View source for Aldehyde

{{Short description|1=Organic compound containing the functional group R–CH=O}}
:[[File:Aldehyde general structure.svg|150px|alt=Aldehyde structure|thumb|class=skin-invert|Aldehyde structure]]

In [[organic chemistry]], an '''aldehyde''' ({{IPAc-en|ˈ|æ|l|d|ᵻ|h|aɪ|d}}) (lat. ''al''cohol ''dehyd''rogenatum,<ref>{{cite web |title=Diccionario de la lengua española|url=https://dle.rae.es/aldeh%C3%ADdo|access-date=2025-03-16}}</ref> [[dehydrogenated]] [[Alcohol (chemistry)|alcohol]]) is an [[organic compound]] containing a [[functional group]] with the structure {{chem2|R\sCH\dO}}.<ref>IUPAC Gold Book, [http://goldbook.iupac.org/A00208.html aldehydes].</ref> The functional group itself (without the "R" [[side chain]]) can be referred to as an '''aldehyde''' but can also be classified as a '''formyl group'''. Aldehydes are a common motif in many chemicals important in technology and biology.<ref name="March"/><ref>{{cite book |title=The Carbonyl Group |year=1966 |editor=Saul Patai |isbn=978-0-470-77105-1 |doi=10.1002/9780470771051|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups|volume=1}}{{cite book |title=The Carbonyl Group|year=1970 |editor=Jacob Zabicky |isbn=978-0-470-77122-8 |doi=10.1002/9780470771228 |publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups|volume=2}}</ref>

==Structure and bonding==
Aldehyde molecules have a central carbon atom that is connected by a double bond to oxygen, a single bond to hydrogen and another single bond to a third substituent, which is carbon or, in the case of formaldehyde, hydrogen. The central carbon is often described as being sp<sup>2</sup>-[[Orbital hybridisation|hybridized]]. The aldehyde group is somewhat [[polar molecule|polar]]. The {{chem2|C\dO}} bond length is about 120–122 [[picometer]]s.<ref>{{cite book |title=The Carbonyl Group|chapter=General and Theoretical Aspects of the Carbonyl Group|author=G. Berthier, J. Serre |year=1966 |editor=Saul Patai |doi=10.1002/9780470771051.ch1|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups|volume=1|pages=1–77|isbn=978-0-470-77105-1}}</ref>

==Physical properties and characterization==
Aldehydes have properties that are diverse and that depend on the remainder of the molecule. Smaller aldehydes such as [[formaldehyde]] and [[acetaldehyde]] are soluble in water, and the volatile aldehydes have pungent odors.

Aldehydes can be identified by spectroscopic methods. Using [[IR spectroscopy]], they display a strong ''ν''<sub>CO</sub> band near 1700&nbsp;cm<sup>−1</sup>. In their [[Proton nuclear magnetic resonance|<sup>1</sup>H NMR]] spectra, the formyl hydrogen center absorbs near ''δ''<sub>H</sub>&nbsp;9.5 to 10, which is a distinctive part of the spectrum. This signal shows the characteristic coupling to any protons on the α carbon with a small coupling constant typically less than 3.0&nbsp;Hz. The <sup>13</sup>C NMR spectra of aldehydes and ketones gives a suppressed (weak) but distinctive signal at ''δ''<sub>C</sub>&nbsp;190 to 205.

==Applications and occurrence==
:[[File:Important Aldehyde Structures.svg|class=skin-invert-image|thumb|center|830px]] Important aldehydes and related compounds. The '''aldehyde group''' (or '''formyl group''') is colored red. From the left: (1) [[formaldehyde]] and (2) its trimer [[1,3,5-Trioxane|1,3,5-trioxane]], (3) [[acetaldehyde]] and (4) its enol [[vinyl alcohol]], (5) [[glucose]] (pyranose form as α-{{sc|D}}-glucopyranose), (6) the flavorant [[cinnamaldehyde]], (7) [[retinal]], which forms with [[opsin]]s [[photoreceptor protein|photoreceptors]], and (8) the vitamin [[pyridoxal]].

===Naturally occurring aldehydes===
Traces of many aldehydes are found in [[essential oil]]s and often contribute to their pleasant odours, including [[cinnamaldehyde]], [[cilantro]], and [[vanillin]]. Possibly due to the high reactivity of the formyl group, aldehydes are not commonly found in organic "building block" molecules, such as amino acids, nucleic acids, and lipids. However, most sugars are derivatives of aldehydes. These [[aldose]]s exist as [[hemiacetal]]s, a sort of masked form of the parent aldehyde. For example, in aqueous solution only a tiny fraction of glucose exists as the aldehyde.

==Synthesis==<!--A reaction that introduces an aldehyde group is known as a ''[[formylation reaction]]''.
-->

===Hydroformylation===
Of the several methods for preparing aldehydes,<ref name="March">{{March6th}}</ref> one dominant technology is [[hydroformylation]].<ref name="Bertleff">Bertleff, W.; Roeper, M. and Sava, X. (2003) "Carbonylation" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH: Weinheim. {{doi|10.1002/14356007.a05_217.pub2}}</ref> Hydroformylation is conducted on a very large scale for diverse aldehydes. It involves treatment of the alkene with a mixture of hydrogen gas and carbon monoxide in the presence of a metal catalyst.  Illustrative is the generation of [[butyraldehyde]] by [[hydroformylation]] of [[propylene]]:
:{{chem2|H2 + CO + CH3CH\dCH2 → CH3CH2CH2CHO}}
One complication with this process is the formation of isomers, such as isobutyraldehyde:
:{{chem2|H2 + CO + CH3CH\dCH2 → CH3CH(CHO)CH3}}

===Oxidative routes===
{{Main|Alcohol oxidation}}
The largest operations involve [[methanol]] and [[ethanol]] respectively to [[formaldehyde]] and [[acetaldehyde]], which are produced on multimillion ton scale annually. Other large scale aldehydes are produced by [[autoxidation]] of hydrocarbons: [[benzaldehyde]] from [[toluene]], [[acrolein]] from [[propylene]], and [[methacrolein]] from [[isobutene]].<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a18_261.pub2 |chapter=Oxidation |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2015 |last1=Teles |first1=J. Henrique |last2=Hermans |first2=Ive |last3=Franz |first3=Gerhard |last4=Sheldon |first4=Roger A. |pages=1–103 |isbn=978-3-527-30385-4 }}</ref><ref name=CH2O/> In the [[Wacker process]], oxidation of ethylene to acetaldehyde in the presence of copper and palladium catalysts, is also used.  "[[Green chemistry|Green]]" and cheap oxygen (or air) is the oxidant of choice.

Laboratories may instead apply a wide variety of specialized [[oxidizing agent]]s, which are often consumed stoichiometrically. [[Oxidation with chromium(VI) complexes|Chromium(VI) reagents are popular]]. Oxidation can be achieved by heating the alcohol with an acidified solution of [[potassium dichromate]]. In this case, excess [[dichromate]] will further oxidize the aldehyde to a [[carboxylic acid]], so either the aldehyde is [[distillation|distilled]] out as it forms (if [[Vapor pressure|volatile]]) or milder reagents such as [[pyridinium chlorochromate|PCC]] are used.<ref>{{OrgSynth | author = Ratcliffe, R. W. | title = Oxidation with the Chromium Trioxide-Pyridine Complex Prepared in situ: 1-Decanal | collvol = 6 | collvolpages = 373 | year = 1988 | prep = cv6p0373}}</ref>

A variety of reagent systems achieve aldehydes under chromium-free conditions.  One such are the [[hypervalent organoiodine compounds]] (i.e., [[2-Iodoxybenzoic acid|IBX acid]], [[Dess–Martin periodinane]]), although these often [[Carbonyl oxidation with hypervalent iodine reagents|also oxidize the α position]].   A [[Lux-Flood acid]] will activate other pre-oxidized substrates: [[Sulfonium-based oxidation of alcohols to aldehydes|various sulfoxides]] (e.g. the [[Swern oxidation]]), or amine oxides (e.g., the [[Ganem oxidation]]).  Sterically-hindered [[nitroxyl]]s (i.e., [[TEMPO]]) can [[Oxoammonium-catalyzed oxidation|catalyze aldehyde formation with a cheaper oxidant]].

Alternatively, [[vicinal diol]]s or their [[Organic redox reaction|oxidized sequelae]] ([[acyloin]]s or [[Alpha hydroxy acid|α-hydroxy acids]]) can be oxidized with [[Bond cleavage|cleavage]] to two aldehydes or an aldehyde and [[carbon dioxide]].<ref>{{cite journal|last1=Ōeda|first1=Haruomi|title=Oxidation of some α-hydroxy-acids with lead tetraacetate|journal=Bulletin of the Chemical Society of Japan|date=1934|volume=9|issue=1|pages=8–14|doi=10.1246/bcsj.9.8|doi-access=free}}</ref><ref>{{cite journal|last1=Nwaukwa|first1=Stephen|last2=Keehn|first2=Philip|title=Oxidative cleavage of α-diols, α-diones, α-hydroxy-ketones and α-hydroxy- and α-keto acids with calcium hypochlorite [Ca(OCl)2]|journal=Tetrahedron Letters|date=1982|volume=23|issue=31|pages=3135–3138|doi=10.1016/S0040-4039(00)88578-0}}</ref>

===Specialty methods===
{| class="wikitable sortable skin-invert" style="background-color: transparent;float: center; border-collapse: collapse; margin: 0em 1em;" border="1" cellpadding="2" cellspacing="0"

! width=220px|Reaction name !! Substrate !! class="unsortable" | Comment
|-
|valign=top | [[Ozonolysis]]
|valign=top|[[Alkene]]s
| Reductive [[Work-up (chemistry)|work-up]]; similar effect with [[singlet oxygen]] and no work-up
|-
|valign=top| [[Carbonyl reduction]]
|[[Ester]]s, [[amides]]
| Reduction of an [[ester]] with diisobutylaluminium hydride ([[DIBAL-H]]) or [[sodium aluminium hydride]]; see also [[amide reduction]].
|-
|valign=top| [[Rosenmund reaction]]
|valign=top|[[Acyl chloride]]s
| Acyl chlorides selectively [[organic redox reaction|reduced]] to aldehydes. Lithium tri-''t''-butoxyaluminium hydride ({{chem2|LiAlH(O^{''t''}Bu)3}}) is an effective reagent.{{citation needed|date=February 2016}}
|-
|valign=top| [[Wittig reaction]]
|valign=top|[[Ketone]]s
| A modified Wittig reaction using [[methoxymethylenetriphenylphosphine]] as a reagent.
|-
|valign=top| [[Formylation reaction]]s
|valign=top| [[Nucleophile|Nucleophilic]] [[Aromatic hydrocarbon|arenes]]
| Various reactions, for example the [[Vilsmeier-Haack reaction]].
|-
|valign=top| [[Nef reaction]]
|valign=top| [[Nitro compound]]s
| The [[acid catalysis|acid]] [[hydrolysis]] of a [[Primary (chemistry)|primary]] nitro compound to form an aldehyde.
|-
|valign=top| [[Kornblum oxidation]]
|valign=top| [[Haloalkane]]s
| The oxidation of primary halide with [[dimethyl sulfoxide]] to form an aldehyde.
|-
|valign=top| [[Zincke reaction]]
|valign=top| [[Pyridine]]s
| [[Zincke aldehyde]]s formed in a reaction variation.
|-
|valign=top| [[Stephen aldehyde synthesis]]
|valign=top| [[Nitrile]]s
| Hydrolysis of an [[iminium]] salt generated by [[tin(II) chloride]] and [[Hydrogen chloride|HCl]] to form an aldehyde.
|-
|valign=top| [[Geminal halide hydrolysis]]
|valign=top| [[Geminal]] [[halocarbon|dihalide]]s
| Hydrolysis of [[Primary (chemistry)|primary]] geminal dihalides to yield aldehydes.
|-
|valign=top| [[Meyers synthesis]]
|valign=top| [[Oxazine]]s
| [[Hemiaminal]] oxazine hydrolysis with water and [[oxalic acid]] to yield an aldehyde.
|-
|valign=top| [[Hofmann rearrangement]] variation<ref>{{cite journal|last1=Weerman|first1=R.A.|title=Einwirkung von Natriumhypochlorit auf Amide ungesättigter Säuren|journal=Justus Liebigs Annalen der Chemie|date=1913|volume=401|issue=1|pages=1–20|doi=10.1002/jlac.19134010102|url=https://zenodo.org/record/1427615}}</ref><ref>{{Cite book|last1=Everett|first1=Wallis|last2=Lane|first2=John|title=The Hofmann Reaction|series=Organic Reactions|date=1946|volume=3|issue=7|pages=267–306|isbn=978-0-471-00528-5|doi=10.1002/0471264180.or003.07}}</ref>
|valign=top| [[Saturated and unsaturated compounds|Unsaturated]] or [[Alpha and beta carbon|α]]-[[Hydroxyl|hydroxy]] [[amides]]
| Aldehydes via the hydrolysis of an [[Reaction intermediate|intermediate]] [[Carbamic acid|carbamate]].
|-
|valign=top| [[McFadyen-Stevens reaction]]
|valign=top| [[Hydrazide]]s
| [[Base (chemistry)|Base]]-[[Catalysis|catalyzed]] [[thermal decomposition]] of acylsulfonylhydrazides.
|-
|valign=top| [[Biotransformation]]
|valign=top| [[Alkene]]s
| [[Lyophilized]] cell cultures of ''[[Trametes hirsuta]]'' in the presence of oxygen.<ref>{{cite book|author1=Sutton, Peter |author2=Whittall, John |title=Practical Methods for Biocatalysis and Biotransformations 2|date=2012|publisher=John Wiley & Sons, Ltd.|location=Chichester, West Sussex|isbn=978-1-119-99139-7|pages=199–202|url=https://books.google.com/books?id=WlODZ-WX8vIC&q=9781119991397}}</ref>
|}

==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 |article-number=57639 |doi=10.3791/57639 |issn=1940-087X |pmc=5933314 |pmid=29658940}}</ref>

===More complex reactions===
{| class="wikitable sortable skin-invert" style="background-color: transparent; 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
|}

== Dialdehydes ==
{{main|Dicarbonyl}}
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 [[Succindialdehyde|butanedial]], which is also called succinaldehyde (from [[succinic acid]]).

==Biochemistry==
Some aldehydes are substrates for [[aldehyde dehydrogenase]] [[enzyme]]s which metabolize aldehydes in the body. There are [[Toxicity|toxicities]] associated with some aldehydes that are related to [[Neurodegeneration|neurodegenerative]] disease, [[heart disease]], and some types of [[cancer]].<ref>{{cite journal|last1=Chen|first1=Che-Hong|last2=Ferreira|first2=Julio Cesar Batista|last3=Gross|first3=Eric R.|first4=Daria Mochly|last4=Rosen|title=Targeting Aldehyde Dehydrogenase 2: New Therapeutic Opportunities|journal=Physiological Reviews|date=1 January 2014|volume=94|issue=1|pages=1–34|doi=10.1152/physrev.00017.2013|pmid=24382882|pmc=3929114}}</ref>

==Examples of aldehydes==
* [[Formaldehyde]] (methanal)
* [[Acetaldehyde]] (ethanal)
* [[Propionaldehyde]] (propanal)
* [[Butyraldehyde]] (butanal)
* [[Isovaleraldehyde]] 
* [[Benzaldehyde]] (phenylmethanal)
* [[Cinnamaldehyde]]
* [[Vanillin]]
* [[Tolualdehyde]]
* [[Furfural]]
* [[Retinaldehyde]]
*[[Glycolaldehyde]]

==Examples of dialdehydes==
* [[Glutaraldehyde]]
* [[Glyoxal]]
* [[Malondialdehyde]]
* [[Phthalaldehyde]]
* [[Succindialdehyde]]

==Uses==
Of all aldehydes, formaldehyde is produced on the largest scale, about {{val|6,000,000|u=tons per year}}. It is mainly used in the production of resins when combined with [[urea]], [[melamine]], and [[phenol]] (e.g., [[Bakelite]]). It is a precursor to [[methylene diphenyl diisocyanate]] ("MDI"), a precursor to [[polyurethane]]s.<ref name=CH2O>Reuss, G.; Disteldorf, W.; Gamer, A. O. and Hilt, A. (2005) "Formaldehyde" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a11_619}}.</ref> The second main aldehyde is [[butyraldehyde]], of which about {{val|2,500,000|u=tons per year}} are prepared by [[hydroformylation]]. It is the principal precursor to [[2-ethylhexanol]], which is used as a [[plasticizer]].<ref name=aliphald>Kohlpaintner, C.; Schulte, M.; Falbe, J.; Lappe, P. and Weber, J. (2008) "Aldehydes, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a01_321.pub2}}.</ref> Acetaldehyde once was a dominating product, but production levels have declined to less than {{val|1,000,000|u=tons per year}} because it mainly served as a precursor to [[acetic acid]], which is now prepared by [[carbonylation]] of [[methanol]]. Many other aldehydes find commercial applications, often as precursors to alcohols, the so-called [[oxo alcohol]]s, which are used in detergents. Some aldehydes are produced only on a small scale (less than 1000 tons per year) and are used as ingredients in flavours and [[perfume]]s such as [[Chanel No. 5]]. Fresh, green, citrusy and nutty notes in perfumes are often due to aldehydes<ref>{{Cite web |title=Роль альдегидов в пищевой промышленности и ароматизаторах |url=https://aldehydes.ru/ |access-date=2025-12-26 |website=aldehydes.ru}}</ref>. These include [[cinnamaldehyde]] and its derivatives, [[citral]], and [[lilial]].

==Nomenclature==

===IUPAC names for aldehydes===
The common names for aldehydes do not strictly follow official guidelines, such as those recommended by [[IUPAC]], but these rules are useful. IUPAC prescribes the following nomenclature for aldehydes:<ref>[http://www.uwc.edu/dept/chemistry/helpful_files/nomenclature.pdf Short Summary of IUPAC Nomenclature of Organic Compounds] {{Webarchive|url=https://web.archive.org/web/20060901144219/http://www.uwc.edu/dept/chemistry/helpful_files/nomenclature.pdf |date=2006-09-01}}, web page, University of Wisconsin Colleges, accessed on line August 4, 2007.</ref><ref>[http://www.acdlabs.com/iupac/nomenclature/93/r93_449.htm §R-5.6.1, Aldehydes, thioaldehydes, and their analogues, A Guide to IUPAC Nomenclature of Organic Compounds: recommendations 1993], IUPAC, Commission on Nomenclature of Organic Chemistry, Blackwell Scientific, 1993.</ref><ref>[http://www.acdlabs.com/iupac/nomenclature/93/r93_480.htm §R-5.7.1, Carboxylic acids, A Guide to IUPAC Nomenclature of Organic Compounds: recommendations 1993], IUPAC, Commission on Nomenclature of Organic Chemistry, Blackwell Scientific, 1993.</ref>
# Acyclic [[aliphatic compound|aliphatic]] aldehydes are named as derivatives of the longest carbon chain containing the aldehyde group. Thus, HCHO is named as a derivative of methane, and {{chem2|CH3CH2CH2CHO}} is named as a derivative of [[butane]]. The name is formed by changing the suffix ''-e'' of the parent [[alkane]] to ''-al'', so that HCHO is named ''[[methanal]]'', and {{chem2|CH3CH2CH2CHO}} is named ''[[butyraldehyde|butanal]]''.
# In other cases, such as when a {{chem2|\sCHO}} group is attached to a ring, the suffix ''-carbaldehyde'' may be used. Thus, {{chem2|C6H11CHO}} is known as ''cyclohexanecarbaldehyde''. If the presence of another functional group demands the use of a suffix, the aldehyde group is named with the prefix ''formyl-''. This prefix is preferred to ''methanoyl-''.
# If the compound is a natural product or a [[carboxylic acid]], the prefix ''oxo-'' may be used to indicate which carbon atom is part of the aldehyde group; for example, {{chem2|CHOCH2COOH}} is named ''2-oxoethanoic acid''.
# If replacing the aldehyde group with a [[carboxyl group]] ({{chem2|\sCOOH}}) would yield a carboxylic acid with a trivial name, the aldehyde may be named by replacing the suffix ''-ic acid'' or ''-oic acid'' in this trivial name by ''-aldehyde''.

===Etymology===
The word ''aldehyde'' was coined by [[Justus von Liebig]] as a contraction of the Latin {{lang|la|'''al'''cohol '''dehyd'''rogenatus}} (dehydrogenated alcohol).<ref>Liebig, J. (1835) [https://books.google.com/books?id=85c5AAAAcAAJ&pg=PA289 "Sur les produits de l'oxidation de l'alcool"] (On the products of the oxidation of alcohol), ''Annales de Chimie et de Physique'', '''59''': 289–327. From page 290: "Je le décrirai dans ce mémoire sous le nom ''d'aldehyde''; ce nom est formé de ''alcool dehydrogenatus''." (I will describe it in this memoir by the name of ''aldehyde''; this name is formed from ''alcohol dehydrogenatus''.)</ref><ref>{{Citation |last=Crosland |first=Maurice P. |title=Historical Studies in the Language of Chemistry |publisher=Courier Dover Publications |year=2004 |url=https://books.google.com/books?id=kwQQaltqByAC&q=alcohol+dehydrogenatus&pg=PA297 |isbn=978-0-486-43802-3}}.</ref> In the past, aldehydes were sometimes named after the corresponding [[Alcohol (chemistry)|alcohol]]s, for example, ''vinous aldehyde'' for [[acetaldehyde]]. (''Vinous'' is from [[Latin]] {{lang|la|vinum}} "wine", the traditional source of [[ethanol]], cognate with ''[[Vinyl group|vinyl]]''.)

The term ''formyl group'' is derived from the [[Latin language|Latin]] word {{lang|la|formica}} "ant". This word can be recognized in the simplest aldehyde, [[formaldehyde]], and in the simplest carboxylic acid, [[formic acid]].

== See also ==
* [[Enol]]
* [[Pseudoacid]]
* [[Semialdehyde]]
* [[Ketone]]

==References==
{{reflist}}

==External links==
{{Wiktionary}}
{{wikiquote}}

{{Functional Groups}}
{{Authority control}}

[[Category:Aldehydes| ]]
[[Category:Functional groups]]
[[Category:1830s neologisms]]