Editing Aldehyde (section)

==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===
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" style="background-color:white;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=9780471005285|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=9781119991397|pages=199–202|url=https://books.google.com/books?id=WlODZ-WX8vIC&q=9781119991397}}</ref>
|}