Citral

Chemical properties

mobile light yellow liquid with a lemon-like smell

Chemical properties

Citral occurs as (2Z)- and (2E)-isomers (citral a and b, respectively) analogous to the corresponding alcohols, geraniol and nerol: geranial (citral a), bp2.7 kPa 118–119 °C, d20 0.8888, n20 D 1.4898; neral (citral b), bp2.7 kPa 120 °C, d20 0.8869, n20 D 1.4869. Natural citral is nearly always a mixture of the two isomers. It occurs in lemongrass oil (up to 85%), in Litsea cubeba oil (up to 75%), and in small amounts inmany other essential oils. The citrals are colorless to slightly yellowish liquids, with an odor reminiscent of lemon.
Since citral is an α,β-unsaturated aldehyde with an additional double bond, it is highly reactive and may undergo reactions such as cyclization and polymerization. Geraniol, citronellol, and 3,7-dimethyloctan-l-ol can be obtained from citral by stepwise hydrogenation. Citral can be converted into a number of addition compounds; the (Z)- and (E)-isomers can be separated via the hydrogen sulfite addition compounds.The condensation of citral with active methylene groups is used on an industrial scale in the synthesis of pseudoionones, which are starting materials for ionones and vitamins.

Chemical properties

Citral has a strong, lemon-like odor and a characteristic bittersweet taste. Commercially, the product is a mixture of two geometric isomers—α-citral and β-citral, each exhibiting cis- and trans-isomers because of the position of the double bond.

Occurrence

Reported found in apricot, clary sage, ginger, grape, grapefruit, lemon, lime, mandarin, orange, raspberry, tamarind, tangerine, tea and tomato.

The Uses of Citral

Citral is an anti-microbial agent found in plants with antibacterial activity against some food pathogens. It is also a fragrance compound with a distinct lemon scent.

The Uses of Citral

citral is a naturally occurring aroma compound used to provide a lemon-type fragrance. Citral is a constituent of lemon oil, lemongrass oil, lime oil, ginger oil, verbena oil, and other plant-derived C essential oils.

The Uses of Citral

Citral is a liquid flavoring agent, light yellow in color with a citrus odor. it occurs in lemon and lemongrass oils. it is usually obtained from citral-containing oils by chemical means but may also be pre- pared synthetically. it is soluble in fixed oils, mineral oil, and pro- pylene glycol. it is moderately stable and should be stored in glass, tin, or resin-lined containers. it is used in flavors for lemon with applications in candy, baked goods, and ice cream at 20–40 ppm. it is also termed 2,6-dimethyl-octadian-2-6-al-8.

What are the applications of Application

Citral is a reagent used in the synthesis of vitamin A, ionone, and methylionone

Definition

Commercial material is a mixture of α and β isomers.

Preparation

Since citral is used in bulk as a starting material for the synthesis of vitamin A, it is produced industrially on a large scale. Smaller quantities are also isolated from essential oils.
1) Isolation from essential oils: Citral is isolated by distillation from lemongrass oil and from L. cubeba oil. It is the main component of these oils. 2) Synthesis from geraniol: Currently, the most important synthetic procedures are vapor-phase dehydrogenation and oxidation of geraniol or geraniol–nerol mixtures. Catalytic dehydrogenation under reduced pressure using copper catalysts is preferred.
3) Synthesis from dehydrolinalool: Dehydrolinalool is produced on a large scale from 6-methyl-5-hepten-2-one and acetylene and can be isomerized to citral in high yield by a number of catalysts. Preferred catalysts include organic orthovanadates, organic trisilyl oxyvanadates, and vanadium catalysts with silanols added to the reaction system.
4) Synthesis from isobutene and formaldehyde: 3-Methyl-3-buten-l-ol, obtained from isobutene and formaldehyde, isomerizes to form 3-methyl- 2-buten-lol. However, it is also converted into 3-methyl-2-butenal by dehydrogenation and subsequent isomerization. Under azeotropic conditions in the presence of nitric acid, 3-methyl-2-buten-l-ol and 3-methyl-2-butenal form an acetal (shown as follows), which eliminates one molecule of 3-methyl-2-buten-l-ol at higher temperatures. The intermediate enol ether undergoes Claisen rearrangement followed by Cope rearrangement to give citral in excellent yield:
Today, this route is performed on a very large industrial scale in a continuous reactive distillation process.

Aroma threshold values

Detection at 1.0%: characterizing lemon-like, distilled lime peel, intense aldehydic citruslike.

Taste threshold values

Taste characteristics at 5 ppm in 5% sugar and 0.1% CA: characteristic lemon, peely, citrus, green floral juicy with woody and candy notes.

Synthesis Reference(s)

Journal of the American Chemical Society, 101, p. 7131, 1979 DOI: 10.1021/ja00517a088
Tetrahedron Letters, 35, p. 4007, 1994 DOI: 10.1016/S0040-4039(00)76726-8

General Description

A clear yellow colored liquid with a lemon-like odor. Less dense than water and insoluble in water. Toxic by ingestion. Used to make other chemicals.

Air & Water Reactions

Insoluble in water.

Reactivity Profile

Citral is an aldehyde. Aldehydes are frequently involved in self-condensation or polymerization reactions. These reactions are exothermic; they are often catalyzed by acid. Aldehydes are readily oxidized to give carboxylic acids. Flammable and/or toxic gases are generated by the combination of aldehydes with azo, diazo compounds, dithiocarbamates, nitrides, and strong reducing agents. Aldehydes can react with air to give first peroxo acids, and ultimately carboxylic acids. These autoxidation reactions are activated by light, catalyzed by salts of transition metals, and are autocatalytic (catalyzed by the products of the reaction). The addition of stabilizers (antioxidants) to shipments of aldehydes retards autoxidation. Citral can react with alkalis and strong acids. Citral can readily isomerize.

Hazard

Questionable carcinogen.

Fire Hazard

Citral is combustible.

Flammability and Explosibility

Non flammable

Contact allergens

Citral is an aldehyde fragrance and flavoring ingredient, a blend of isomers cis (Neral) and trans (geranial). As a fragrance allergen, citral has to be mentioned by name in cosmetics within the EU.

Safety Profile

Moderately toxic by intraperitoneal route. Mildly toxic by ingestion. Experimental reproductive effects. A severe human and experimental skin irritant. Mutation data reported. Combustible liquid. When heated to decomposition it emits acrid smoke and irritating fumes.

Synthesis

Citral is usually isolated from the citral-containing oil by chemical means or by chemical synthesis (from β-pinene, isoprene, etc.).