Aldehyde
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222677095
2008-06-30T15:51:57Z
130.101.90.163
/* Other nomenclature */
[[Image:aldehyde2.png|thumb|100px|right|An aldehyde.<br>-R is the group attached to the aldehyde group.]]
An '''aldehyde''' is an organic compound containing a terminal [[carbonyl group]]. This [[functional group]], which consists of a [[carbon]] atom bonded to a [[hydrogen]] atom and [[double bond|double-bonded]] to an [[oxygen]] atom (chemical formula O=CH-), is called the '''aldehyde group'''. The aldehyde group is also called the '''formyl''' or '''methanoyl''' group.
The word ''aldehyde'' seems to have arisen from '''''al'''cohol '''dehyd'''rogenated''. In the past, aldehydes were sometimes named after the corresponding [[alcohol]]s, for example, ''vinous aldehyde'' for [[acetaldehyde]]. (''Vinous'' is from [[Latin]] ''vinum'' = wine, the traditional source of [[ethanol]]; compare ''[[vinyl]]''.)
The aldehyde group is [[polar molecule|polar]]. [[Oxygen]], more [[electronegative]] than carbon, pulls the electrons in the [[carbon-oxygen bond]] towards itself, creating an electron deficiency at the carbon atom.
Owing to resonance stabilization of the conjugate base, an [[alpha hydrogen|α-hydrogen]] in an aldehyde is more acidic than a hydrogen atom in an alkane, with a typical [[pKa]] of 17.
==Nomenclature==
===IUPAC names for aldehydes===
[[Image:Iupac-aldehyde.png|frame|right|244px|Examples of IUPAC nomenclature for aldehydes.]]
[[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], 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 CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CHO 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 CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CHO is named ''[[butyraldehyde|butanal]]''.
# In other cases, such as when a -CHO group is attached to a ring, the suffix ''-carbaldehyde'' may be used. Thus, C<sub>6</sub>H<sub>11</sub>CHO 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, CHOCH<sub>2</sub>COOH is named ''3-oxopropanoic acid''.
# If replacing the aldehyde group with a carboxyl (-COOH) group 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''. For example:
:*HCHO may be called ''[[formaldehyde]]''.
:*CH<sub>3</sub>CHO may be called ''[[acetaldehyde]]''.
:*C<sub>6</sub>H<sub>5</sub>CHO may be called ''[[benzaldehyde]]''.
===Other nomenclature===
The carbon atom adjacent to a carbonyl group is called the α carbon. Carbon atoms further away from
the group may be named β for the carbon atom bonded to the α carbon, γ for the next, and so on.
Hydrogen atoms bonded to these carbon atoms are named likewise:
an α hydrogen is a hydrogen atom bonded to the α carbon and so on.
A reaction that introduces an aldehyde group is known as a ''[[formylation reaction]]''.
==Chemistry==
===Synthesis===
There are several methods for preparing aldehydes:
* Reacting a [[Alcohol#Primary.2C Secondary.2C and Tertiary Alcohols|primary alcohol]] with an [[oxidizing agent]]. In the laboratory, this may be achieved by heating the alcohol with a chromium(VI) reagent an acidified solution of [[potassium dichromate]], which is [[redox|reduced]] to green Cr<sup>3+</sup> during the reaction. Excess [[dichromate]] will further oxidise the aldehyde to form a [[carboxylic acid]], so either the aldehyde is [[distillation|distilled]] out as it forms (if [[Vapor pressure|volatile]]) or milder methods and reagents such as [[pyridinium chlorochromate|PCC]] oxidation, [[IBX acid]], [[Dess-Martin periodinane]], or [[Swern oxidation]] are used. The reaction is illustrated below with [[propan-1-ol]] being oxidised to form [[propionaldehyde]], and again with [[pentan-1-ol]] being oxidized to form [[pentanal]].
:CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>OH —→ CH<sub>3</sub>CH<sub>2</sub>CHO
:[[Image:Oxidation of pentan-1-ol to pentanal.svg]]
* Reacting an [[alkene]] (if there is a [[vinyl]]ic hydrogen) with [[ozone]] will form an ozonide (an unstable, explosive intermediate), which yields an aldehyde upon reduction with zinc and acid at reduced temperatures. This process is called [[ozonolysis]].
* Reacting an [[ester]] with diisobutyl aluminium hydride ([[DIBAL-H]]) or sodium aluminium hydride can cause reduction, yielding an aldehyde.
* Reduction of an [[acid chloride]] using the [[Rosenmund reduction]] or using lithium tri-t-butoxyaluminium hydride (LiAlH(O-t-C<sub>4</sub>H<sub>9</sub>)<sub>3</sub>).
* Reaction of ketones with [[methoxymethylenetriphenylphosphine]] in a modified [[Wittig reaction]].
* Various [[formylation reaction]]s, such as the [[Vilsmeier-Haack reaction]], can be used to introduce an aldehyde group.
* In the [[Nef reaction]], aldehydes form by hydrolysis of salts of primary nitro compounds.
* [[Zincke aldehyde]]s form by reaction of pyridinium salts with secondary amines followed by hydrolysis.
* In the [[Stephen aldehyde synthesis]] aldehydes form from [[nitrile]]s, [[tin(II) chloride]], and [[hydrochloric acid]].
* In the [[Meyers synthesis]] they form by hydrolysis of an oxazine.
* The [[McFadyen-Stevens reaction]] is a base-catalyzed thermal decomposition of acylsulfonylhydrazides
===Keto-enol tautomerism===
Aldehydes can exist in either the keto or [[enol]] [[tautomers]]. [[Keto-enol tautomerism]] is catalyzed by either acid or base.
===Common reactions===<!-- This section is linked from [[Organic reaction]] -->
====Reduction and oxidation====
* The aldehyde group can be reduced to the group -CH<sub>2</sub>OH, changing the aldehyde into a [[primary alcohol]].
* The aldehyde group can be oxidized to the group -COOH, yielding a [[carboxylic acid]]. Suitable oxidizing agents include [[potassium permanganate]], [[nitric acid]], [[Chromic acid|chromium(VI) oxide]], and [[acid (chemistry)|acidified]] [[potassium dichromate]].
* The combination of [[manganese dioxide]], [[acetic acid]] and [[methanol]] will convert the aldehyde to a methyl [[ester]] <ref>Called '''Corey-Gilman-Ganem oxidation''': ''New methods for the oxidation of aldehydes to carboxylic acids and esters'' [[Elias J. Corey]], Norman W. Gilman, and B. E. Ganem [[J. Am. Chem. Soc.]] 1968; 90(20) pp 5616 - 5617; {{DOI|10.1021/ja01022a059}}</ref>
* Another oxidation reaction is 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 [Ag(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> complex. This reagent will convert aldehydes to carboxylic acids without attacking carbon-carbon double-bonds. The name ''silver mirror test'' arises because this reaction will produce a precipitate of silver whose presence can be used to test for the presence of an aldehyde.
* If the aldehyde can not form an enolate (e.g. benzaldehyde), addition of strong base causes the [[Cannizzaro reaction]] to occur, producing a mixture of alcohol and carboxylic acid.
====Nucleophilic addition reactions====
In nucleophilic addition reactions a [[nucleophile]] can add to the carbon atom in the carbonyl group, yielding an addition compound in which this carbon atom has [[tetrahedral molecular geometry]]. Together with protonation of the oxygen atom in the carbonyl group (which can take place either before or after addition); this yields a product where the carbon atom in the carbonyl group is bonded to the nucleophile, a hydrogen atom, and a hydroxyl group.
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 various examples of nucleophilic addition reactions.
* In the [[acetalisation]] reaction, under [[acid]]ic or [[base (chemistry)|basic]] conditions, an [[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 (under acidic or basic conditions) to form [[hydrate]]s, R-C(H)(OH)(OH), although these are only stable when strong [[electron withdrawing group]]s are present, as in [[chloral hydrate]]. The mechanism is identical to hemiacetal formation.
* 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 to yield an [[imine]]. This reaction is catalyzed by acid.
* The cyano group in [[Hydrogen cyanide|HCN]] can add to the carbonyl group to form [[cyanohydrin]]s, R-C(H)(OH)(CN).
* In the [[Grignard reaction]], a [[Grignard reagent]] adds to the group, eventually yielding an alcohol with a substituted group from the Grignard reagent. Related reactions are the [[Barbier reaction]] and the [[Nozaki-Hiyama-Kishi reaction]].
* In the [[aldol reaction]], the metal [[enolates]] of [[ketone]]s, [[ester]]s, [[amide]]s, and [[carboxylic acids]] will add to aldehydes to form β-hydroxycarbonyl compounds ([[aldol]]s). Acid or base-catalyzed dehydration will then lead to α,β-unsaturated carbonyl compounds. The combination of these two steps is known as the [[aldol condensation]].
* [[Hydroxylamine]] (NH<sub>2</sub>OH) can add to the carbonyl group. After the elimination of water, this will result in an [[oxime]].
* An [[ammonia]] derivative of the form H<sub>2</sub>NNR<sub>2</sub> such as [[hydrazine]] (H<sub>2</sub>NNH<sub>2</sub>) or [[2,4-dinitrophenylhydrazine]] can add to the carbonyl group. After the elimination of water, this will result in the formation of a [[hydrazone]]. This forms the basis of a test for aldehydes and [[ketones]].
====More complex reactions====
* If an aldehyde is converted to a simple hydrazone (RCH=NHNH<sub>2</sub>) and this is heated with a base such as [[Potassium hydroxide|KOH]], the terminal carbon is fully reduced via the [[Wolff-Kishner reaction]] to a [[methyl]] group. The Wolff-Kishner reaction may be performed as a [[One-pot synthesis|one-pot reaction]], giving the overall conversion RCH=O → RCH<sub>3</sub>.
* Reaction of aldehydes with [[reducing agent]]s such as [[magnesium]] gives [[diol]]s in a [[Pinacol coupling reaction]].
* The [[Wittig reaction]] takes aldehydes to [[alkene]]s and the [[Corey-Fuchs reaction]] takes aldehydes to [[alkyne]]s. Both use a [[triphenylphosphine]] reagent. The [[Takai reaction]] also converts aldehydes to alkenes. The [[Corey-Chaykovsky reagent]] is a [[sulfonium ylide]] which converts aldehydes to [[epoxide]]s.
==Examples of aldehydes==
* [[Methanal]] (Formaldehyde)
* [[Ethanal]] (Acetaldehyde)
* [[Propanal]] (Propionaldehyde)
* [[Butanal]] (butyraldehyde)
* [[Glucose]]
* [[Benzaldehyde]]
* [[Cinnamaldehyde]]
* [[tolualdehyde]]
==Related compounds==
Other kinds of organic compounds containing [[carbonyl group]]s include
* [[Dialdehyde]]s
* [[Ketone]]s
* [[Carboxylic acid]]s
* [[Amide]]s
==References==
{{Refimprove|date=December 2007}}
{{reflist}}
{{Wiktionary}}
{{Functional Groups}}
[[Category:Aldehydes]]
[[Category:functional groups]]
[[ar:ألدهيد]]
[[bg:Алдехид]]
[[cs:Aldehydy]]
[[da:Aldehyd]]
[[de:Aldehyde]]
[[et:Aldehüüdid]]
[[es:Aldehído]]
[[eo:Aldehido]]
[[fa:آلدهید]]
[[fo:Aldehyd]]
[[fr:Aldéhyde]]
[[ko:알데하이드]]
[[id:Alkanal]]
[[it:Aldeidi]]
[[he:אלדהיד]]
[[la:Aldehydum]]
[[lv:Aldehīdi]]
[[mk:Алдехид]]
[[nl:Aldehyde]]
[[ja:アルデヒド]]
[[no:Aldehyd]]
[[nn:Aldehyd]]
[[pl:Aldehydy]]
[[pt:Aldeído]]
[[ru:Альдегиды]]
[[sk:Aldehyd]]
[[sl:Aldehid]]
[[sr:Алдехид]]
[[sh:Aldehid]]
[[su:Aldehida]]
[[fi:Aldehydi]]
[[sv:Aldehyd]]
[[vi:Anđehyt]]
[[tg:Алдегид]]
[[tr:Aldehitler]]
[[uk:Альдегіди]]
[[zh:醛]]