Industry expert Tony O'Lenick asks Mark Garrison of Avon to explain the difference between esters and lactones.
An ester is a molecule that contains the functional group shown in Figure 1. An ester is formed from the reaction between an alcohol and an acid. A molecule of water is eliminated and an ester linkage is formed. The reaction is reversible; i.e., a molecule of water can hydrolyze an ester, giving back the original acid and alcohol, as shown in Figure 2.
R and R’ can be the same or different. When R is H, it is still an ester. When R’ is H, it is no longer an ester; the compound is then a carboxylic acid. The structures of esters, and hence their physical properties can vary widely. For example, the starting molecule of carboxylic acid may have any number of acid groups that can react. Likewise, the starting alcohol molecule may contain any number of hydroxyl groups. Consider also that either starting material can contain any number of carbon atoms, as well as branching, rings, unsaturation, etc.—and the possibilities are fairly limitless.
There are literally thousands of esters in use in the cosmetic industry. Esters provide emolliency and contribute considerably to the overall feel of a cosmetic formula. They can help to solubilize actives or sunscreens. The feel of cosmetic esters varies from the extremely light and “dry,” to oily, heavy, unctuous and everything in between. They vary in viscosity and consistency—from thin liquids, to very viscous liquids, to solid waxes. Volatile esters find use in the flavor and fragrance industry.
Since the esterification reaction is reversible, one may wonder why esters can be used in cosmetic emulsions; in other words, why is the hydrolysis of esters not a widespread stability problem? This is because hydrolysis generally is not an issue due to the poor water solubility of most cosmetic esters and the relatively mild pH levels of most cosmetic products. However, water-soluble esters can hydrolyze in aqueous formulations, and such reactions are accelerated at extremes of pH and elevated temperatures.
Lactones are also esters, but in lactones the ester forms a ring structure. Thus, the acid and alcohol groups are part of the same molecule. When a lactone hydrolyzes, only one molecule forms, which contains both functional groups, as shown in Figure 3. Lactones are classified as alpha, beta, gamma and delta based on the position of the OH group relative to the acid group, as shown in Figure 4.
Most lactones are gamma or delta, having five or six membered rings. These lactones are more stable than the lower members of the series because they have minimal ring strain. Lactones in the cosmetic and personal care industry are not nearly as common as the conventional esters described earlier. Considering the starting materials required for each, this makes sense. There exists a large variety of alcohols and carboxylic acids available to synthesize esters, and they are relatively inexpensive. However, starting materials for lactones are hydroxy acids, which are not nearly as abundant or inexpensive. So it is not surprising that there are not an abundance of lactones available for use as cosmetic emollients.
As with volatile esters, some lactones are used as flavors and fragrances. For example, gamma undecalactone has a peach aroma while gamma nonalactone smells like coconut (see Figure 5). Outside of the personal care industry are some interesting natural compounds that are lactones. For example, nepetalactone is the active constituent in catnip. Artemesinin, a complex structure, is a sesquiterpene lactone, which is used to treat malaria.
To summarize, esters and lactones contain the same functional group but lactones are in a ring structure. Esters are quite common in the personal care industry, useful as emollients and solubilizers. Lactones are much less common in personal care/cosmetics, but there are many interesting and useful natural compounds that are lactones. Volatile members of both groups find use as flavor and fragrance components.
F Fischetti, Flavors, in: Kirk-Othmer Encyclopedia of Chemical Technology, DOI: 10.1002/0471238961.0612012206091903.a01, posted online (Dec. 4, 2000)
WL Schreiber, Perfumes, in: Kirk-Othmer Encyclopedia of Chemical Technology, DOI: 10.1002/ 0471238961.1605180619030818.a01.pub2 . (Sep 23, 2005)
S Budavari, MJ O’Neil, A Smith, PE Heckelman and JF Kinneary, eds, The Merck Index, 12th edn., Whitehouse Station (NJ): Merck; (1996)
R Hall, D Klemme D and J Nienhaus, PhD, The H&R Book: Guide to Fragrance Ingredients, Hamburg: Verlagsgesellschaft R. Gloss & Co. (1985)