Esters

Esters – general information

Esters constitute one of the most important groups of compounds in organic chemistry. Mainly, they are the products of the reaction of carboxylic acids (but also inorganic acids) with alcohols. The general formula for  esters is R₁COOR₂ (where R is an alkyl group). The names of esters are created based on the names of the appropriate acids from which they were obtained or the names of acid residues and radicals that are part of the ester, e.g. formic acid ethyl ester or ethyl formate. Their characteristic feature are their odours, often described as fruity.

Esters are obtained in the esterification reaction. It involves monocarboxylic acids (having only one -COOH group in the molecule), in which the hydrogen atom is substituted with an alkyl radical. Esters also undergo hydrolysis reactions, i.e. disintegration under the influence of water molecules, as a result of which the carboxylic acid and alcohol are reconstituted. The hydrolysis of esters is relatively easy and it can be accelerated by the introduction of hydrogen or hydroxide ions into the system. Another characteristic reaction is the saponification of esters. By treating them with sodium hydroxide, a sodium salt of the acid and the corresponding alcohol are obtained. In chemistry, the salts of higher carboxylic acids obtained this way are called soaps.

The esterification reaction

The reaction between carboxylic (or inorganic) acids and alcohols is called esterification. This process results in the formation of an appropriate ester. The hydrogen atom from the carboxyl group and the hydroxyl group combine to form a water molecule. At low temperatures, the esterification reaction proceeds very slowly and never finishes, even after a long period of time. After the reaction of a significant amount of the acid and the alcohol, an equilibrium state is established (the rate of product formation is equal to the rate of their decomposition and re-conversion to substrate molecules). Thus, it is a reversible process. The speed at which this reaction reaches the state of chemical equilibrium depends primarily on the strength of the acid involved, as well as on the temperature in which the entire process takes place. Sulphuric (VI) acid is often used as a catalyst for esterification. The acid is a hydrogen ion donor and a binding agent for the water that is formed. Thanks to this, it is possible to shift the state of chemical equilibrium in favour of product formation.

Properties of esters

Physical and chemical properties of organic acid esters:

• Esters of lower carboxylic acids are highly volatile liquids. They are poorly soluble in water and have a lower density in comparison to water.
• The boiling points of esters are lower in comparison to other organic compounds due to the lack of hydrogen bonds between the molecules. Short-chain esters, e.g. ethyl acetate, which is well soluble in water due to the presence of hydrogen bonds, are exceptions
• As the carbon chain in the ester molecules grows, their density and boiling point increase, and their physical states change from oily liquids to solids.
• All esters are colourless.
• The vast majority of esters have a characteristic, fruity odour, e.g. ethyl acetate like a pear, and 3-methylbutyl acetate like a banana. Compounds obtained from higher carboxylic acids are odourless.
• Esters are good solvents for a wide range of organic compounds.

Esters of inorganic acids

The esterification reaction of inorganic acids with alcohols leads to the formation of inorganic esters.

Esters of hydracids

Hydracids, such as hydrochloric acid (HCl) or hydrobromic acid (HBr), react with alcohols, e.g. ethanol, which leads to the formation of appropriate ester molecules (ethyl chloride, ethyl bromide) and a water molecule. The reaction proceeds strongly towards the formation of products due to the high volatility of hydracid esters (they quickly evaporate from the reaction environment).

Sulphuric acid esters

Esters of sulphuric (VI) acid are of great importance in chemistry. A concentrated acid is used for the esterification reaction. The simplest in this group of compounds is sulphuric acid methyl ester. It is easily hydrolysed and reacts with alkalis. Distillation of this compound under reduced pressure leads to the formation of the highly poisonous sulphuric acid dimethyl ester. Esters of H₂SO₄ and higher alcohols are used as surfactants.

Nitric acid esters

These compounds are unique, primarily because of their explosive properties. Glycerol trinitrate, i.e. the popular nitroglycerin, is obtained by the reaction of glycerin (polyhydroxy alcohol) with a mixture of nitric and sulphuric acids. It is a colourless, oily liquid. It explodes even under the influence of a weak shock or impact. A mixture of nitroglycerin and diatomaceous earth forms dynamite. Ethyl and amyl nitrite are used in medicine.

Fats

Fats are of great practical importance in chemistry. From a chemical point of view, they are esters (of natural origin). They are formed as a result of the reaction of glycerin (a polyhydroxyl alcohol, containing three hydroxyl groups in the molecule) with higher fatty acids. Esters are often known as glycerides. Their chemical structure is diverse. In nature, there are homogeneous glycerides, i.e. those in which all three hydroxyl groups have been esterified with the same acid, or mixed glycerides, where the acid residues come from two, and sometimes even three fatty acids. Fats include saturated acids (e.g. palmitic and stearic acid) and unsaturated acids (e.g. oleic acid).

Usually fats are classified as animal fats and plant fats.

Animal fats are made of saturated fatty acid residues, which make them solid. This group includes suet, cod liver oil (exception: liquid state), lard, butter, etc.

On the other hand, the molecules of plant fats contain unsaturated acid residues, which is why they are usually liquid. Examples of plant fats are olive oil, rapeseed oil, linseed oil, etc.

Fats, like other esters, undergo hydrolysis. Under the influence of light, moisture, bacteria and enzymes, they go rancid. They do not dissolve in water, but they dissolve very well in organic solvents. Liquid fats of higher unsaturated acids can be converted into saturated fats in the process of fat hardening. It is a hydrogenation reaction occurring in the presence of nickel as a catalyst. Glycerides also undergo hydrolysis, as a result of which they break down into glycerin and the corresponding fatty acids.