An aldehyde /ˈældɪhaɪd/ or alkanal is an organic compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen and to an R group, which is any generic alkyl or side chain. The group—without R—is the aldehyde group, also known as the formyl group. Aldehydes are common in organic chemistry. Many fragrances are aldehydes.
1 Structure and bonding 2 Nomenclature
3 Physical properties and characterization 4 Applications and occurrence
4.1 Naturally occurring aldehydes
5.1 Oxidative routes 5.2 Specialty methods
6 Common reactions
6.3.1 Oxygen nucleophiles 6.3.2 Nitrogen nucleophiles 6.3.3 Carbon nucleophiles 6.3.4 Bisulfite reaction
6.4 More complex reactions
7 Dialdehydes 8 Biochemistry 9 Examples of aldehydes 10 Examples of dialdehydes 11 Uses 12 See also 13 References 14 External links
Structure and bonding
Aldehydes feature an sp2-hybridized, planar carbon center that is
connected by a double bond to oxygen and a single bond to hydrogen.
The C–H bond is not ordinarily acidic. Because of resonance
stabilization of the conjugate base, an α-hydrogen in an aldehyde
(not shown in the picture above) is far more acidic, with a pKa near
17, compared to the acidity of a typical alkane (pKa about 50).
This acidification is attributed to (i) the electron-withdrawing
quality of the formyl center and (ii) the fact that the conjugate
base, an enolate anion, delocalizes its negative charge. Related to
(i), the aldehyde group is somewhat polar.
Aldehydes (except those without an alpha carbon, or without protons on
the alpha carbon, such as formaldehyde and benzaldehyde) can exist in
either the keto or the enol tautomer.
Acyclic 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 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 CH3CH2CH2CHO is named butanal.9 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 3-oxopropanoic 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.
The word aldehyde was coined by
Justus von Liebig
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) the visual pigment retinal, and (8) the vitamin pyridoxal. Naturally occurring aldehydes Traces of many aldehydes are found in essential oils and often contribute to their favorable odors, e.g. cinnamaldehyde, cilantro, and vanillin. Possibly because of the high reactivity of the formyl group, aldehydes are not common in several of the natural building blocks: amino acids, nucleic acids, lipids. Most sugars, however, are derivatives of aldehydes. These aldoses exist as hemiacetals, 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 There are several methods for preparing aldehydes, but the dominant technology is hydroformylation. Illustrative is the generation of butyraldehyde by hydroformylation of propene:
H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO
Oxidative routes Aldehydes are commonly generated by alcohol oxidation. In industry, formaldehyde is produced on a large scale by oxidation of methanol. Oxygen is the reagent of choice, being "green" and cheap. In the laboratory, more specialized oxidizing agents are used, but 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 distilled out as it forms (if volatile) or milder reagents such as PCC are used.
[O] + CH3(CH2)9OH → CH3(CH2)8CHO + H2O
Oxidation of primary alcohols to form aldehydes can be achieved under milder, chromium-free conditions by employing methods or reagents such as IBX acid, Dess–Martin periodinane, Swern oxidation, TEMPO, or the Oppenauer oxidation. Another oxidation route significant in industry is the Wacker process, whereby ethylene is oxidized to acetaldehyde in the presence of copper and palladium catalysts (acetaldehyde is also produced on a large scale by the hydration of acetylene). On the laboratory scale, α-hydroxy acids are used as precursors to prepare aldehydes via oxidative cleavage. Specialty methods
Reaction name Substrate Comment
Organic reduction Esters Reduction of an ester with diisobutylaluminium hydride (DIBAL-H) or sodium aluminium hydride.
Rosenmund reaction Acyl chlorides Acyl chlorides selectively reduced to aldehydes. Lithium tri-t-butoxyaluminium hydride (LiAlH(OtBu)3) is an effective reagent.
Formylation reactions Nucleophilic 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
McFadyen-Stevens reaction Hydrazides Base-catalyzed thermal decomposition of acylsulfonylhydrazides.
Aldehydes are highly reactive and participate in many reactions.
From the industrial perspective, important reactions are (a)
condensations, e.g. to prepare plasticizers and polyols, and (b)
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
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.
In the acetalisation reaction, under acidic or basic conditions, an
alcohol adds to the carbonyl group and a proton is transferred to form
a hemiacetal. Under acidic 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 hydrates, R−CH(OH)2.
These diols are stable when strong electron withdrawing groups are
present, as in chloral hydrate. The mechanism of formation 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
intermediate to yield an imine or its trimer, a hexahydrotriazine This
reaction is catalyzed by acid.
RCHO + HSO− 3 → RCH(OH)SO− 3
This reaction is used as a test for aldehydes. 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 diacid from which they can be derived. An example is butanedial, which is also called succinaldehyde (from succinic acid). Biochemistry Some aldehydes are substrates for aldehyde dehydrogenase enzymes which metabolize aldehydes in the body. There are toxicities associated with some aldehydes that are related to neurodegenerative disease, heart disease, and some types of cancer. Examples of aldehydes
Examples of dialdehydes
Glyoxal Malondialdehyde Succindialdehyde Glutaraldehyde Phthalaldehyde
Of all aldehydes, formaldehyde is produced on the largest scale, about
6,000,000 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
polyurethanes. The second main aldehyde is butyraldehyde, of which
about 2,500,000 tons per year are prepared by hydroformylation. It is
the principal precursor to 2-ethylhexanol, which is used as a
Look up aldehyde in Wiktionary, the free dictionary.
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v t e
Only carbon, hydrogen and oxygen
Only one element apart from C, H, O
Disulfide Sulfone Sulfonic acid Sulfoxide Thial Thioester Thioether Thioketone Thiol
Selenol Selenonic acid Seleninic acid Selenenic acid
Isothiocyanate Phosphoramide Sulfenyl chloride Sulfonamide Thiocyanate
See also chemical classification, chemical nomenclature (inorganic, organic)
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