What are esters?
Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).
A common ester - ethyl ethanoate
The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:
Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these.
another example
For example, acetic acid esterification in excess Ethanol in the presence of concentrated H2SO4 as a catalyst results in an ester (ethyl actate) as well as carboxylic acids, with the removal of water molecule. Due to this, esterification reaction is also called 'Dehydration reaction'. The chemical composition of the acid, alcohol and catalyst used also affect the rate of reaction. Strong acid makes the rate of formation of acids very fast.
Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).
A common ester - ethyl ethanoate
The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:
Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these.
The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:
So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:
another example
For example, acetic acid esterification in excess Ethanol in the presence of concentrated H2SO4 as a catalyst results in an ester (ethyl actate) as well as carboxylic acids, with the removal of water molecule. Due to this, esterification reaction is also called 'Dehydration reaction'. The chemical composition of the acid, alcohol and catalyst used also affect the rate of reaction. Strong acid makes the rate of formation of acids very fast.
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