Editing Aldehyde (section)

==Common reactions==<!-- This section is linked from [[Organic reaction]] -->
Aldehydes participate in many reactions.<ref name="March"/> From the industrial perspective, important reactions are:
* condensations, e.g., to prepare [[plasticizers]] and [[polyols]], and
* reduction to produce alcohols, especially "oxo-alcohols". From the biological perspective, the key reactions involve addition of nucleophiles to the formyl carbon in the formation of imines (oxidative deamination) and hemiacetals (structures of aldose sugars).<ref>{{Cite web |date=2023-07-29 |title=Aldehyde and Ketone - NEB Class 12 Chemistry 2080 |url=https://www.iswori.com.np/2023/07/aldehyde-and-ketone-class-12-chemistry.html |access-date=2023-07-29 |website=Iswori Education |language=en}}</ref><ref name="March"/>

===Acid-base reactions===
Because of [[Resonance (chemistry)|resonance stabilization]] of the conjugate base, an [[alpha hydrogen|α-hydrogen]] in an aldehyde is weakly [[acid]]ic with a [[pKa|p''K''<sub>a</sub>]] near 17. Note, however, this is much more acidic than an alkane or ether hydrogen, which has [[pKa|p''K''<sub>a</sub>]] near 50 approximately, and is even more acidic than a ketone α-hydrogen which has [[pKa|p''K''<sub>a</sub>]] near 20. This acidification of the α-hydrogen in aldehyde is attributed to:
* the electron-withdrawing quality of the formyl center and
* the fact that the conjugate base, an [[enolate]] anion, delocalizes its negative charge.
The formyl proton itself does not readily undergo deprotonation.

===Enolization===
Aldehydes (except those without an alpha carbon, or without protons on the alpha carbon, such as formaldehyde and benzaldehyde) can exist in either the [[Ketone|keto]] or the [[enol]] [[tautomer]]. [[Keto–enol tautomerism]] is catalyzed by either acid or base. In neutral solution, the enol is the minority tautomer, reversing several times per second.<ref>{{cite encyclopedia|url=https://www.britannica.com/science/aldehyde/Tautomerism|encyclopedia=Encyclopedia Britannica|date=4 June 2024 |article=Aldehyde Tautomerism}}</ref> But it becomes the dominant tautomer in strong acid or base solutions, and enolized aldehydes undergo [[carbonyl alpha-substitution reactions|nucleophilic attack at the α position]].<ref>{{cite book|title=Organic synthesis: the disconnection approach|edition=2nd|first1=Stuart|last1=Warren|first2=Paul|last2=Wyatt|publisher=Wiley|year=2008|isbn=978-0-470-71236-8|pages=129–133}}</ref><ref>{{cite book|pages=601–608|title=Advanced Organic Chemistry|edition=5th|volume=A: Structure and Mechanisms|first1=Francis&nbsp;A.|last1=Carey|first2=Richard&nbsp;J.|last2=Sundberg|publisher=Springer|isbn=978-0-387-44899-2|year=2007}}</ref>

===Reduction===
{{Main|Aldehyde reduction}}
The formyl group can be readily reduced to a [[primary alcohol]] ({{chem2|\sCH2OH}}). Typically this conversion is accomplished by catalytic [[hydrogenation]] either directly or by [[transfer hydrogenation]]. [[Stoichiometry|Stoichiometric]] reductions are also popular, as can be effected with [[sodium borohydride]].

===Oxidation===
The formyl group readily oxidizes to the corresponding [[carboxyl group]] ({{chem2|\sCOOH}}). The preferred oxidant in industry is oxygen or air. In the laboratory, popular oxidizing agents include [[potassium permanganate]], [[nitric acid]], [[Chromium trioxide|chromium(VI) oxide]], and [[chromic acid]]. The combination of [[manganese dioxide]], [[cyanide]], [[acetic acid]] and [[methanol]] will convert the aldehyde to a methyl [[ester]].<ref name="March"/>

Another oxidation reaction is the basis of the ''silver-mirror test''. In this test, an aldehyde is treated with [[Tollens' reagent]], which is prepared by adding a drop of [[sodium hydroxide]] solution into [[silver nitrate]] solution to give a precipitate of silver(I) oxide, and then adding just enough dilute [[ammonia]] solution to redissolve the precipitate in aqueous ammonia to produce {{chem2|[Ag(NH3)2]+}} complex. This reagent converts aldehydes to carboxylic acids without attacking carbon–carbon double bonds. The name ''silver-mirror test'' arises because this reaction produces a precipitate of silver, whose presence can be used to test for the presence of an aldehyde.

A further oxidation reaction involves [[Fehling's reagent]] as a test. The {{chem2|Cu(2+)}} complex ions are reduced to a red-brick-coloured {{chem2|[[Copper(I) oxide|Cu2O]]}} precipitate.

If the aldehyde cannot form an enolate (e.g., [[benzaldehyde]]), addition of strong base induces the [[Cannizzaro reaction]]. This reaction results in [[disproportionation]], producing a mixture of alcohol and carboxylic acid.

===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>

===More complex reactions===
{| class="wikitable sortable" style="background-color: white; float: center; border-collapse: collapse; margin: 0em 1em;"
! Reaction name !! Product !!class="unsortable"| Comment
|-
|valign=top| [[Wolff–Kishner reduction]]
|valign=top| [[Alkane]]
| If an aldehyde is converted to a simple hydrazone ({{chem2|RCH\dNHNH2}}) and this is heated with a base such as KOH, the terminal carbon is fully reduced to a methyl group. The Wolff–Kishner reaction may be performed as a [[One-pot synthesis|one-pot reaction]], giving the overall conversion {{chem2|RCH\dO -> RCH3}}.
|-
|valign=top| [[Pinacol coupling reaction]]
| [[Diol]]
| With reducing agents such as magnesium
|-
|valign=top| [[Wittig reaction]]
|valign=top| [[Alkene]]
| Reagent: an [[ylide]]
|-
|valign=top| [[Takai reaction]]
|valign=top| [[Alkene]]
| Diorganochromium reagent
|-
|valign=top| [[Corey–Fuchs reaction]]s
|valign=top| [[Alkyne]]
| Phosphine-dibromomethylene reagent
|-
|valign=top| [[Ohira–Bestmann reaction]]
|valign=top| [[Alkyne]]
| Reagent: dimethyl (diazomethyl)phosphonate
|-
|valign=top| [[Johnson–Corey–Chaykovsky reaction]]
|valign=top| [[Epoxide]]
| Reagent: a [[sulfonium]] [[ylide]]
|-
|valign=top| [[Oxo-Diels–Alder reaction]]
|valign=top| [[Pyran]]
| Aldehydes can, typically in the presence of suitable catalysts, serve as partners in [[cycloaddition]] reactions. The aldehyde serves as the dienophile component, giving a pyran or related compound.
|-
|valign=top| [[Hydroacylation]]
|valign=top| [[Ketone]]
| In hydroacylation an aldehyde is added over an unsaturated bond to form a [[ketone]].
|-
|valign=top| [[Decarbonylation]]
|valign=top| Alkane
| Catalysed by transition metals
|}