Citral Chemical Properties

Melting point:

<-10°C

Boiling point:

229 °C (lit.)

Density 

0.888 g/mL at 25 °C (lit.)

vapor density 

5 (vs air)

vapor pressure 

0.2 mm Hg ( 200 °C)

refractive index 

n20/D 1.488(lit.)

FEMA 

2303 | CITRAL

Flash point:

215 °F

storage temp. 

2-8°C

solubility 

0.42g/l

form 

Liquid

color 

colorless to light yellow

Odor

at 100.00 %. sweet lemon citral

Odor Type

citrus

explosive limit

4.3-9.9%(V)

Water Solubility 

PRACTICALLY INSOLUBLE

Merck 

14,2322

JECFA Number

1225

BRN 

1721871

Exposure limits

ACGIH: TWA 5 ppm (Skin)

Stability:

Stable. but readily isomerizes. Incompatible with alkalies, strong oxidizing agents, strong acids. Combustible. Air and light sensitive.

LogP

2.76 at 25℃

CAS DataBase Reference

5392-40-5(CAS DataBase Reference)

NIST Chemistry Reference

Citral(5392-40-5)

EPA Substance Registry System

Citral (5392-40-5)

Safety Information

Hazard Codes 

Xi

Risk Statements 

38-43

Safety Statements 

24/25-37

RIDADR 

1760

WGK Germany 

1

RTECS 

RG5075000

Autoignition Temperature

225 °C

TSCA 

Yes

HS Code 

2912 19 00

HazardClass 

8

PackingGroup 

III

Hazardous Substances Data

5392-40-5(Hazardous Substances Data)

Toxicity

LD50 orally in rats: 4.96 g/kg (Opdyke)

MSDS

• Language:English Provider:SigmaAldrich
• Language:English Provider:ACROS
• Language:English Provider:ALFA

Citral Usage And Synthesis

Overview

Citral (C10H16O), also called 3,7-dimethyl-2,6-octadienal, a pale yellow liquid, with a strong lemon odour, that occurs in the essential oils of plants. It is insoluble in water but soluble in ethanol (ethyl alcohol), diethyl ether, and mineral oil. It is used in perfumes and flavourings and in the manufacture of other chemicals. Chemically, citral is a mixture of two aldehydes that have the same molecular formula but different structures.
Content analysis
Accurately weigh about 1g of the sample, and then perform the determination by the hydroxylamine method (OT-7, method one) used in aldehyde and ketone determination. The equivalent factor (e) in the calculation is 76.12.

Toxicity

ADI 0~0.5mg/kg (FAO/WHO, 1994-). LD50 4960 mg/kg (rat, oral); MNL 500 mg/kg.

Usage limits

FEMA (mg/kg): soft drinks 9.2; cold drinks 23; candy 41; baked goods 43; chewing gums 170

Chemical Properties

Colorless or slightly yellow liquid; strong lemon flavor; no optical rotation; boiling point 228 °C; flash point 92 °C;
There are cis and trans two isomers. With sodium bisulfite treatment, cis isomer solubility is minimal, while the trans isomer solubility is very large, so the two isomers can be separated.
Cis citral: relative density (d20) 0.8898, refractive index (nD20) 1.4891, boiling point 118~119℃ (2666Pa).
Trans citral: relative density (d20) 0.8888, refractive index (nD20) 1.4891, boiling point 117~118℃ (2666Pa).
Soluble in non-volatile oils, volatile oils, propylene glycol and ethanol; insoluble in glycerol and water; unstable in alkaline and strong acids
Natural products present in lemon grass oil (70% to 80%), litsea cubeba oil (about 70%), lemon oil, white lemon oil, citrus leaf oil and so on.

Application

Citral is an artificial flavor permitted to use in China, which can be used to prepare strawberries, apples, apricots, sweet orange, lemon and other fruit-based flavors. According to normal production needs, the citrals amount used in chewing gums is 1.70mg/kg; baked goods 43mg/kg; candy 41mg/kg; cold drinks 23mg/kg; soft drinks 9.2mg/kg.
It is also widely used in dishwashing detergents and the flavoring agents of soap and toilet water. Citral can be used as the raw material to synthesize ionone, methyl ionone and dihydro damascene. As organic raw material, it can also be reduced to generate citronellol, nerol alcohol and geraniol, and be converted into lemonile. In the pharmaceutical industry, it can be used for the manufacture of vitamin A and E, and also as the raw material of chlorophyll.

Production method

Citral natural exists in the litsea cubeba oil (about 80%), lemon grass oil (80%), clove basil oil (65%), sour lemon oil (35%) and lemon oil. In industry, citral can be derived from natural essential oils, or be prepared by chemical.
Synthesis based on methyl heptenone as raw material
Ethoxyacetylene magnesium bromide and methyl heptenone performed condensation reaction to form 3,7-dimethyl-1-ethoxy-3-hydroxy-6-octene-1-yne, which was then partly hydrogenated in the presence of catalysis to generate enol ether. And the enol ether was then hydrolyzed with phosphoric acid and dehydrated to obtain citral, with a yield of 68% calculated by methyl heptenone. In addition, acetylene and methyl heptenone could perform condensation reaction to form dehydrogenation linalool, which was then rearranged in the presence of silicon sulfone catalysis at 140~150 °C in inert solvent to get citral.
Derived from litsea cubeba oil (which is the main method to product citral in China)
Add 30 kg of cubeba oil containing about 75% of citral into a mixture under fully stirring, which was prepared with 18 kg of sodium bicarbonate, 38 kg of sodium sulfite and about 165 kg of water, and then continually stir for 5 to 6 h at room temperature. After standing overnight for stratification, the lower citral precipitated in the form of adduct. And the adduct was then washed with a small amount of toluene to remove oil and dried. And then add 10% sodium hydroxide solution to decompose citral at room temperature, and extract it with benzene. The extract was first distilled at atmospheric pressure (80-82°C) to recover benzene and then distilled under reduced pressure to collect fractions of 110-111°C (1.47kPa) to obtain pure product of 98% citral in an amount of about 15 to 16 kg.

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.

Uses

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.

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.

Uses

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.

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.

Definition

Commercial material is a mixture of α and β isomers.

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.).

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