Esters are common chemical compounds that derivate from acids, either organic or inorganic ones. It is an umbrella term for a wide range of organic compounds that are reactive in contact with water. The reaction generates the initial organic or inorganic acids, as well as alcohols. Leopold Gmelin, a German chemist, is credited with the introduction of the term ester, early in the 19th century.
An ester is defined as an acid derivative where one or more hydroxyl groups (-OH) have been replaced by an alkoxy (-O-alkyl) group. Most esters are formed after a reaction between carboxylic acids and alcohols. Some of the most important esters are the fatty acid compounds of glycerol, named glycerides, which represent most of the vegetable oils and animal fats. As one of the main lipid classes, glycerides are a key part of human and animal biology. Esters are also important for their fragrance and those with a reduced molecular weight are included in both pheromones and essential oils. Another key role of these compounds is in the structure of DNA, as phosphoesters. Many other esters are useful in industry, for example nitroglycerin and other nitrate esters are powerful explosives, while monomers linked by ester moieties form polyesters that are some of the most common types of plastics.
The process named esterification is the most common way to prepare esters. It happens as a reaction between carboxylic acids and alcohols, with strong acids such as hydrochloric acid or sulphuric acid acting as catalysts. This produces compounds with the formula RCOOR′, where R and R′ can be any organic groups, named carboxylic acid esters. The hydroxyl group (OH) in carboxylic acid is substituted with alcohol’s alkoxy group (R′O) as part of the reaction.
Esterification can also happen in reverse, which is a type of hydrolysis. Many other reactions are known. An alternative way to produce esters is reacting salts of carboxylic acids with alkyl halides or acid halides or acid anhydrides with alcohols. Transesterification is the transformation of an ester into another one, in the presence of a catalyst. The reaction can be with a carboxylic acid, an alcohol or even another ester.
Saponification is an important reaction in industry that produces soap using various fats and oils as raw materials. It is also useful to estimate the overall amount of esters and is a hydrolysis of esters in the presence of alkalies.
As long as they have a low molecular weight, carboxylic acid esters are volatile liquids soluble in water, with no color and nice fragrance. They give the flavour of most varieties of flowers and fruits and are widely found in nature. Every fruit is usually associated with an ester that grants it the specific fragrance, for example pineapples have a content of ethyl butyrate, while methyl salicylate is found in wintergreen and isopentyl acetate in bananas. Fragrant esters are also used industrially to produce perfumes, cosmetics and artificial flavours.
Many types of esters are found in nature. Various types of wax produced by both plants and some animals actually consist of esters, produced in reactions between carboxylic acids and alcohols with long molecular chains. Long-chain carboxylic acids and glycerol form all types of vegetable oils and animal fats.
Other volatile esters such as ethyl acetate and butyl acetate are produced in large amounts for use in industry. These serve as solvents in the manufacturing of paints, lacquers and varnishes.
Esters in liquid form with low volatility are used in the production of resin and plastic, as softening agents. Polymers, which are also esters, have numerous industrial applications. Lucite and Plexiglas are the commercial names of polymethyl methacrylate, a polymer that can replace glass. Terylene, Fortrel or Dacron are polymer textile fibers, while Mylar is a film made from polyethylene terephthalate.
Alcohols can also react with sulphuric, phosphoric, nitric and other types of inorganic acids, forming esters. Nitrate esters are particularly important because they serve as explosives, the best known is nitroglycerin or glyceryl trinitrate. Phosphate esters are important in both biology and industry. They include nucleic acids and many compounds that serve as plasticizers, solvents, gasoline and oil additives, insecticides or flame retardants.
Sulphuric and sulphurous esters also have industrial applications, especially in pharmacy and the production of dyes. The most widely found is dimethyl sulphate, which is extremely poisonous.
Solubility in water
Esters with a simple structure are soluble in water but this property decreases for those with longer chains. Esters are soluble because they are able to hydrogen bond with molecules of water, even if they are unable to do it with themselves. The bond can only form between a hydrogen atom with a slightly positive charge from a water molecule and a lone pair from one of the oxygen atoms in an ester. The attraction only exists between these two atoms.
Other forces are also present, for example the ester and the water molecules are dipole-dipole attracted and there are also dispersion forces. The energy released by these attractions is what breaks the molecules of ester and water, allowing them to react with each other.
The reason why esters with a longer chain become less soluble is that the large hydrocarbon parts start to prevent this reaction. These hydrocarbon segments fit between the molecules of water and destroy the strong hydrogen bonds that bind them together. However, no other bonds replace them so a lower amount of energy is produced. As a result, long-chain esters become less and less soluble in water.
Esters are key compounds both in biology and in industry. Parabens are a type of esters that kill mold, yeast and other similar micro organisms, which makes them widely used as preservatives for medicine and food. Because of their low molecular mass and pleasant fruity smell, esters are a very common ingredient in perfumes and cosmetic products in general. Esters with a lower molecular mass have a stronger fragrance than the ones with a larger structure. The sweet smell is also a valuable addition to processed food, improving the taste and flavour. These are the very common artificial flavours present on the labels of many of today’s foods.
Ethyl ethanoate and other esters with a simple structure are known as excellent organic solvents. They are used in numerous industrial products, such as nail polish removers, sun protection lotions, glues or plasticizers. Dissolving polystyrene in ethyl ethanoate produces the special polystyrene cement, which unites two surfaces with a solid layer of plastic, after the ester solvent evaporates. Larger esters are the raw materials for all types of detergents and soaps, which are used to clean skin and solid objects after being dissolved in water.
Polyester, which is one of the most widespread forms of plastic, is made using esters. It is a plastic with high durability and no chemical reactivity, due to the extremely long molecular chain. Many types of synthetic fibers used in the production of clothing and other items are made of polyester. It is an extremely resilient material, able to stretch and shrink with minimal abrasion, while not reacting with most chemicals. Since it repels water, it also dries very quickly, which makes it a useful hollow fiber material used in insulation works.
Most plants found in nature have a content of esters, which is behind both the fragrance of their flowers and the taste of their fruits. This effect has been copied in industrial manufacturing, where esters serve as artificial flavours or fragrances. It is now possible to replicate the smell and taste of most fruits, sometimes using a single ester. Some more complex fragrances require a mix of esters with other compounds.
Another interesting modern use of esters is the scratch and sniff spots usually found on paper items, such as stickers, advertisements, magazines or books for kids. In order to achieve this effect, a layer of esters packaged in tiny capsules is applied. Scratching it with a fingernail or coin destroys the microcapsules, releasing the volatile esters.