Aldehyde
In organic chemistry, an aldehyde (/ˈældɪhaɪd/) is an organic compound containing a functional group with the structure R−CH=O.[1] 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.[2][3]
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 sp2-
Physical properties and characterization
Aldehydes have properties that are diverse and that depend on the remainder of the molecule. Smaller aldehydes are more soluble in water, formaldehyde and acetaldehyde completely so. The volatile aldehydes have pungent odors.
Aldehydes can be identified by spectroscopic methods. Using
Applications and occurrence
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, (3) acetaldehyde and (4) its enol vinyl alcohol, (5) glucose (pyranose form as α-D-glucopyranose), (6) the flavorant cinnamaldehyde, (7) retinal, which forms with opsins photoreceptors, and (8) the vitamin pyridoxal.
Naturally occurring aldehydes
Traces of many aldehydes are found in
Synthesis
There are several methods for preparing aldehydes,
- H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO
Oxidative routes
Aldehydes are commonly generated by
Laboratories may instead apply a wide variety of specialized
- [O] + CH3(CH2)9OH → CH3(CH2)8CHO + H2O
A variety of reagent systems achieve aldehydes under chromium-free conditions. One such are the
Alternatively,
Specialty methods
Reaction name | Substrate | Comment |
---|---|---|
Ozonolysis | Alkenes | Reductive work-up; similar effect with singlet oxygen and no work-up
|
Carbonyl reduction | amides
|
Reduction of an DIBAL-H) or sodium aluminium hydride; see also amide reduction .
|
Rosenmund reaction
|
Acyl chlorides | Acyl chlorides selectively reduced to aldehydes. Lithium tri-t-butoxyaluminium hydride (LiAlH(OtBu)3) is an effective reagent.[citation needed] |
Wittig reaction | Ketones | A modified Wittig reaction using methoxymethylenetriphenylphosphine as a reagent.
|
Formylation reactions
|
arenes
|
Various reactions, for example the Vilsmeier-Haack reaction .
|
Nef reaction | Nitro compounds | The acid hydrolysis of a primary nitro compound to form an aldehyde. |
Kornblum oxidation | Haloalkanes | The oxidation of primary halide with dimethyl sulfoxide to form an aldehyde. |
Zincke reaction | Pyridines | Zincke aldehydes formed in a reaction variation. |
Stephen aldehyde synthesis | Nitriles | Hydrolysis of an iminium salt generated by tin(II) chloride and HCl to form an aldehyde. |
Geminal halide hydrolysis | Geminal dihalides | Hydrolysis of primary geminal dihalides to yield aldehydes. |
Meyers synthesis | Oxazines
|
Hemiaminal oxazine hydrolysis with water and oxalic acid to yield an aldehyde. |
Hofmann rearrangement variation[10][11] | amides
|
Aldehydes via the hydrolysis of an intermediate carbamate. |
McFadyen-Stevens reaction
|
Hydrazides | Base-catalyzed thermal decomposition of acylsulfonylhydrazides. |
Biotransformation | Alkenes | Lyophilized cell cultures of Trametes hirsuta in the presence of oxygen.[12]
|
Common reactions
Aldehydes participate in many reactions.[2] From the industrial perspective, important reactions are:
- condensations, e.g., to prepare 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)[13].[2]
Acid-base reactions
Because of
- 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
Reduction
The formyl group can be readily reduced to a primary alcohol (−CH2OH). Typically this conversion is accomplished by catalytic hydrogenation either directly or by transfer hydrogenation. Stoichiometric reductions are also popular, as can be effected with sodium borohydride.
Oxidation
The formyl group readily oxidizes to the corresponding
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 [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
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
Nucleophiles add readily to the carbonyl group. In the product, the carbonyl carbon becomes sp3-hybridized, being bonded to the nucleophile, and the oxygen center becomes protonated:
- RCHO + Nu− → RCH(Nu)O−
- 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–elimination or addition–condensation reaction. There are many variations of nucleophilic addition reactions.
Oxygen nucleophiles
In the
Nitrogen nucleophiles
In
Carbon nucleophiles
The
Organometallic compounds, such as organolithium reagents, Grignard reagents, or acetylides, undergo nucleophilic addition reactions, yielding a substituted alcohol group. Related reactions include organostannane additions, Barbier reactions, and the Nozaki–Hiyama–Kishi reaction.
In the
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 bisulfites:
- RCHO + HSO−3 → RCH(OH)SO−3
This reaction is used as a test for aldehydes and is useful for separation or purification of aldehydes.[17][18]
More complex reactions
Reaction name | Product | Comment |
---|---|---|
Wolff–Kishner reduction | Alkane | If an aldehyde is converted to a simple hydrazone (RCH=NHNH2) 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 reaction, giving the overall conversion RCH=O → RCH3. |
Pinacol coupling reaction | Diol | With reducing agents such as magnesium |
Wittig reaction | Alkene | Reagent: an ylide |
Takai reaction
|
Alkene | Diorganochromium reagent |
Corey–Fuchs reactions | Alkyne | Phosphine-dibromomethylene reagent |
Ohira–Bestmann reaction
|
Alkyne | Reagent: dimethyl (diazomethyl)phosphonate |
Johnson–Corey–Chaykovsky reaction | Epoxide | Reagent: a sulfonium ylide |
Oxo-Diels–Alder reaction | 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. |
Hydroacylation | Ketone | In hydroacylation an aldehyde is added over an unsaturated bond to form a ketone. |
Decarbonylation | Alkane | Catalysed by transition metals |
Dialdehydes
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
Biochemistry
Some aldehydes are substrates for
Examples of aldehydes
- Formaldehyde (methanal)
- Acetaldehyde (ethanal)
- Propionaldehyde (propanal)
- Butyraldehyde (butanal)
- Isovaleraldehyde
- Benzaldehyde (phenylmethanal)
- Cinnamaldehyde
- Vanillin
- Tolualdehyde
- Furfural
- Retinaldehyde
- Glycolaldehyde
Examples of dialdehydes
- Glyoxal
- Malondialdehyde
- Succindialdehyde
- Glutaraldehyde
- Phthalaldehyde
Uses
Of all aldehydes, formaldehyde is produced on the largest scale, about 6000000 tons per year. It is mainly used in the production of resins when combined with
Nomenclature
IUPAC names for aldehydes
The common names for aldehydes do not strictly follow official guidelines, such as those recommended by
- Acyclic methanal, and CH3CH2CH2CHO is named butanal.
- In other cases, such as when a −CHO group is attached to a ring, the suffix -carbaldehyde may be used. Thus, 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, CHOCH2COOH is named 2-oxoethanoic acid.
- If replacing the aldehyde group with a carboxyl group(−COOH) 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 alcohol dehydrogenatus (dehydrogenated alcohol).[24][25] In the past, aldehydes were sometimes named after the corresponding alcohols, for example, vinous aldehyde for acetaldehyde. (Vinous is from Latin vinum "wine", the traditional source of ethanol, cognate with vinyl.)
The term formyl group is derived from the
See also
References
- ^ IUPAC Gold Book, aldehydes.
- ^ ISBN 978-0-471-72091-1
- ISBN 9780470771228.
- ISBN 9780470771051.
- ^ .
- ^ Ratcliffe, R. W. (1988). "Oxidation with the Chromium Trioxide-Pyridine Complex Prepared in situ: 1-Decanal". Organic Syntheses; Collected Volumes, vol. 6, p. 373.
- doi:10.1246/bcsj.9.8.
- .
- .
- ISBN 9780471005285.
- ISBN 9781119991397.
- ^ "Aldehyde and Ketone - NEB Class 12 Chemistry 2080". Iswori Education. 2023-07-29. Retrieved 2023-07-29.
- ^ "Aldehyde Tautomerism". Encyclopedia Britannica.
- ISBN 978-0-470-71236-8.
- ISBN 978-0-387-44899-2.
- ^ ISBN 978-0-471-59748-3.
- PMID 29658940.
- PMID 24382882.
- .
- ^ Short Summary of IUPAC Nomenclature of Organic Compounds Archived 2006-09-01 at the Wayback Machine, web page, University of Wisconsin Colleges, accessed on line August 4, 2007.
- ^ §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.
- ^ §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.
- ^ Liebig, J. (1835) "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.)
- ISBN 9780486438023.