Citral

Structural formula
	left Geranial (Citral A), right Neral (Citral B)
General
Surname	Citral
other names	3,7-dimethylocta-2,6-dienal; ( E ) -3,7-dimethylocta-2,6-dienal (geranial); ( Z ) -3,7-Dimethylocta-2,6-dienal (Neral);
Molecular formula	C 10 H 16 O
Brief description	light yellow liquid with a lemon-like odor
External identifiers / databases
EC number	226-394-6
ECHA InfoCard	100.023.994
PubChem	8843
Wikidata	Q60176530
properties
Molar mass	152.24 g mol −1
Physical state	liquid
density	0.89 g cm −3
Melting point	<−20 ° C
boiling point	225 ° C
Vapor pressure	0.046 h Pa (20 ° C)
solubility	almost insoluble in water (420 mg l −1 at 20 ° C)
Refractive index	1.4898 (20 ° C)
safety instructions
GHS hazard labeling from Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary Caution
H and P phrases	H: 315-317-319
	P: 280-305 + 351 + 338
Toxicological data	6000 mg kg −1 ( LD 50 , mouse , oral ) ; 4960 mg kg −1 ( LD 50 , rat , oral ) ;
	As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Citral is the mixture of the cis - trans isomers geranial (Citral A) and neral (Citral B). Geranial is used as a perfume u. a. to be found in tomatoes in small proportions as a breakdown product of lycopene . Citral is the main component of lemongrass oil . It is the alarm pheromone of the leaf cutter ant .

Extraction and presentation

In addition to being extracted from Litsea cubeba oil (content 60 to 80%), citral can also be obtained through various chemical syntheses. For example, through the reaction of isobutylene with formaldehyde to form isoprenol and its reaction to isoprenal with the help of a silver catalyst and then subsequent reactions with prenol to form citral. Other known syntheses start from isoprene or pinenes or are carried out by pyrolysis of limonene .

properties

Citral and neral and geranial include chemically to the group of acyclic mono terpene - aldehydes . Citral is a pale yellowish liquid with an intensely fresh lemon scent. The mixture boils at 228 ° C and is almost insoluble in water. Citral is irritating to the skin in its pure form and in a mixture in concentrations of 1% or more.

use

Citral is used as a fragrance and aroma. According to the 7th amendment to the EU Cosmetics Directive 76/768 / EEC, it must be declared as a cosmetic ingredient due to its allergenic potential.

Reactions

Neral and geranial can be made from nerol and geraniol by dehydration . Citral reacts with acetone in an alkaline environment (for example barium hydroxide ), initially releasing water to form pseudoionone . This can then be converted into the isomer ionone in the presence of acids and at higher temperatures .

In an alkaline environment it can also break down into acetaldehyde and 2-methylhept-2-en-6-one (retro- aldol reaction ).

The heterogeneously catalyzed hydrogenation in the presence of palladium catalysts leads via the intermediate citronellal to dihydrocitronellal with about 86% yield .

By reaction with hydroxylamine to Citraloxim and its dehydration can geranylnitrile be won.

Risk assessment

In 2014, citral was included in the EU's ongoing action plan ( CoRAP ) in accordance with Regulation (EC) No. 1907/2006 (REACH) as part of substance evaluation . The effects of the substance on human health and the environment are re-evaluated and, if necessary, follow-up measures are initiated. The reasons for the inclusion of citral were concerns about consumer use , exposure of workers , high (aggregated) tonnage and widespread use as well as the suspected hazards from sensitizing properties. The re-evaluation took place from 2015 and was carried out by Sweden . A final report was then published.

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Entry on citral in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .
↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-118.
↑ Entry on citral in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
↑ E. Boyland: Experiments on the chemotherapy of cancer: Further experiments with aldehydes and their derivatives. In: The Biochemical journal. Volume 34, Numbers 8-9, September 1940, pp. 1196-1201, PMID 16747303 , PMC 1265400 (free full text).
↑ ^a ^b Entry on citral in the ChemIDplus database of the United States National Library of Medicine (NLM), accessed on February 2, 2018.
^ PM Jenner, EC Hagan, Jean M. Taylor, EL Cook, OG Fitzhugh: Food flavors and compounds of related structure I. Acute oral toxicity. In: Food and Cosmetics Toxicology. 2, 1964, pp. 327-343, doi : 10.1016 / S0015-6264 (64) 80192-9 .
^ Ralf Günter Berger: Flavors and Fragrances Chemistry, Bioprocessing and Sustainability . Springer Science & Business Media, 2007, ISBN 978-3-540-49339-6 , pp. 288 ( limited preview in Google Book search).
↑ Volker Hessel: Design and Engineering of Microreactor and Smart-Scaled Flow Processes . MDPI, 2018, ISBN 978-3-03842-038-5 , pp. 128 ( limited preview in Google Book search).
^ Charles S. Sell: A Fragrant Introduction to Terpenoid Chemistry . Royal Society of Chemistry, 2007, ISBN 978-1-84755-001-9 , pp. 295 ( limited preview in Google Book search).
↑ Juliane Daphi-Weber, Heike Raddatz, Rainer Müller: Investigation of Fragrances - Controlled Fragrances , pp. 94–95, in Volume V of the series HighChem hautnah - News from food chemistry (published by the Society of German Chemists ) 2010, ISBN 978- 3-936028-64-5 .
↑ NODA, C., ALT, GP, WERNECK, RM et al .: Aldol Condensation of Citral with Acetone on Basic Solid Catalysts, in: Braz. J. Chem. Eng. , 1998 , 15 ; doi : 10.1590 / S0104-66321998000200004 ; Abstract .
^ A. Russell, RL Kenyon: Pseudoionone In: Organic Syntheses . 23, 1943, p. 78, doi : 10.15227 / orgsyn.023.0078 ; Coll. Vol. 3, 1955, p. 747 ( PDF ).
↑ P. Claus, J. Arras, D. Ruppert: Influence of ionic liquids with functionalized cations on the palladium-catalyzed liquid-phase hydrogenation of citral , in: Chem. Ing. Techn. 81 (2009), pp. 2007-2011; doi : 10.1002 / cite.200900085 .
^ Charles Sell: The Chemistry of Fragrances From Perfumer to Consumer . Royal Society of Chemistry, 2006, ISBN 978-0-85404-824-3 , pp. 67 ( limited preview in Google Book search).
↑ European Chemicals Agency (ECHA): Substance Evaluation Conclusion and Evaluation Report .
↑ Community rolling action plan ( CoRAP ) of the European Chemicals Agency (ECHA): Citral , accessed on March 26, 2019.

[GESTIS-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Entry on citral in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .

[CRC90_3_118-2] David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-118.

[CLP_100.023.994-3] Entry on citral in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .

[4] E. Boyland: Experiments on the chemotherapy of cancer: Further experiments with aldehydes and their derivatives. In: The Biochemical journal. Volume 34, Numbers 8-9, September 1940, pp. 1196-1201, PMID 16747303 , PMC 1265400 (free full text).

[chemid-5] Entry on citral in the ChemIDplus database of the United States National Library of Medicine (NLM), accessed on February 2, 2018.

[6] PM Jenner, EC Hagan, Jean M. Taylor, EL Cook, OG Fitzhugh: Food flavors and compounds of related structure I. Acute oral toxicity. In: Food and Cosmetics Toxicology. 2, 1964, pp. 327-343, doi : 10.1016 / S0015-6264 (64) 80192-9 .

[Ralf_Günter_Berger-7] Ralf Günter Berger: Flavors and Fragrances Chemistry, Bioprocessing and Sustainability . Springer Science & Business Media, 2007, ISBN 978-3-540-49339-6 , pp. 288 ( limited preview in Google Book search).

[Volker_Hessel-8] Volker Hessel: Design and Engineering of Microreactor and Smart-Scaled Flow Processes . MDPI, 2018, ISBN 978-3-03842-038-5 , pp. 128 ( limited preview in Google Book search).

[Charles_S._Sell-9] Charles S. Sell: A Fragrant Introduction to Terpenoid Chemistry . Royal Society of Chemistry, 2007, ISBN 978-1-84755-001-9 , pp. 295 ( limited preview in Google Book search).

[Weber-10] Juliane Daphi-Weber, Heike Raddatz, Rainer Müller: Investigation of Fragrances - Controlled Fragrances , pp. 94–95, in Volume V of the series HighChem hautnah - News from food chemistry (published by the Society of German Chemists ) 2010, ISBN 978- 3-936028-64-5 .

[11] NODA, C., ALT, GP, WERNECK, RM et al .: Aldol Condensation of Citral with Acetone on Basic Solid Catalysts, in: Braz. J. Chem. Eng. , 1998 , 15 ; doi : 10.1590 / S0104-66321998000200004 ; Abstract .

[12] A. Russell, RL Kenyon: Pseudoionone In: Organic Syntheses . 23, 1943, p. 78, doi : 10.15227 / orgsyn.023.0078 ; Coll. Vol. 3, 1955, p. 747 ( PDF ).

[13] P. Claus, J. Arras, D. Ruppert: Influence of ionic liquids with functionalized cations on the palladium-catalyzed liquid-phase hydrogenation of citral , in: Chem. Ing. Techn. 81 (2009), pp. 2007-2011; doi : 10.1002 / cite.200900085 .

[Charles_Sell-14] Charles Sell: The Chemistry of Fragrances From Perfumer to Consumer . Royal Society of Chemistry, 2006, ISBN 978-0-85404-824-3 , pp. 67 ( limited preview in Google Book search).

[15] European Chemicals Agency (ECHA): Substance Evaluation Conclusion and Evaluation Report .

[16] Community rolling action plan ( CoRAP ) of the European Chemicals Agency (ECHA): Citral , accessed on March 26, 2019.