Aldehydes

Aldehydes are carbonyl compounds in which the carbonyl carbon carries at least one hydrogen atom, giving the functional group –CHO. This lesson explains the structure of the carbonyl group and how aldehydes are named, by common names derived from carboxylic acids and by IUPAC names ending in -al.

In an aldehyde, R–CHO, the carbonyl carbon is sp² hybridised: its three σ bonds lie in one plane at about 120°, and a π bond joins it to oxygen. Because oxygen is more electronegative, the C=O bond is polar, with a partial positive charge on carbon. Common names come from the related carboxylic acids (formic acid gives formaldehyde, acetic acid gives acetaldehyde), with Greek letters marking substituent positions. IUPAC names replace the final -e of the alkane by -al; the –CHO carbon is always C-1, so no locant is needed. Special rules cover dialdehydes (-dial), unsaturated aldehydes (-enal) and –CHO groups attached to rings (-carbaldehyde). Aldehydes are functional isomers of ketones and show chain isomerism among themselves.

What makes a compound an aldehyde

Aldehydes and ketones both contain the carbonyl group, C=O. In an aldehyde the carbonyl carbon is bonded to at least one hydrogen atom, so the functional group is –CHO, written this way and not as –COH. The general formula is R–CHO, where R is a hydrogen atom, an alkyl group or an aryl group. Methanal, HCHO, is the only aldehyde whose carbonyl carbon carries two hydrogen atoms.

Structure of the carbonyl group

• The carbonyl carbon is sp² hybridised. Its three σ bonds, to oxygen, to hydrogen and to R, lie in one plane at angles of about 120°, so this part of the molecule is trigonal planar.
• The unhybridised p orbital of carbon overlaps sideways with a p orbital of oxygen to form a π bond, so C=O is a double bond made of one σ and one π bond.
• Oxygen also carries two lone pairs.
• Oxygen is more electronegative than carbon, so the bond is polar: carbon carries a partial positive charge and oxygen a partial negative charge. This can be shown by two resonance structures, C=O and C⁺–O⁻.
• As a result, the carbonyl carbon is attacked by nucleophiles, the oxygen behaves as a weak base, and aldehydes are polar molecules with sizeable dipole moments.

Common names

Common names are taken from the common names of the corresponding carboxylic acids, replacing "-ic acid" by "-aldehyde":

In common names, the positions of substituents are shown by Greek letters. The carbon next to the –CHO group is α, the next one β, then γ and δ. So CH₃CH(Br)CHO is α-bromopropionaldehyde.

IUPAC names

1. Choose the longest carbon chain that includes the –CHO carbon.
2. Number the chain from the –CHO carbon, which is always C-1, so the locant 1 is not written.
3. Replace the final -e of the alkane name by -al, and name the substituents with their locants in alphabetical order.
4. For a –CHO group at each end of the chain, keep the -e and add -dial: OHC–CHO is ethanedial and OHC–CH₂–CHO is propanedial.
5. For a C=C double bond, use -en- with its locant: CH₂=CH–CHO is prop-2-enal and CH₃–CH=CH–CHO is but-2-enal.
6. When the –CHO group is attached to a ring, use the suffix -carbaldehyde, and do not count the –CHO carbon in the ring: C₆H₁₁CHO is cyclohexanecarbaldehyde. For C₆H₅CHO, the name benzaldehyde is accepted by IUPAC (systematically, benzenecarbaldehyde).

Worked naming

• CH₃CH₂CH(CH₃)CH₂CHO: the chain runs from the –CHO carbon (C-1) through CH₂ (C-2) and CH(CH₃) (C-3) to the end, five carbons in all, with a methyl group on C-3. The name is 3-methylpentanal (common name β-methylvaleraldehyde).
• CH₃CH(OH)CH₂CHO: the aldehyde group takes priority over –OH, which becomes the prefix hydroxy. The name is 3-hydroxybutanal.
• (CH₃)₃C–CHO: the longest chain containing the –CHO carbon has three carbons, with two methyl groups on C-2. The name is 2,2-dimethylpropanal.

Isomerism

• Functional isomers: saturated open-chain aldehydes and ketones with the same number of carbon atoms share the formula CₙH₂ₙO. For example, propanal, CH₃CH₂CHO, and propanone, CH₃COCH₃, are both C₃H₆O.
• Chain isomers: C₄H₈O gives two aldehydes, butanal and 2-methylpropanal; butanone is a functional isomer of both.

Common mistakes

• Writing butan-1-al: the –CHO carbon is always C-1, so the name is simply butanal.
• Numbering from the wrong end: numbering always starts at the –CHO carbon.
• Calling C₆H₁₁CHO "cyclohexanal": the –CHO carbon is outside the ring, so the correct name is cyclohexanecarbaldehyde.
• Mixing systems, as in "isobutanal": use either isobutyraldehyde (common) or 2-methylpropanal (IUPAC).