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-hybridized. The aldehyde group is somewhatpolar. TheC=O bond length is about 120–122picometers.[5]
Aldehydes have properties that are diverse and that depend on the remainder of the molecule. Smaller aldehydes such asformaldehyde andacetaldehyde are soluble in water, and the volatile aldehydes have pungent odors.
Aldehydes can be identified by spectroscopic methods. UsingIR spectroscopy, they display a strongνCO band near 1700 cm−1. In their1H NMR spectra, the formyl hydrogen center absorbs nearδH 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. The13C NMR spectra of aldehydes and ketones gives a suppressed (weak) but distinctive signal atδC 190 to 205.
Traces of many aldehydes are found inessential oils and often contribute to their pleasant odours, includingcinnamaldehyde,cilantro, andvanillin. 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. Thesealdoses exist ashemiacetals, a sort of masked form of the parent aldehyde. For example, in aqueous solution only a tiny fraction of glucose exists as the aldehyde.
Of the several methods for preparing aldehydes,[3] one dominant technology ishydroformylation.[6] 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 ofbutyraldehyde byhydroformylation ofpropylene:
H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO
One complication with this process is the formation of isomers, such as isobutyraldehyde:
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).[15][3]
Because ofresonance stabilization of the conjugate base, anα-hydrogen in an aldehyde is weaklyacidic with apKa near 17. Note, however, this is much more acidic than an alkane or ether hydrogen, which haspKa near 50 approximately, and is even more acidic than a ketone α-hydrogen which haspKa 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, anenolate anion, delocalizes its negative charge.
The formyl proton itself does not readily undergo deprotonation.
Aldehydes (except those without an alpha carbon, or without protons on the alpha carbon, such as formaldehyde and benzaldehyde) can exist in either theketo or theenoltautomer.Keto–enol tautomerism is catalyzed by either acid or base. In neutral solution, the enol is the minority tautomer, reversing several times per second.[16] But it becomes the dominant tautomer in strong acid or base solutions, and enolized aldehydes undergonucleophilic attack at the α position.[17][18]
Another oxidation reaction is the basis of thesilver-mirror test. In this test, an aldehyde is treated withTollens' reagent, which is prepared by adding a drop ofsodium hydroxide solution intosilver nitrate solution to give a precipitate of silver(I) oxide, and then adding just enough diluteammonia 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 namesilver-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 involvesFehling's reagent as a test. TheCu2+ complex ions are reduced to a red-brick-colouredCu2O precipitate.
If the aldehyde cannot form an enolate (e.g.,benzaldehyde), addition of strong base induces theCannizzaro reaction. This reaction results indisproportionation, producing a mixture of alcohol and carboxylic acid.
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 anaddition–elimination oraddition–condensation reaction. There are many variations of nucleophilic addition reactions.
In theacetalisation reaction, underacidic orbasic conditions, analcohol adds to the carbonyl group and a proton is transferred to form ahemiacetal. Underacidic conditions, the hemiacetal and the alcohol can further react to form anacetal and water. Simple hemiacetals are usually unstable, although cyclic ones such asglucose can be stable. Acetals are stable, but revert to the aldehyde in the presence of acid. Aldehydes can react with water to formhydrates,R−CH(OH)2. These diols are stable when strongelectron withdrawing groups are present, as inchloral 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.
Inalkylimino-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 acarbinolamine. In the case of a primary amine, a water molecule can be eliminated from the carbinolamine intermediate to yield animine or its trimer, ahexahydrotriazine This reaction is catalyzed by acid.Hydroxylamine (NH2OH) can also add to the carbonyl group. After the elimination of water, this results in anoxime. Anammonia derivative of the formH2NNR2 such ashydrazine (H2NNH2) or2,4-dinitrophenylhydrazine can also be the nucleophile and after the elimination of water, resulting in the formation of ahydrazone, which are usually orange crystalline solids. This reaction forms the basis of a test for aldehydes andketones.[19]
Thecyano group inHCN can add to the carbonyl group to formcyanohydrins,R−CH(OH)CN. In this reaction theCN− ion is thenucleophile that attacks the partially positive carbon atom of thecarbonyl 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 withH+, such as from the HCN molecule, to form the alcohol group of the cyanohydrin.
ThePrins reaction occurs when a nucleophilicalkene oralkyne reacts with an aldehyde as electrophile. The product of the Prins reaction varies with reaction conditions and substrates employed.
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 aone-pot reaction, giving the overall conversionRCH=O → RCH3.
Aldehydes can, typically in the presence of suitable catalysts, serve as partners incycloaddition reactions. The aldehyde serves as the dienophile component, giving a pyran or related compound.
Adialdehyde 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 thediacid from which they can be derived. An example isbutanedial, which is also called succinaldehyde (fromsuccinic acid).
Of all aldehydes, formaldehyde is produced on the largest scale, about6000000 tons per year. It is mainly used in the production of resins when combined withurea,melamine, andphenol (e.g.,Bakelite). It is a precursor tomethylene diphenyl diisocyanate ("MDI"), a precursor topolyurethanes.[8] The second main aldehyde isbutyraldehyde, of which about2500000 tons per year are prepared byhydroformylation. It is the principal precursor to2-ethylhexanol, which is used as aplasticizer.[22] Acetaldehyde once was a dominating product, but production levels have declined to less than1000000 tons per year because it mainly served as a precursor toacetic acid, which is now prepared bycarbonylation ofmethanol. Many other aldehydes find commercial applications, often as precursors to alcohols, the so-calledoxo alcohols, 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 andperfumes such asChanel No. 5. These includecinnamaldehyde and its derivatives,citral, andlilial.
The common names for aldehydes do not strictly follow official guidelines, such as those recommended byIUPAC, but these rules are useful. IUPAC prescribes the following nomenclature for aldehydes:[23][24][25]
Acyclicaliphatic aldehydes are named as derivatives of the longest carbon chain containing the aldehyde group. Thus, HCHO is named as a derivative of methane, andCH3CH2CH2CHO is named as a derivative ofbutane. The name is formed by changing the suffix-e of the parentalkane to-al, so that HCHO is namedmethanal, andCH3CH2CH2CHO is namedbutanal.
In other cases, such as when a−CHO group is attached to a ring, the suffix-carbaldehyde may be used. Thus,C6H11CHO is known ascyclohexanecarbaldehyde. If the presence of another functional group demands the use of a suffix, the aldehyde group is named with the prefixformyl-. This prefix is preferred tomethanoyl-.
If the compound is a natural product or acarboxylic acid, the prefixoxo- may be used to indicate which carbon atom is part of the aldehyde group; for example,CHOCH2COOH is named2-oxoethanoic acid.
If replacing the aldehyde group with acarboxyl 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 wordaldehyde was coined byJustus von Liebig as a contraction of the Latinalcoholdehydrogenatus (dehydrogenated alcohol).[26][27] In the past, aldehydes were sometimes named after the correspondingalcohols, for example,vinous aldehyde foracetaldehyde. (Vinous is fromLatinvinum "wine", the traditional source ofethanol, cognate withvinyl.)
The termformyl group is derived from theLatin wordformica "ant". This word can be recognized in the simplest aldehyde,formaldehyde, and in the simplest carboxylic acid,formic acid.
^G. Berthier, J. Serre (1966). "General and Theoretical Aspects of the Carbonyl Group". In Saul Patai (ed.).The Carbonyl Group. PATAI'S Chemistry of Functional Groups. Vol. 1. John Wiley & Sons. pp. 1–77.doi:10.1002/9780470771051.ch1.ISBN978-0-470-77105-1.
^Bertleff, W.; Roeper, M. and Sava, X. (2003) "Carbonylation" inUllmann's Encyclopedia of Industrial Chemistry, Wiley-VCH: Weinheim.doi:10.1002/14356007.a05_217.pub2
^abReuss, G.; Disteldorf, W.; Gamer, A. O. and Hilt, A. (2005) "Formaldehyde" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim.doi:10.1002/14356007.a11_619.
^Nwaukwa, Stephen; Keehn, Philip (1982). "Oxidative cleavage of α-diols, α-diones, α-hydroxy-ketones and α-hydroxy- and α-keto acids with calcium hypochlorite [Ca(OCl)2]".Tetrahedron Letters.23 (31):3135–3138.doi:10.1016/S0040-4039(00)88578-0.
^Warren, Stuart; Wyatt, Paul (2008).Organic synthesis: the disconnection approach (2nd ed.). Wiley. pp. 129–133.ISBN978-0-470-71236-8.
^Carey, Francis A.; Sundberg, Richard J. (2007).Advanced Organic Chemistry. Vol. A: Structure and Mechanisms (5th ed.). Springer. pp. 601–608.ISBN978-0-387-44899-2.
^abShriner, R. L.; Hermann, C. K. F.; Morrill, T. C.; Curtin, D. Y.; Fuson, R. C. (1997).The Systematic Identification of Organic Compounds. John Wiley & Sons.ISBN978-0-471-59748-3.
^Kohlpaintner, C.; Schulte, M.; Falbe, J.; Lappe, P. and Weber, J. (2008) "Aldehydes, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim.doi:10.1002/14356007.a01_321.pub2.
^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 nomd'aldehyde; ce nom est formé dealcool dehydrogenatus." (I will describe it in this memoir by the name ofaldehyde; this name is formed fromalcohol dehydrogenatus.)
