Citral

Other comments :

Citral is one of the 26 allergens in perfumery.

Stability :

Aldehydes may form diethylacetals in alcoholic perfumes, with no real impact on their smell.
Terpenes tend to polymerize by oxydation.
Very instable in most of perfumed products, except shampoos, hair conditioners, soaps and other neutral products.

Uses in perfumery :

Citral is used in citrus notes and especially lemon notes. Can be mixed with D-Limonene for a sweet facet. Brings a fresh facet to ginger and shades the camphorated facet of cardamom.

Year of discovery :

Discovered in 1889.

Isomerism :

Citral is actually a mixture of two molecules: Geranial and Neral, with a similar smell. Both are enantiomers, as Citral contains a double bond. Thus, Geranial is the trans (E) isomer and Neral is the cis (Z) isomer of Citral. Both can be separated by adding hydrogen sulfite salts in the mixture. This allows to separate them and convert them back to their original form afterwards. Isocyclocitral® and Camphor are constitutional isomers of Citral. Isocyclocitral has a citrus smell, although greener and earthier.

Synthesis precursor :

Citral is an unsaturated aldehyde, capable of forming many compounds with an organoleptic interest or for other applications. For example, it may be subjected to a condensation of the aldehyde function with another carbonyl function, or be cyclized, or even polymerized. In addition, hydrogenation of Citral leads to Geraniol, Citronellol or 1,3-Dimethyl-1-octanol. It is also at the origin of the synthesis of pseudoionones, by condensation with an active methylene group. These compounds allow to obtain ionones (Alpha-Ionone, Beta-Ionone) or vitamins (vitamin A for example). Finally, Citral forms a Schiff base by reaction with Methyl Anthranilate or Indole for example.

Natural availability :

Several essential oils are called 'Citral essential oils'. Lemongrass EO can contain up to 85% Citral, Litsea Cubeba EO up to 75% and Verbena up to about 40%. Citral can therefore be obtained naturally from these essential oils.

Synthesis route :

The synthetic production of Citral takes many forms. A copper-catalysed dehydrogenation in vapor phase allows Citral to be obtained from Geraniol or from a mixture of Geraniol and Nerol. Another synthetic route is made from Dehydrolinalool (Linalool containing an alcyne function instead of an alcene) simply by puting it in the presence of a vanadium catalyst such as sodium orthovanadate.