SO3-}}
This reaction is used as a test for aldehydes and is useful for separation or purification of aldehydes.<ref name=Fuson>{{cite book |title=The Systematic Identification of Organic Compounds |first1=R. L. |last1=Shriner |first2=C. K. F. |last2=Hermann |first3=T. C. |last3=Morrill |first4=D. Y. |last4=Curtin |first5=R. C. |last5=Fuson |publisher=John Wiley & Sons|year=1997 |isbn=978-0-471-59748-3}}</ref><ref>{{Cite journal |last1=Furigay |first1=Maxwell H. |last2=Boucher |first2=Maria M. |last3=Mizgier |first3=Nikola A. |last4=Brindle |first4=Cheyenne S. |date=2018-04-02 |title=Separation of Aldehydes and Reactive Ketones from Mixtures Using a Bisulfite Extraction Protocol |journal=Journal of Visualized Experiments |issue=134 |pages=57639 |doi=10.3791/57639 |issn=1940-087X |pmc=5933314 |pmid=29658940}}</ref>