Aldehyde

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An aldehyde.
-R is the group attached to the aldehyde group.

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom which is bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula O=CH-), is called the aldehyde group. The aldehyde group is also called the formyl or methanoyl group.

The word aldehyde seems to have arisen from alcohol dehydrogenated. 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; compare vinyl.)

The aldehyde group is polar. Oxygen, more electronegative than carbon, pulls the electrons in the carbon-oxygen bond towards itself, creating an electron deficiency at the carbon atom.

Owing to resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is more acidic than a hydrogen atom in an alkane, with a typical pKa of 17.

Contents

  • 1 Nomenclature
    • 1.1 IUPAC names for aldehydes
    • 1.2 Other nomenclature
  • 2 Chemistry
    • 2.1 Synthesis
    • 2.2 Keto-enol tautomerism
    • 2.3 Common reactions
      • 2.3.1 Reduction and oxidation
      • 2.3.2 Nucleophilic addition reactions
      • 2.3.3 More complex reactions
  • 3 Examples of aldehydes
  • 4 Related compounds
  • 5 References

[edit] Nomenclature

[edit] IUPAC names for aldehydes

Examples of IUPAC nomenclature for aldehydes.

IUPAC prescribes the following nomenclature for aldehydes:[1][2][3]

  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 (-COOH) group 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. For example:
  • HCHO may be called formaldehyde.
  • CH3CHO may be called acetaldehyde.
  • C6H5CHO may be called benzaldehyde.

[edit] Other nomenclature

The carbon atom adjacent to a carbonyl group is called the α carbon. Carbon atoms further away from the group may be named β for the carbon atom bonded to the α carbon, γ for the next, and so on. Hydrogen atoms bonded to these carbon atoms are named likewise: an α hydrogen is a hydrogen atom bonded to the α carbon and so on.

A reaction that introduces an aldehyde group is known as a formylation reaction.

[edit] Chemistry

[edit] Synthesis

There are several methods for preparing aldehydes:

CH3CH2CH2OH —→ CH3CH2CHO

[edit] Keto-enol tautomerism

Aldehydes can exist in either the keto or enol tautomers. Keto-enol tautomerism is catalyzed by either acid or base.

[edit] Common reactions

[edit] Reduction and oxidation

[edit] Nucleophilic addition reactions

In nucleophilic addition reactions a nucleophile can add to the carbon atom in the carbonyl group, yielding an addition compound where this carbon atom has tetrahedral molecular geometry. Together with protonation of the oxygen atom in the carbonyl group (which can take place either before or after addition), this yields a product where the carbon atom in the carbonyl group is bonded to the nucleophile, a hydrogen atom, and a hydroxyl group.

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 various examples of nucleophilic addition reactions.

[edit] More complex reactions

[edit] Examples of aldehydes

[edit] Related compounds

Other kinds of organic compounds containing carbonyl groups include