Alcohols, carboxylic acids and esters
Another group of molecules can be made by reacting alcohols with carboxylic acids. These reactions are called condensation reactions and the products are esters. Esters often have a fruity smell. They are used as flavouring compounds in the food industry and as part of fragrances for the cosmetic industry.
Condensation reactions
The reactions of alcohols with carboxylic acids to form esters are relatively simple condensation reactions, and we will consider these in this section.
Consider the conversion of ethanol and ethanoic acid to ethyl ethanoate (an ester) shown below.
Click on the image to enlarge
Count up the total number of C, H and O atoms in the reactants and the number of these atoms in the product.
By doing this you should find that the by-product for this reaction is H2O (water) because the product has lost one oxygen atom and one hydrogen atom.
Now write a balanced equation for this reaction. Click below to check your answer.
Click here for the formation of ethyl ethanoate equation
CH3CH2OH + CH3COOH → CH3CH2OOCCH3 + H2O
All condensation reactions will produce a small molecule, such as water, as a by-product.
Condensation reactions are the reactions used in industry to produce polyesters, and by our bodies to produce proteins from amino acids. You will encounter them again when looking at polymers in a later topic.
The diagram in the previous section shows the structure of an ester (ethyl ethanoate). As you have seen, esters can be made by combining an alcohol and a carboxylic acid. The functional group found in all esters is shown here.
On the structure of the ethyl ethanoate below, highlight in green the section of the molecule that came from the alcohol and then use the pink highlighter to show the part of the molecule from the carboxylic acid.
Text alternative to the ester activity
1. The part of the ester formed from the alcohol is called an alkyl group. In this case it is the ethyl group.
2. The part formed from the carboxylic acid is called a carboxylate group. In this case the group is called the ethanoate group.
Click here to open the ester activity in a new window
You should see that the ester molecule can be divided into two parts. This will help you to work out how esters can be made and also how the esters are named. The two parts are described below.
- The part formed from the alcohol is called an alkyl group. In this case it is the ethyl group.
- The part formed from the carboxylic acid is called a carboxylate group. In this case the group is called the ethanoate group.
Hence the name of this ester is ethyl ethanoate.
In reality, the alcohol provides one of the oxygen atoms in the ester functional group and the carboxylic acid provides the other, but to help with naming, it is easier to split the ester molecule into the two halves as shown above.
The ethanoate group is sometimes called acetate, so an alternative name for the above ester would be ethyl acetate. For the rest of this topic we will use the more formal name ethanoate for the -OOCCH3 group.
As you have seen, esters can be formed by combining an alcohol and a carboxylic acid, with water being produced as a by-product of the condensation reaction. (The examples that you see here will involve primary alcohols, but all alcohols can be involved in these reactions). Concentrated sulfuric acid is often used as a catalyst for these reactions.
The interaction below shows the steps involved in the process. Click next to reveal each step.
Text alternative to the formation of propyl methanoate interaction
An animation that demonstrates the formation of an ester from propan-1-ol and methanoic acid. The H is lost from the alcohol and the OH is lost from the acid molecule. These combine to form a water molecule and the two remaining sections from the acid and alcohol molecules move together to form the ester molecule (propyl methanoate).
Click here to open the interaction in a new window
Now answer the following questions relating to this reaction.
1 |
The alcoholWhat is the correct name for the alcohol used in this reaction? |
2 |
The carboxylic acidWhat is the systematic name for the carboxylic acid used in this reaction? |
3 |
The esterWhat is the correct name for the ester formed in this reaction? |
You have seen the names of esters are found from the name of the alkyl group (from the alcohol) and the carboxylate group (from the carboxylic acid). Use this rule to complete the following questions.
4 |
What is the name of the ester shown above? |
5 |
Identifying an esterWhich of these is the correct formula for ethyl pentanoate? |
6 |
Forming an esterWhich of the two compounds would be required to produce the ester CH3COOCH(CH3)CH2CH3? |
