Aldehyde

An aldehyde is an organic compound containing a characteristic carbonyl functional group in which the carbonyl carbon is bonded to at least one hydrogen atom. In general, aldehydes have the structure R–CHO, where R represents a hydrogen or an organic substituent, and –CHO is called the aldehyde (formyl) group. Aldehydes are widely important in synthetic organic chemistry, industrial chemistry and biological systems, and they display rich and distinctive reactivity centred on the highly polar carbonyl group.

Definition, Nomenclature and Functional Group

The aldehyde functional group consists of a carbon atom double-bonded to oxygen (C=O) and single-bonded to hydrogen (–CH=O). When considered without the attached R group, the –CHO unit is often referred to as a formyl group.
In IUPAC nomenclature:

• Aldehydes are generally named by replacing the -e of the corresponding alkane with -al, e.g. methanal (formaldehyde), ethanal (acetaldehyde), propanal, etc.
• When –CHO is treated as a substituent on a more complex structure, the term formyl is used.

The term “aldehyde” derives historically from the Latin phrase alcohol dehydrogenatum (“dehydrogenated alcohol”), reflecting their classical preparation by oxidation of primary alcohols.

Structure and Bonding

In aldehydes, the carbonyl carbon is approximately sp²-hybridised, giving the –CHO centre a trigonal planar geometry with bond angles close to 120°. The key structural features are:

• A C=O double bond between carbon and oxygen, with a bond length of about 120–122 pm.
• A C–H bond to the formyl hydrogen.
• A C–R bond to the remainder of the molecule, which may be another hydrogen (in formaldehyde) or an alkyl/aryl group.

The C=O bond is highly polar, since oxygen is more electronegative than carbon. This polarity renders the carbonyl carbon strongly electrophilic, making aldehydes susceptible to nucleophilic attack. The formyl hydrogen itself is not significantly acidic, but α-hydrogens (on the carbon next to the carbonyl) are relatively acidic (pKₐ ≈ 17), much more so than in alkanes (pKₐ ≈ 50), owing to stabilisation of the resulting enolate anion by resonance.
Many aldehydes can exist in equilibrium with their enol tautomers, especially under acidic or basic conditions (keto–enol tautomerism). In neutral solution the keto (aldehyde) form is usually favoured, but under strong acid or base conditions the enol form can become significant and undergo further reactions such as α-substitution.

Physical Properties and Spectroscopic Characterisation

The physical and spectroscopic behaviour of aldehydes is strongly influenced by both the carbonyl group and the rest of the molecule.
Physical properties

• Low-mass aldehydes (e.g. formaldehyde, acetaldehyde) are highly soluble in water due to hydrogen bonding and dipole–dipole interactions.
• Many volatile aldehydes have sharp, often pungent odours; some, however, contribute to pleasant fragrances.
• Boiling points are typically higher than alkanes but lower than alcohols of similar molecular mass, because of dipole–dipole forces but the absence of strong hydrogen bonding between aldehyde molecules.

IR spectroscopy

• Aldehydes show a strong C=O stretching band near 1700 cm⁻¹ (often around 1720 cm⁻¹, shifting slightly with conjugation or substitution).
• Often, a weak but characteristic C–H stretch for the formyl hydrogen appears around 2720–2820 cm⁻¹.

¹H NMR spectroscopy

• The formyl proton (–CHO) gives a distinctive signal at δ ≈ 9.5–10 ppm, usually downfield of most other proton signals.
• This proton often shows small coupling (J < 3 Hz) to any protons on the adjacent α-carbon.

¹³C NMR spectroscopy

• The carbonyl carbon of aldehydes (and ketones) usually appears at δ ≈ 190–205 ppm, a weak but characteristic signal in the downfield region of the spectrum.

These spectroscopic features make aldehydes relatively straightforward to identify in the laboratory.

Occurrence in Nature and Important Examples

Although free aldehydes are less common as stable building blocks in biomacromolecules, they occur widely in nature, especially in low concentrations:

• Essential oils often contain aldehydes that contribute to characteristic odours, e.g.
  • cinnamaldehyde in cinnamon,
  • vanillin in vanilla,
  • aldehydes contributing to cilantro/coriander aroma.
• Many sugars (aldoses) are technically derivatives of aldehydes. In aqueous solution, however, they mainly exist as cyclic hemiacetals, which “mask” the aldehyde; only a small fraction is present as the free –CHO form (e.g. in aqueous glucose).
• Biologically important molecules such as retinal (involved in vision) and pyridoxal (a form of vitamin B₆) contain aldehyde groups that play critical roles in enzyme mechanisms and light reception.

In industrial chemistry, simple aldehydes like formaldehyde, acetaldehyde and butyraldehyde are basic feedstocks for resins, plastics, plasticisers, solvents and other products.

Synthesis of Aldehydes

A reaction that introduces an aldehyde group is known as a formylation. There are several important synthetic routes:
1. Hydroformylation (oxo process)Hydroformylation is a major industrial method. In this process, an alkene reacts with synthesis gas (CO + H₂) in the presence of a transition metal catalyst (commonly cobalt or rhodium) to form an aldehyde:

• Example: propene → butanal (butyraldehyde), along with some isobutanal as an isomer.

Hydroformylation is carried out on a very large scale and underpins the production of many bulk aldehydes used as intermediates for alcohols, plasticisers and detergents.
2. Oxidation of primary alcohols
In the laboratory, aldehydes are commonly prepared by controlled oxidation of primary alcohols:

• Reagents include acidified potassium dichromate (K₂Cr₂O₇), PCC (pyridinium chlorochromate), Dess–Martin periodinane, IBX, and TEMPO-based systems.
• With strong oxidants such as dichromate, over-oxidation to carboxylic acids can occur. To isolate the aldehyde, conditions are controlled by distilling the aldehyde as it forms or by using milder, more selective reagents.

3. Industrial oxidation routes
Industrially, aldehydes can be produced by the oxidation of simple alcohols or hydrocarbons:

• Methanol → formaldehyde,
• Ethanol → acetaldehyde,
• Toluene → benzaldehyde,
• Propene → acrolein,
• Isobutene → methacrolein.

The Wacker process oxidises ethene to ethanal (acetaldehyde) using palladium and copper catalysts. In green chemistry, oxygen or air is preferred as the terminal oxidant.

Reactivity and Common Reactions of Aldehydes

Aldehydes are among the most reactive carbonyl compounds owing to the strong electrophilicity of the formyl carbon and the absence of steric hindrance relative to ketones.
1. Acid–base and enolisation behaviour

• α-Hydrogens are weakly acidic (pKₐ ≈ 17) due to stabilisation of the enolate anion by resonance.
• Under acidic or basic conditions, aldehydes (with α-hydrogens) can undergo keto–enol tautomerism and react via their enol or enolate forms in α-substitution and condensation reactions (e.g. aldol reactions).

2. Oxidation

• Aldehydes are readily oxidised to carboxylic acids, both in the laboratory and in industry.
• Common laboratory oxidants include potassium permanganate, nitric acid, chromic acid (H₂CrO₄) and related chromium(VI) reagents.
• Oxidation can also lead to other products, e.g. conversion to methyl esters under specific conditions such as manganese dioxide/cyanide/acetic acid/methanol systems.

Qualitative tests

• Tollens’ test (silver mirror test):
  • Aldehydes reduce the diamminesilver(I) complex to metallic silver while being oxidised to carboxylates.
  • A shiny silver deposit on the glass test tube confirms the presence of an aldehyde.
• Fehling’s test:
  • Aldehydes (especially reducing sugars) reduce copper(II) complex ions to a red brick-coloured precipitate of Cu₂O.

Aldehydes that cannot form an enolate (e.g. benzaldehyde) may undergo the Cannizzaro reaction in strong base, where one molecule is reduced to an alcohol and another oxidised to a carboxylate.
3. Reduction
The formyl group in an aldehyde can be reduced to a primary alcohol:

• By catalytic hydrogenation using H₂ and a metal catalyst (e.g. Pd, Pt, Ni).
• By hydride reagents such as NaBH₄ or LiAlH₄.

This conversion is widely used in synthesis for transforming aldehydes into alcohols with retention of the carbon skeleton.
4. Nucleophilic addition reactions
The most characteristic reactions of aldehydes involve nucleophilic addition to the carbonyl carbon:

• The nucleophile attacks the electrophilic carbonyl carbon, converting it from sp² to sp³, and the oxygen is protonated.
• In many cases, subsequent loss of a small molecule (often water) leads to addition–elimination (condensation) products.

Key nucleophilic additions include:

• Addition of alcohols:
  • One equivalent of alcohol gives a hemiacetal.
  • Under acidic conditions, further reaction with another alcohol molecule forms an acetal and water.
  • Simple hemiacetals are often unstable, but cyclic hemiacetals (e.g. in sugars) are stable.
  • Acetals are relatively stable protecting groups for aldehydes and can be reverted to the aldehyde in acid.
• Addition of water:
  • Aldehydes can form geminal diols (hydrates).
  • These are stabilised when strong electron-withdrawing groups are present (e.g. chloral hydrate).
• Aldol-type reactions:
  • Another aldehyde molecule can act as a nucleophile via its enolate, leading to aldol products (β-hydroxy aldehydes) and, after dehydration, α,β-unsaturated aldehydes.