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To make an aldehyde, both the carbonyl group (=0) and a hydrogen atom (-H) attach themselves to a carbon atom in either a chain or a ring molecule. See Figure 3.15.

Aldehydes are very important to the perfumer because they very often have a powerful aroma. There is no mistaking an aldehyde as they are referred to by the use of that word, or their names end in ‘al’.

The properties of aldehydes fall somewhere between alcohols and ketones though they are not such consistent and anti-infectious agents as alcohols and tend to be skin irritants to some degree (see chapter 6).

FIGURE 3.15: Benzaldehyde

A few aldehydes are skin sensitizers, which means they may, on certain people, cause a reaction such as a skin rash, which may appear whenever the same aldehyde is met with in any other oil.

FIGURE 3.16: Cinnamic aldehyde

Cinnamic aldehyde (Figure 3.16), when isolated from cinnamon bark oil, can be a skin sensitizer, but when the whole oil (diluted and tested on humans) is used, it has been found not to be so.⁹ It should nevertheless be used with care, as on certain hyper-sensitive skins, unless well diluted, it can be a skin irritant. Citral, an aldehyde (consisting of geranial and neral) present in lemon oil (5 per cent), is a powerful irritant when isolated; in the whole oil its possible hazards are cancelled by the presence of the other 95 per cent of constituents, most of which are terpenes. Many of the known (and unknown) constituents of an oil act as ‘quenchers’ (i.e. they prevent side effects occurring). Mixing an oil containing citral with an oil containing an equal amount of dextro-limonene can negate its irritant properties altogether (see under monoterpenes earlier in this chapter).

Aldehydes are anti-inflammatory, anti-infectious, tonic, hypotensive, calming to the nervous system and generally temperature reducing.

If an oil containing aldehydes is stored for any length of time or in poor conditions, unwanted acids can form from the precious therapeutically active aldehydes, rendering the oil almost useless for aromatherapy purposes.