Microscale – Compounds for the fragrance industry (ester synthesis)

Written by J.A Dobado | Last Updated on April 22, 2024

Objective

To prepare various esters at a microscale with applications in the fragrance industry by combining different carboxylic acids and alcohols that are selected because they are common in Organic Chemistry laboratory experiments.

Background

Esters are compunds with the following general formula R-C(=O)O-R’ formula. They result from by a reaction between a carboxylic acid and an alcohol in the presence of a mineral acid that is used as a catalyst. This process is the very well-known Fischer-Speier reaction (Fischer-Speier esterification). Most esters have pleasant odors, and in fact many of them produce the aromas of many fruits. This property implies that many esters are used in the perfumery industry or in food as fragrances or flavorings. In the case of the food industry, different legislation in different countries regulates their use, although the esters are produced in high purity and have the same properties as those isolated from natural sources. The carboxylic acids and alcohols used in this experiment are frequently available in an undergraduate Organic Chemistry laboratory, and it is possible to prepare a wide range of esters with recognizable flavors or odors. Table 1 lists the products that result from esterification reaction between different carboxylic acids and alcohols generally available in an Organic Chemistry laboratory.

Microscale experimental procedure

In a 5 ml conical vial provided with a spin vane, place 2.5 mmol of carboxylic acid, 1.25 mmol alcohol, and 1 drop of concentrated H₂SO₄. Fit a water condenser, and heat the mixture for 30 min in a water bath at 70 ºC.

After heating, allow the mixture to cool to r.t., remove the condenser, and add 1 ml of deionized water and 1 ml of diethyl ether. Set a stopper and shake by hand a few times for 30 s, open the stopper, and let the two layers separate. Using a Pasteur pipette, separate the aqueous layer (lower) and the organic one, and then wash the organic layer with a solution of NaHCO₃ 5 % in portions of 0.5 ml to ensure that all acid has been removed. Dry the organic layer by passing it through a Pasteur pipette with anhydrous Na₂SO₄ on a tared 5 ml conical vial. Remove the solvent by gentle heating or by passing an air stream (perform this evaporation in a fume hood). Weigh the crude product and calculate the yield. The ester may be contaminated with traces of unreacted alcohol. If the b.p. of the ester is less than 150 ºC, it can be purified by distillation with a Hickman column.

Physico-chemical properties

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

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
Acetic acid
Benzoic acid
Butan-1-ol
Butyl acetate
Butyl benzoate
Diethyl ether
EtOH
Ethyl benzoate	Non-hazardous
H2SO4
Isoamyl salicylate	Non-hazardous
Isopentyl acetate
Isopropyl benzoate
Na2SO4	Non-hazardous
NaHCO3	Non-hazardous
Octan-1-ol
Octyl acetate	Non-hazardous
Phenethyl acetate	Non-hazardous
Phenethyl alcohol
Phenethyl salicylate
Propan-2-ol
Salicylic acid
3-Methyl-butan-1-ol

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.

References

• Isac-García, J.; Dobado, J. A.; Calvo-Flores, F. G.; and Martínez-García, H. (2015). Experimental Organic Chemistry Laboratory Manual. Elsevier Science & Technology. ISBN: 978-0-12-803893-2
• Vogel, A.I., Furniss, B.S., Hannaford, A.J., Tatchell, A.R., and Smith, P.W.G. (1989). Vogel’s Textbook of Practical Organic Chemistry (Vogel’s Textbook series). Longman. ISBN: 9780470214145