Uses
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.
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.
Reactivity Profile
CITRAL(5392-40-5) 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. This compound can react with alkalis and strong acids. CITRAL(5392-40-5) can readily isomerize.
Air & Water Reactions
Insoluble in water.
Hazard
Questionable carcinogen.
Fire Hazard
This chemical is combustible.
Occurrence
Reported found in apricot, clary sage, ginger, grape, grapefruit, lemon, lime, mandarin, orange, raspberry,
tamarind, tangerine, tea and tomato.
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.
Flammability and Explosibility
Nonflammable
Synthesis
Citral is usually isolated from the citral-containing oil by chemical means or by chemical synthesis (from β-pinene, isoprene,
etc.).
