Esters- Structure and formation

Esters are organic compounds with the general formula RCOOR', where R and R' are alkyl or aryl groups. You will probably have heard of esters for the first time during your study of gcse chemistry. You may even have had the opportunity to make some esters by carrying out a condensation reaction; that is a reaction between two molecules which combine to make a larger one and release a small molecule; which is usually water. In the lab esters can be made quickly by the reaction between a carboxylic acid and an alcohol.

Esters are very easy to make, simply mix equal amounts of an alcohol and a carboxylic acid in a test-tube as shown opposite, a few drops of concentrated sulfuric acid is added to act as a catalyst, since the preparation of esters from alcohols and carboxylic acids is a reversible reaction addition of the concentrated sulfuric acid, a powerful dehydrating agent, it will help push the position of equilibrium toward the products. The test-tube is then placed in a beaker of hot water. The carboxylic acid and the alcohol then react in a condensation reaction or esterification reaction to produce an ester and release the small molecule water. A general equation for this esterification reaction can be written as:

Esterification reactions

The ester ethyl ethanoate can be made by reacting the carboxylic acid ethanoic acid with the alcohol ethanol in the presence of a sulfuric acid catalyst. Ethyl ethanoate can be used as nail varnish remover; it also smells of pear drops! An equation for this reaction is shown below.

ethanoic acid_(aq) + ethanol_(aq) ⇌ ethyl ethanoate_(l) + water_(l)

CH ₃COOH_(aq) + CH₃CH₂OH_(aq) ⇌ CH₃COOCH₂CH_3(l) + H₂O_(l)

The diagram below shows an outline of the reactive functional groups in the alcohol and carboxylic acid that take part in the reaction.

Preparation of ethyl ethanoate

The Small scale lab preparation of the ester ethyl ethanoate is outlined below.

• Step1. Add equal amounts of ethanoic acid and ethanol to the pear shaped flask containing a few anti-bumping granules, next a few mls of concentrated sulfuric acid is slowly added with cooling if necessary down through the Liebig condenser. Care should be taken to ensure the "liquids" in the flask are well mixed and the solution is homogeneous. Concentrated sulfuric acid is much denser than the other two solutions and may simply settle on the bottom of the flask. The flask is then heated gently for around 10 minutes using an electric heater until the mixture starts to boil and drips are seen falling from the Liebig back into the pear shaped flask.
• Step2. The ester ethyl ethanoate has a boiling point of 77^oC. The mixture in the flask is set-up for distillation and any fraction produced at or around 77^oC should be collected in the conical flask.
• Step3. The ester from step2 is collected in a conical flask but it is likely to have some impurities present in it, probably some of the ethanoic acid or even some sulfuric acid. To remove these acidic impurities the mixture is added to separating funnel containing an excess of sodium hydrogencarbonate solution or sodium carbonate solution. These solutions are basic and will react with any acidic impurities. This reaction will also release CO₂ gas. The mixture is the separating funnel is shaken and once no more fizzing is seen it can be assumed that any acidic impurities have been neutralised. The contents of the funnel should be left to settle into two layers. The top organic layer containing the ester and the bottom aqueous layer containing the sodium hydrogencarbonate solution. Once the contents have settled into two layers the bottom layer should be run off and discarded to leave the ester layer in the funnel.

• Ste4. Next the ester layer from the separating funnel is added to a conical flask. The ester is bound to be wet from any residual water in the separating funnel. A drying agent such as anhydrous sodium sulfate or calcium chloride should be added to the ester to remove any traces of water.
• The dry ester should now be distilled using dry apparatus. The collecting flask should be chilled in iced water. Collect any distillate between the temperatures of 74 and 77⁰C. This should be the ester ethyl ethanoate. Note a by product of this reaction is an ether (diethylether CH₃CH₂0CH₂CH₃), which boils between 35-40^oC. This fraction containing the ether should be discarded.

Ester from acid chlorides

Using carboxylic acids and alcohols is not a particularly good method for preparing esters since the reaction is a reversible one and will produce an equilibrium mixture of carboxylic acid, alcohol, ester and water which will result in a reduced yield and separation problems if the ester is to be isolated from the mixture. A much more efficient way to produce esters is by using the reaction of an acid chloride or an acid anhydride with alcohols to form esters.

Using acid chlorides or acid anhydrides is an excellent method of preparing esters and good yields are easily obtainable since this avoids the equilibrium reaction of a carboxylic acid/alcohol esterification reaction. However acid chlorides are expensive as well as being hazardous volatile liquids to use while acid anhydrides can also produce mixtures of products which will need to be separated out, which will result in more expense. The mechanism below shows the reaction of an acid chloride with an alcohol to form an ester (for more detail check out the pages on acid chlorides and acid anhydrides.)

As an alternative the ester ethyl ethanoate can also be made using the acid anhydride and the alcohol ethanol as shown below:

Properties and uses of esters

You may perhaps be familiar with esters as the sweet smelling compounds found in many perfumes, scents and flavourings. However esters have many uses and are found in a wide variety of other substances such as:

• Many drug molecules such as aspirin, anaesthetics such as novocaine and chloroprocaine as well as drug molecules such as cocaine all contain the ester functional group (RCOOR).
• Insecticides such as malathion, pyrethin and Cypermethrin also contain the ester functional group.
• Fats and oils are also esters of the alcohol glycerol.
• Esters being polar molecules are good solvents. Esters such as ethyl ethanoate and butyl ethanoate are commonly used in the pharmaceutical industry as solvents.
• Esters are commonly used as plasticisers. Plasticisers are molecules which slip between the long polymer chains found in many thermoplastics such as PVC. The plasticisers weaken the intermolecular forces between the polymer chains and allow them to slip past each other, this gives some flexibility in the polymer or plastic. However these plasticiser molecules in time evaporate which is why, for example bag handles and shoe soles appear to crack with age. Here the plasticiser evaporates and the flexibility of the polymer reduces.

• Perhaps the best known use of esters is as artificial food flavourings and scents. Some esters have sweet fruit odours and flavours and are used in many foodstuffs.

Esters used in food flavouring include:

Naming esters

If you study the image below you will see that the ester group (-COO-) is made when the hydroxyl group (-OH) is removed from a carboxylic acid and the -H is removed from the an alcohol. The acid and alcohol molecular residues then join together to form the ester. The -OH and -H which are removed join to form a molecule of water.
Esters are named by changing the ending of the name of the alcohol from -ol to -yl, we simply use the alkyl stem name e.g. methanol would be methyl, ethanol would be ethyl, propanol would be propyl. The acid name ending in -ic is changed to -ate. The name of the alcohol is placed first when naming the ester, so butyl ethanoate is a common ester made from the alcohol butanol and the carboxylic acid ethanoic acid, other common esters include:

Physical properties of esters

Esters are volatile liquids since they cannot form hydrogen bonds. Short chain length esters are sparingly soluble in water, since esters contain polar C=O and C-O bonds they are able to form dipole-dipole bonds with neighbouring ester molecules and also solvent molecules such as water. However as the non-polar covalent chain length of the ester increases in size the attractive forces between the C=O and C-O bonds in the ester and any solvent water molecules will diminish overall in size and the solubility of the ester will reduce.

Key Points

• Esters contain the functional group RC00R', where R can be alkyl or aryl.
• Esters can be prepared in a condensation reaction by reacting:
• a carboxylic acid and an alcohol
• an acid chloride and an alcohol
• an acid anhydride and an alcohol
• Esters are named by combining the name of the alcohol and the acid together. The alcohol name is taken from the alkyl stem and the last few letters in the acid name is changed to -oate.
• Common uses of esters include: plasticisers, solvents, food flavourings and odours.

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