Aldehydes

In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . 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 common and play important roles in the technology and biological spheres.

Structure and bonding

Aldehydes feature a carbon center that is connected by a double bond to oxygen and a single bond to hydrogen and 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 C=O 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 are more soluble in water, formaldehyde and acetaldehyde completely so. 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 (weak) but distinctive signal at ''δ''_C 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 opsins potoreceptors, and (8) the vitamin pyridoxal.

Naturally occurring aldehydes

Traces of many aldehydes are found in essential oils and often contribute to their favorable odours, 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.Bertleff, W.; Roeper, M. and Sava, X. (2003) "Carbonylation" in ''Ullmann’s Encyclopedia of Industrial Chemistry'', Wiley-VCH: Weinheim. 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.
: + 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,.

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

Common reactions

Aldehydes 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 sugars).

Acid-base reactions

Because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is weakly acidic, with a p''K''_a near 17. 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. 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 keto or the enol tautomer. Keto–enol tautomerism is catalyzed by either acid or base. Usually the enol is the minority tautomer, but it is more reactive. The enolization typically reverses several times per second.

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 carboxyl group (−COOH). 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 g(NH₃)₂sup>+ 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 Cu²⁺ complex ions are reduced to a red-brick-coloured Cu₂O 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

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 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.

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 (NH₂OH) can also add to the carbonyl group. After the elimination of water, this results in an oxime. An ammonia derivative of the form H₂NNR₂ such as hydrazine (H₂NNH₂) 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, R−CH(OH)CN. In this reaction the CN⁻ 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 H⁺, 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 (aldols). 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 sodium bisulfite:
: RCHO + →
This reaction is used as a test for aldehydes.

More complex reactions

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

* Formaldehyde (methanal)
* Acetaldehyde (ethanal)
* Propionaldehyde (propanal)
* Butyraldehyde (butanal)
* Isovaleraldehyde
* Benzaldehyde (phenylmethanal)
* Cinnamaldehyde
* Vanillin
* Tolualdehyde