Aldehyde

In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure. The functional group itself 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 sp²-hybridized. The aldehyde group is somewhat polar. The bond length is about 120–122 picometers.

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 ν_CO band near 1700 cm⁻¹. In their ¹H NMR spectra, the formyl hydrogen center absorbs near δ_H 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 but distinctive signal at δ_C 190 to 205.

Applications and occurrence

Naturally occurring aldehydes

Traces of many aldehydes are found in essential oils 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 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

Hydroformylation

Of the several methods for preparing aldehydes, one dominant technology is hydroformylation. 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" and cheap oxygen is the oxidant of choice.
Laboratories may instead apply a wide variety of specialized oxidizing agents, which are often consumed stoichiometrically. Chromium 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 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, although these often also oxidize the α position. A Lux-Flood acid will activate other pre-oxidized substrates: various sulfoxides, or amine oxides. Sterically-hindered nitroxyls can catalyze aldehyde formation with a cheaper oxidant.
Alternatively, vicinal diols or their oxidized sequelae can be oxidized with cleavage to two aldehydes or an aldehyde and carbon dioxide.

Specialty methods

Common reactions

Aldehydes participate in many reactions. From the industrial perspective, important reactions are:

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 and hemiacetals.
Acid-base reactions

Because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is weakly acidic with a pK_a near 17. Note, however, this is much more acidic than an alkane or ether hydrogen, which has pK_a near 50 approximately, and is even more acidic than a ketone α-hydrogen which has pK_a 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 can exist in either the keto 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. But it becomes the dominant tautomer in strong acid or base solutions, and enolized aldehydes undergo nucleophilic attack at the α position.

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. Stoichiometric 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 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 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.
Aromatic aldehydes, such as benzaldehyde, do not give a positive Fehling's test, because they are not easily oxidized by mild oxidizing agents like Fehling's reagent. The benzene ring provides stability, while steric hindrance prevents the formation of the required hydrated anion intermediate.
If the aldehyde cannot form an enolate, addition of strong base induces the Cannizzaro reaction. This reaction results in disproportionation, producing a mixture of alcohol and carboxylic acid.

Nucleophilic addition reactions

s 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 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,. 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 cyano group in HCN can add to the carbonyl group to form cyanohydrins,. In this reaction the ion is the nucleophile that attacks the partially positive carbon atom of the carbonyl 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, 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 aldol reaction, the metal enolates of ketones, esters, amides, 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.