9.3.F - Esters


Esters are sweet smelling chemical compounds usually derived by reacting an alkanoic acid with a hydroxyl compound such as an akanol.

Esters are found everywhere in nature. Most naturally occurring fats and oils are the fatty acid esters of glycerol. Esters with low molecular weights are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. Nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester bonds.

Esters have widespread use in the cosmetic industry as fragrance and are also use in industry as solvents for non-polar organic compounds.

In this topic students:

  • Name alkanols and alkanoic acids using the IUPAC nomenclature
  • Distinguish between the homologous series of alkanols and alkanoic acids on the basis of their functional groups
  • Account for the trends in boiling point for alkanols and alkanoic acids,
  • Describe the reaction mechanism for esterification,
  • Describe the production of esters in the school laboratory and account for the materials and apparatus used, and
  • Describe some applications and uses of esters.


Alkanols and Alkanoic Acids

Structure

The structure of the functional
group for the homologous
series of alkanols (above) and
alkanoic acids (below).

Alkanols are one of the homologous series of carbon compounds that all have a hydroxyl group (-OH) attached to the molecule. Each molecule in the homologous series differs by the addition of one extra carbon atom and creates a different compound. The common hydroxyl group is called the functional group for the homologous series of alkanols. Because the functional group is the same for each molecule in the series, they have similar chemical properties.

There are many homologous series of carbon compounds, each having their own functional group and with their compounds differing by the addition of a single carbon atom. Another series, known as the alkanoic acids, has the carboxyl group (-COOH) as their functional group. Again, each compound in the series differs by the addition of one carbon atom. This functional group consists of a carbon atom with a doubale covalent bond to an oxygen atom and another single covalent bond to the oxygen of the hydroxyl group.

The structure, formulae and naming conventions for the first eight compounds in each homologous series are outlined in the table below. By convention, the letter R is used to denote a carbon chain of any length. You should note that when an extra carbon atom is added to the chain, two hydrogens must also be added to ensure each carbon has four covlaent bonds.

The alkanols in the table below are known as primary alkanols because the hydroxyl group is attached to the first carbon in the chain in each case. If the hydroxyl group was attached to any carbon other than the first or last in the chain, they would be collectively called secondary alkanols.

The formulae in the table below are known as condensed structural formula. They show some aspects of the bonding in each molecule. While the condensed structural formula for butanoic acid is CH3-(CH2)2-COOH, its molecular formula could be written as C4H8O2.

The general formula for alkanoic acids is CnH2n-1COOH and for alkanols it is CnH2n+1OH, where n is the number of carbon atoms in the molecule.

No. Carbon Atoms Alkanols Alkanoic Acids
Name Formula Name Formula
R hydroxyl group R-CH2-OH carboxyl group R-COOH
1 methanol CH3-OH methanoic acid HCOOH
2 ethanol CH3-CH2-OH ethanoic acid CH3-COOH
3 propan-1-ol CH3-(CH2)2-OH propanoic acid CH3-CH2COOH
4 butan-1-ol CH3-(CH2)3-OH butanoic acid CH3-(CH2)2-COOH
5 pentan-1-ol CH3-(CH2)4-OH pentanoic acid CH3-(CH2)3-COOH
6 hexan-1-ol CH3-(CH2)5-OH hexanoic acid CH3-(CH2)4-COOH
7 heptan-1-ol CH3-(CH2)6-OH heptanoic acid CH3-(CH2)5-COOH
8 octan-1-ol CH3-(CH2)7-OH octanoic acid CH3-(CH2)6-COOH

 

Physical Properties

A graph showing the relative boiling points of the the homologous
series of alkanes, alkenes, alkanols (alcohols) and alkanoic acids
(carboxylic acids).

The graph on the right is rich in information about the homologous series it represents.

Firstly, the boiling points of each homologous series increases with the number of carbon atoms in the chain. You should remember that intermolecular dispersion forces increase with the the number of electrons in a molecule. Each time a carbon atom is added to a molecule in the series, the strength of the dispersion forces increases and the boiling point increases.

Secondly, the boiling points of alkanoic acids are higher than their corresponding alkanols, which are higher again than the alkanes and alkenes. Alkanoic acids and alkanols have hydrogen bonding between their molecules while the alkanes and alkenes have weaker dispersion forces. This would account for the alkanoic acids and alkanols having higher boiling points than the alkanes and alkenes, but not tell us why the boiling point of the alkanoic acids is higher than the alkanols. For this we need to look more closely at the structure of the molecules and the bonds that form between them.

Structure of (a) the head-to-tail dimer formed between two carboxylic
acid molecules, and (b) a typical network of a primary alcohol in the
liquid phase. The dashed line in each case represents hydrogen
bonding.

If we were to consider molecules of ethanol in the liquid phase, there is only one place where a hydrogen bond can occur between the two adjacent molecules. You should also remember that hydrogen bonding occurs when a hydrogen atom on one molecule sits between either a nitrogen, oxygen or fluorine on another molecule. The diagram of ethanol (b) on the right shows the hydrogen in the hydrogen bond between two atoms of oxygen.

Dimers (two molecules) of ethanoic acid (a) are able to orient themselves so that there are two sites where hydrogen bonding can occur. The presence of two hydrogen bonds per molecule accounts for the higher boiling point of the alkanoic acids.

 

Esterification

Chemistry

Esters are produced when alkanoic acids are heated with alkanols in the presence of a a concentrated acid catalyst. The catalyst is usually sulfuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring).

The esterification reaction is both slow and reversible. The general equation for the reaction between an acid R-COOH and an alcohol R'-OH (where R and R' can be the same or different) is:

So, for example, if you were making propyl ethanoate from ethanoic acid and propan-1-ol, the equation would be:

The hydroxyl group from the ethanoic acid and the hydrogen from the propanol are removed and they bond together to make a molecule of water. For this reason, esterification reactions are also condensation reactions. The chemical equation for the above reaction would be:

CH3CH2COOH (l) + CH3CH2CH2OH (l) ⇔ H2O (l) + CH3COOCH2CH3

Naming

The structure of some common esters and their names.

Esters are named according to the alkanol and and the alkanoic acid from which they are made. The general name for an ester is an alkyl alkanoate where the alkyl part represents the alkanol and the alkanoate part represents the alkanoic acid. For example, ethyl propanoate was made from ethanol and propanoic acid while propyl methanoate was made from propanol and methanoic acid. The names and structure of some common esters are shown in are shown on the right.

Many esters have distinctive odours, which has led to their widespread use as artificial flavourings and fragrances. For example:

  • Methyl butanoate and methyl propanoate smell of pineapple
  • Ethyl methanoate smells of raspberry
  • Propyl ethanoate smells of pears
  • Pentyl ethanoate smells of banana
  • Pentyl pentanoate smells of apple
  • Pentyl butanoate smells of pear or apricot
  • Octyl ethanoate smells of orange

Production

On a test tube scale

Alkanoic acids and alkanols can be warmed together in the presence of a few drops of concentrated sulfuric acid in order to observe the smell of the esters formed. You would normally use small quantities of everything heated in a test tube stood in a hot water bath for a couple of minutes. Because the reactions are slow and reversible, you don't get a lot of ester produced in this time. The ester formed evaporates quickly and this stops the reverse reaction and drives the equilibrium in the direction of the products. It works well because the ester has the lowest boiling point of anything present. The ester is the only thing in the mixture which doesn't form hydrogen bonds, and so it has the weakest intermolecular forces.

The smell of the ester is often masked or distorted by the smell of the alkanoic acid. A simple way of detecting the smell of the ester is to pour the mixture into some water in a small beaker. Esters are virtually insoluble in water and tend to form a thin layer on the surface. Excess acid and alkanol both dissolve and are tucked safely away under the ester layer. 

Small esters like ethyl ethanoate smell like typical organic solvents (ethyl ethanoate is a common solvent in glues for example). As the esters get bigger, the smells tend towards artificial fruit flavourings.

On a larger scale

Typical reflux apparatus used in the preparation of larger esters.

If you want to make a reasonably large sample of an ester, the method used depends to some extent on the size of the ester. Larger esters tend to form more slowly. In these cases, it may be necessary to heat the reaction mixture under reflux for some time to produce an equilibrium mixture. The ester can be separated from the alkanoic acid, alkanol, water and sulfuric acid in the mixture by fractional distillation.

Sulfuric acid acts as a dehydrating agent. It effectively removes water from the system and, therefore, drives the reaction towards the products because water is one of the products (Le Chatelier's Principle). Although it is referred to as a catalyst, it doesn't really work in the same way that catalysts do.

The reflux apparatus shown above is used to allow heating of the volatile mixture of liquid reactants without the escape of the reactants into the atmosphere. Volatile substances evaporate easily and are flammable. Heating this mixture in a test tube would simply cause it to evaporate very quickly before any reaction could form. The reflux system allows a reasonable rate of reaction at a high temperature without the loss of the reactants and products. You shoudl also note that a heating coil or water bath should be used for safety reasons. The volatile reactants are flammable and should not be heated with a naked flame.

 

Uses

Esters are widespread in nature and are widely used in industry. In nature, fats are a type of ester derived from glycerol and fatty acids. Esters are responsible for the aroma of many fruits, including apples, pears, bananas, pineapples, and strawberries. Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate (plastic) and cellulose acetate (fibre).

Esters have no free hydroxyl groups so they tend towards being non-polar and insoluble in water. However, they are good solvents for polar organic (carbon) compounds such as lacquers, printing inks, polystyrene and cement. They are also used significantly as solvents in the pharmaceutical industry.

Esters are also added to plastics to enhance their flexibility. In this capacity they are known as plasticisers. Esters of benzene and 1,2-dicarboxylic acid are used in polyvinyl chloride (PVC).

Because of their sweet smells, esters are also used as food flavourings and cosmetic ingredients.

Click here to see a list of esters and their uses.