INTRODUCTION TO ALDEHYDES

Aldehydes

All aldehydes, except for formaldehyde, have a carbonyl group (C&dbond;O) attached to a hydrogen on one side and an alkyl or aryl group on the other side. Formaldehyde, the simplest aldehyde, has a hydrogen on both sides of the carbonyl group.

Nomenclature

Aldehydes are named using either the common system or the IUPAC system. Aldehyde common names are derived from the common names of the corresponding carboxylic acid. For example:

The IUPAC system employs a series of rules to formulate compound names. The following rules explain how to name aliphatic aldehydes:

1. Pick out the longest continuous chain of carbon atoms that contains the carbonyl group.

2. The parent name comes from the alkane name of the same number of carbons.

3. Drop the ‐e ending of the alkane name and add ‐al.

4. Number the chain so that the carbonyl carbon has the lower possible number.

5. Locate and name substituents.

The following examples illustrate IUPAC naming:

In the IUPAC system, cyclic, aliphatic, and aromatic aldehydes are named by adding the word carbaldehyde to the name of the ring system.

Most chemists use the name benzaldehyde instead of benzenecarbaldehyde. Many texts and articles also use this alternate name.

Physical properties

Both aldehydes and ketones, which are described later in this chapter, have higher boiling points than hydrocarbons of similar chain lengths due to a greater degree of polarity and greater dipole‐dipole interaction between molecules. Carbonyl groups cannot form strong intermolecular hydrogen bonds, so both aldehydes and ketones generally boil at lower temperatures than their alcohol analogues. However, aldehydes and ketones of low molecular weight do form strong hydrogen bonds with water, leading to good solubility.

Structure of the carbonyl group

In both aldehydes and ketones, the carbonyl group, with its attached hydrogen atom, alkyl group, or aryl group, lies in a plane with all bond angles close to 120°. The bond length of the carbon‐oxygen double bond is significantly shorter than the average bond length of a carbon‐oxygen single bond in alcohols or ethers. This factor makes the carbonyl group polar. Both aldehydes and ketones have dipole moments that are substantially greater than those of analogues with carbon‐carbon double bonds. For example, 1‐butene shows a dipole moment of 0.3 debye units (0.3 D), while propanol has a dipole moment of 2.5 D.