Conversion of citral to α- and β-ionone

Objective

To produce α- and β-ionone (violet perfume) from citral. These are cyclic terpenoid that occur in many essential oils.

Conversion of citral to α- and β-ionone

Background

Citral (3,7-dimethyl-2,6-octadienal) is the main constituent of the essential oil of lemon grass (citronella). This natural product readily undergoes aldol condensation with acetone to give geranylacetone, which can be converted by cyclization under acidic conditions into a mixture of α- and β-ionones used as an artificial violet flavor. The nature of the catalyst, its concentration, and its temperature play decisive roles in the ratio between the two ionones.

Experimental procedure

A) preparation of geranylacetone

In a 100 ml round-bottom flask, prepare a solution of freshly distilled citral (5 g, b.p. = 91–93 ºC at 2.6 mm Hg) in dry acetone (20 ml). Cool the mix ture to −5 ºC (ice/salt), and add dropwise a solution of sodium ethoxide prepared from 230 mg of sodium and 5 ml of absolute EtOH with magnetic stirring. Continue stirring for another 10 min at −5 ºC, and afterward, neutralize the excess alkali with stirring by adding a solution of tartaric acid in water, prepared from 900 mg tartaric acid in 25 ml deionized water. Remove excess acetone and EtOH in a rotary evaporator. Isolate geranylacetone from the remaining aqueous solution and extract with diethyl ether (2 × 5 ml).

Combine the organic extracts and dry over Na2SO4 anhydrous, remove the desiccant by gravity filtration, and the evaporation generates a liquid that is composed mainly of ψ-ionone. Weigh and calculate the yield. Purify by vacuum distillation (b.p. = 145–150 ºC at 12 mm Hg).

B) Synthesis of α- and β-ionone

To a 100 ml round-bottom flask containing 15 g of a solution of phosphoric acid 85%, add dropwise 2 g of ψ-ionone with magnetic stirring. After the addition, maintain the stirring at r.t. for 1 h. After this time, pour the reaction crude into a 100 ml beaker containing 50 ml of cold water, and then extract the reaction mixture with diethyl ether (2 × 20 ml). Wash the combined ether extracts several times with water (until no acid pH reaction ocurrs). Dry the organic layer over Na2SO4 anhydrous, separate the desiccant by gravity filtration, and remove the solvent on a rotary evaporator. Weigh the resulting liquid and calculate the yield.

Physico-chemical properties

This table collects data for the molecular weight (Mw), melting point (M.p.) boiling point (B.p.) and density of the reactives and compounds used in this laboratory experiment.

Name Mw (g/mol) M.p. (ºC) B.p. (ºC) Density (g/ml)
Citral 152.23 - 229 0.888
α-Ionone 192.30 - 259-263 0.930
β-Ionone 192.30 -35 126-128 0.945
Geranylacetone 192.3 <25 114-116 0.900
H3PO4 98.00 40 158 1.685
Acetone 58.08 -94 56 0.791
L-(+)-Tartaric acid 150.09 170-172 - -
Diethyl ether 74.12 -116 34.6 0.71
Na2SO4 142.04 884 - 2.630
Na 22.99 97.8 883 0.968
EtOH 46.07 -114.1 78.5 0.790

GHS pictograms

Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and are collected in the followinf Table for the chemical compounds used in this experiment.

Name GHS
Citral ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
α-Ionone ghs08  Danger Warning Systemic health hazards pictogram
β-Ionone ghs09  Warning (for cat. 1) (for cat. 2 no signal word) Environmental hazard pictogram
Geranylacetone Non-hazardous
H3PO4 ghs05  Danger Warning Corrosive cat. 1 pictogram
Acetone ghs02  Danger Warning Flammable pictogram ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
L-(+)-Tartaric acid ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
Diethyl ether ghs02  Danger Warning Flammable pictogram ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
Na2SO4 Non-hazardous
Na ghs02  Danger Warning Flammable pictogram ghs05  Danger Warning Corrosive cat. 1 pictogram
EtOH ghs02  Danger Warning Flammable pictogram

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.

Citral WTEVQBCEXWBHNA-JXMROGBWSA-N
α-Ionone UZFLPKAIBPNNCA-BQYQJAHWSA-N
β-Ionone PSQYTAPXSHCGMF-BQYQJAHWSA-N
Geranylacetone HNZUNIKWNYHEJJ-FMIVXFBMSA-N
H3PO4 NBIIXXVUZAFLBC-UHFFFAOYSA-N
Acetone CSCPPACGZOOCGX-UHFFFAOYSA-N
L-(+)-Tartaric acid FEWJPZIEWOKRBE-JCYAYHJZSA-N
Diethyl ether RTZKZFJDLAIYFH-UHFFFAOYSA-N
Na2SO4 PMZURENOXWZQFD-UHFFFAOYSA-L
Na KEAYESYHFKHZAL-UHFFFAOYSA-N
EtOH LFQSCWFLJHTTHZ-UHFFFAOYSA-N

References

Back to the Organic Synthesis Experiments.

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