In organic chemistry, an aldehyde () (lat. alcohol dehydrogenatum,[IUPAC Gold Book, aldehydes.] 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.
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
2-hybridized. The aldehyde group is somewhat
polar molecule. The bond length is about 120–122
.
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 νCO band near 1700 cm−1. In their 1H NMR spectra, the formyl hydrogen center absorbs near δH 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 Hz. The 13C NMR spectra of aldehydes and ketones gives a suppressed (weak) but distinctive signal at δC 190 to 205.
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--glucopyranose), (6) the flavorant
cinnamaldehyde, (7)
retinal, which forms with
photoreceptors, and (8) the vitamin
pyridoxal.
Naturally occurring aldehydes
Traces of many aldehydes are found in
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
exist as
, 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
Hydroformylation
Of the several methods for preparing aldehydes,
one dominant technology is
hydroformylation.
[Bertleff, W.; Roeper, M. and Sava, X. (2003) "Carbonylation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH: Weinheim. ] 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:
One complication with this process is the formation of isomers, such as isobutyraldehyde:
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.
[ In the Wacker process, oxidation of ethylene to acetaldehyde in the presence of copper and palladium catalysts, is also used. "Green chemistry" and cheap oxygen (or air) is the oxidant of choice.
]
Laboratories may instead apply a wide variety of specialized , which are often consumed stoichiometrically. 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 out as it forms (if Vapor pressure) or milder reagents such as PCC are used.
A variety of reagent systems achieve aldehydes under chromium-free conditions. One such are the hypervalent organoiodine compounds (i.e., IBX acid, Dess–Martin periodinane), although these often also oxidize the α position. A Lux-Flood acid will activate other pre-oxidized substrates: various sulfoxides (e.g. the Swern oxidation), or amine oxides (e.g., the Ganem oxidation). Sterically-hindered (i.e., TEMPO) can catalyze aldehyde formation with a cheaper oxidant.
Alternatively, or their oxidized sequelae ( or α-hydroxy acids) can be oxidized with Bond cleavage to two aldehydes or an aldehyde and carbon dioxide.
Specialty methods
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| Ozonolysis | | Reductive work-up; similar effect with singlet oxygen and no work-up |
| Carbonyl reduction | , amides | Reduction of an ester with diisobutylaluminium hydride (DIBAL-H) or sodium aluminium hydride; see also amide reduction. |
| Rosenmund reaction | | Acyl chlorides selectively reduced to aldehydes. Lithium tri- t-butoxyaluminium hydride () is an effective reagent. |
| Wittig reaction | | A modified Wittig reaction using methoxymethylenetriphenylphosphine as a reagent. |
| Formylation reactions | Nucleophile arenes | Various reactions, for example the Vilsmeier-Haack reaction. |
| Nef reaction | | The acid catalysis hydrolysis of a primary nitro compound to form an aldehyde. |
| Kornblum oxidation | | The oxidation of primary halide with dimethyl sulfoxide to form an aldehyde. |
| Zincke reaction | | formed in a reaction variation. |
| Stephen aldehyde synthesis | | Hydrolysis of an iminium salt generated by tin(II) chloride and HCl to form an aldehyde. |
| Geminal halide hydrolysis | Geminal halocarbon | Hydrolysis of primary geminal dihalides to yield aldehydes. |
| Meyers synthesis | | Hemiaminal oxazine hydrolysis with water and oxalic acid to yield an aldehyde. |
Hofmann rearrangement variation | Unsaturated or α-Hydroxyl amides | Aldehydes via the hydrolysis of an intermediate Carbamic acid. |
| McFadyen-Stevens reaction | | Base-Catalysis thermal decomposition of acylsulfonylhydrazides. |
| Biotransformation | | Lyophilized cell cultures of Trametes hirsuta in the presence of oxygen. |
Common reactions
Aldehydes participate in many reactions.
-
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).
Acid-base reactions
Because of resonance stabilization of the conjugate base, an alpha hydrogen in an aldehyde is weakly with a pKa near 17. Note, however, this is much more acidic than an alkane or ether hydrogen, which has pKa near 50 approximately, and is even more acidic than a ketone α-hydrogen which has pKa 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 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.
Reduction
The formyl group can be readily reduced to a primary alcohol (). Typically this conversion is accomplished by catalytic hydrogenation either directly or by transfer hydrogenation. Stoichiometry reductions are also popular, as can be effected with sodium borohydride.
Oxidation
The formyl group readily oxidizes to the corresponding carboxyl group (). The preferred oxidant in industry is oxygen or air. In the laboratory, popular oxidizing agents include potassium permanganate, nitric acid, chromium(VI) oxide, and chromic acid. The combination of manganese dioxide, cyanide, acetic acid and methanol will convert the aldehyde to a methyl ester.
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 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 complex ions are reduced to a red-brick-coloured 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
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:
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 acetalisation reaction, under or basic conditions, an alcohol adds to the carbonyl group and a proton is transferred to form a hemiacetal. Under 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 , . These diols are stable when strong electron withdrawing groups 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 () can also add to the carbonyl group. After the elimination of water, this results in an oxime. An ammonia derivative of the form such as hydrazine () 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.[
]
Carbon nucleophiles
The Cyanide group in Hydrogen cyanide can add to the carbonyl group to form , . In this reaction the ion is the nucleophile that attacks the partially positive carbon atom of the Carboxyl 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 , such as from the HCN molecule, to form the alcohol group of the cyanohydrin.
Organometallic compounds, such as organolithium reagents, , or , undergo nucleophilic addition reactions, yielding a substituted alcohol group. Related reactions include organostannane additions, , and the Nozaki–Hiyama–Kishi reaction.
In the aldol reaction, the metal enolates of , , , and carboxylic acids add to aldehydes to form β-hydroxycarbonyl compounds (). 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 :
This reaction is used as a test for aldehydes and is useful for separation or purification of aldehydes.
More complex reactions
|
| Wolff–Kishner reduction | Alkane | If an aldehyde is converted to a simple hydrazone () 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 . |
| 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 Succindialdehyde, which is also called succinaldehyde (from succinic acid).
Biochemistry
Some aldehydes are substrates for aldehyde dehydrogenase which metabolize aldehydes in the body. There are Toxicity associated with some aldehydes that are related to neurodegenerative disease, heart disease, and some types of cancer.
Examples of aldehydes
Examples of dialdehydes
Uses
Of all aldehydes, formaldehyde is produced on the largest scale, about . 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 .[Reuss, G.; Disteldorf, W.; Gamer, A. O. and Hilt, A. (2005) "Formaldehyde" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. .] The second main aldehyde is butyraldehyde, of which about are prepared by hydroformylation. It is the principal precursor to 2-ethylhexanol, which is used as a plasticizer.[Kohlpaintner, C.; Schulte, M.; Falbe, J.; Lappe, P. and Weber, J. (2008) "Aldehydes, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. .] Acetaldehyde once was a dominating product, but production levels have declined to less than 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 , 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 such as Chanel No. 5. 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:[ Short Summary of IUPAC Nomenclature of Organic Compounds , 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.]
-
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 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 is named butyraldehyde.
-
In other cases, such as when a group is attached to a ring, the suffix -carbaldehyde may be used. Thus, 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, is named 2-oxoethanoic acid.
-
If replacing the aldehyde group with a carboxyl 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.
Etymology
The word aldehyde was coined by Justus von Liebig as a contraction of the Latin alcohol dehydrogenatus (dehydrogenated alcohol).[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.)][.] 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 group.)
The term formyl group is derived from the Latin language word formica "ant". This word can be recognized in the simplest aldehyde, formaldehyde, and in the simplest carboxylic acid, formic acid.
See also
External links
