In the present discussion, Tony O’Lenick recruits Ismail Walele of Phoenix Chemical to explain the differences between fatty alcohols, fatty acids and esters.
Alcohols are alkanes with a hydroxyl group on the terminal carbon, which makes them primary alcohols. These are also called 1-alcohols, an example being 1-butanol or n-butanol. Some alcohols have their hydroxyl group on the c-chain, excluding the terminal primary carbon and making them secondary alcohols. Butanol has three isomers: n-butanol (1-butanol), 2-butanol (secondary butanol) and t-butanol, meaning tert-butanol with hindered hydroxyl on the same carbon with three methyl groups.
Fatty alcohols are aliphatic alcohols derived from natural fats and oils originating in plants and animals. Fatty alcohols are derived from fatty acids and have an even number of carbon atoms. The production of fatty alcohols from fatty acids yields normal-chain alcohols wherein the –OH group attaches to the terminal carbon. Fatty alcohols, due to their amphipatic nature, act as non-ionic surfactants/co-surfactants. Fatty alcohols can be used in cosmetic formulations as emulsifiers, emollients and thickeners.
Generally, alcohols are normal alcohols from natural fats and oils, meaning that they all have an even number of carbons. They can be saturated or unsaturated alcohols. Another type of alcohol is a branched chain alcohol, which is termed a synthetic higher alcohol or an oxo alcohol. Branched alcohols can be mono-methyl branched or multi-carbon chained on the side at any or specific interior carbon of the main carbon chain. Table 1 provides the common names, carbon numbers and the synthetic branched counterparts of alcohols.
Named after inventor M. Guerbet, Guerbet alcohols are alkaline condensation reaction products of primary alcohols. They are primary, alpha branched dimeric alcohols and are 100% defined branched at the second carbon position.
Oxo alcohols and iso-alcohols are alpha-olefin based and are approximately 50% branched at the second carbon position. Oxo alcohols are about 50% linear. Iso-alcohols are 100% multiple methyl branched.
Melting points or pour points are much lower for branched/Guerbet alcohols than for their linear counterparts of the same number of c-chains. Linear unsaturated alcohols are liquid; however, they suffer from poor heat stability due to unsaturation. The saturated Guerbet alcohols or branched iso alcohols offer fluidity and also thermal stability and oxidation stability. These differentiating physico-chemical properties of branched chain alcohols make them immensely important in the synthesis and derivatization into cosmetics and personal care emollients.
Fatty acids are organic acids comprised of carbon chains with a carboxyl group at the end. Saturated fatty acids have all carbons with a full quota of hydrogens. There is a single bond between adjacent carbon atoms. Unsaturated fatty acids have one or more carbon-carbon double bond in the molecule. Chemically, these double bonds will take up hydrogen, a process termed hydrogenation, that yields saturated fatty acids. Table 2 gives common names, IUPAC names, chemical structures and abbreviation designating presence or absence of unsaturation for fatty acid.
Saturated and unsaturated fatty acids are different in their form, as unsaturated fatty acids have one or more alkenyl functional group along the chain. Each alkene substitutes a single bonded CH2-CH2 segment of the chain with a double bonded CH2=CH2 segment, thus a carbon double-bonded to another carbon. Unsaturated fatty acids such as oleic acid can show two of their distinct forms (isomers), i.e. cis and trans forms. The cis form has adjacent carbons on the same side of the double bond. The trans form has adjacent carbons bound to the opposite side of the double bond. The trans form is more rigid than the cis form. Oleic acid has one double bond whereas linoleic acid has two double bonds and liolenic acid has three double bonds.
Fatty acids react just like any other carboxylic acid, meaning they can undergo esterification and acid-base reactions. Reduction of fatty acids gives corresponding fatty alcohols. Unsaturated fatty acids undergo addition reactions, with the most prominent being hydrogenation. Such hydrogenation is used to convert vegetable oils into margarines. Partial hydrogenation of unsaturated fatty acids gives isomers mainly converting cis form to trans form.
Esterification is a condensation reaction where an acid molecule reacts with an alcohol molecule, producing an ester and water, as shown in Figure 1.
Esterification is analogous to neutralization in the way that the resultant ester is named as if it is the alkyl salt of the acid. For example, sodium benzoate is the sodium salt of benzoic acid while lauryl benzoate is the ester of benzoic acid and lauryl alcohol.
There are a wide variety of esters due to the wide range of fatty acids and fatty alcohols available. The properties can be variable due to this wide array of variations.