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An aldehyde

Formaldehyde, the simplest of the aldehydes

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,^[1] 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 and many fragrances are aldehydes.

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 pK_a near 17, compared to the acidity of a typical alkane (pK_a about 50).^[2] 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. The formyl proton itself does not readily undergo deprotonation. The anionic species formally derived from deprotonation of an aldehyde proton, known as an acyl anion, is highly unstable and must be kept at low temperatures. In fact, with the exception of certain hindered dialkylformamides, the synthesis of acyl anions by direct deprotonation is not a feasible route, since the deprotonated species will almost immediately add to the highly reactive carbonyl of the starting material to form an acyloin compound. For this reason, the acidity of the formyl proton is difficult to measure. In the case of HCONⁱPr₂, the acidity of the formyl group was found to be very close to that of diisopropylamine (pK_a ~ 36).^[3] The gas phase acidity of formaldehyde was found to be 393 kcal/mol, making it more acidic than hydrogen (400. kcal/mol) and ammonia (402 kcal/mol), but less acidic than water (390 kcal/mol) in the gas phase.^[4]

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.

At around 85 kcal/mol, the formyl C-H bond is weaker than that of a typical bond between hydrogen and an sp²-hybridized carbon. Thus, aldehydes are prone to undergo hydrogen atom abstraction in the presence of free radicals, a fact accounts for the ease with which aldehydes undergo autoxidation.

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:^[5][6][7]

1. 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.


2. 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-.


3. 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.


4. 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.

Etymology

Formic acid

The word aldehyde was coined by Justus von Liebig as a contraction of the Latin alcohol dehydrogenatus (dehydrogenated alcohol).^[8][9] 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.)

The term formyl group is derived from the Latin word formica "ant". This word can be recognized in the simplest aldehyde, formaldehyde, and in the simplest carboxylic acid, formic acid.

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 degrade in air via the process of autoxidation.

The two aldehydes of greatest importance in industry, formaldehyde and acetaldehyde, have complicated behavior because of their tendency to oligomerize or polymerize. They also tend to hydrate, forming the geminal diol. The oligomers/polymers and the hydrates exist in equilibrium with the parent aldehyde.

Aldehydes are readily 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, which is a distinctive part of the spectrum. This signal shows the characteristic coupling to any protons on the alpha carbon.

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-_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,^[10] but the dominant technology is hydroformylation.^[11] 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.^[12] 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.^[13]

[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.^[14][15]

Specialty methods

Common reactions

Aldehydes are highly reactive and participate in many reactions.^[10] 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).^[10]

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.^[19]

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 [Ag(NH₃)₂]⁺ complex. This reagent will convert aldehydes to carboxylic acids without attacking carbon–carbon double bonds. The name silver mirror test arises because this reaction will produce 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 will result 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.^[20]

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 will add to aldehydes to form β-hydroxycarbonyl compounds (aldols). Acid or base-catalyzed dehydration will then lead 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 + HSO⁻
₃ → RCH(OH)SO⁻
₃

This reaction is used as a test for aldehydes.^[20]

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.^[21]

Examples of aldehydes

• 
  Formaldehyde (methanal)


• 
  Acetaldehyde (ethanal)


• 
  Propionaldehyde (propanal)


• 
  Butyraldehyde (butanal)


• 
  Benzaldehyde (phenylmethanal)


• Cinnamaldehyde


• Vanillin


• Tolualdehyde


• Furfural


• Retinaldehyde

Examples of dialdehydes

• Glyoxal


• Malondialdehyde


• Succindialdehyde


• Glutaraldehyde


• Phthalaldehyde

Uses

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.^[12] 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 plasticizer.^[22] Acetaldehyde once was a dominating product, but production levels have declined to less than 1,000,000 tons per year 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 oxo 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 and perfumes such as Chanel No. 5. These include cinnamaldehyde and its derivatives, citral, and lilial.

See also

• Enol


• Pseudoacid

References

External links

Look up aldehyde in Wiktionary, the free dictionary.

Wikiquote has quotations related to: Aldehyde

• 
  Aldehyde synthesis – Synthetic protocols from organic-reaction.com