Ester
A concise revision guide to ester functional groups, ester naming, the alkyl part from the alcohol, the carboxylate part from the carboxylic acid, and ester priority in IUPAC names.
What Is an Ester?
An ester is an organic compound containing the ester functional group, written as RCOOR’. This contains a carbonyl group, C=O, directly bonded to an oxygen atom that is also bonded to another carbon group.
The ester group is often shown as -COO-. The carbonyl carbon belongs to the carboxylate part of the name, while the group attached to the single-bonded oxygen gives the alkyl part of the name.
Key idea: An ester has the pattern carbonyl carbon, oxygen, carbon. This is different from a carboxylic acid, which has the pattern -COOH.
The ester functional group contains a C=O bond next to an oxygen atom bonded to another carbon group.
Naming Esters: Two Parts
An ester name is made of two parts. The first part is the alkyl group from the alcohol. The second part is the carboxylate part from the carboxylic acid.
The carboxylate part is named second and ends in -oate. For example, an ester made from methanol and ethanoic acid is named methyl ethanoate.
| Part of ester name | Where it comes from | Usual naming pattern | Example in methyl ethanoate |
|---|---|---|---|
| Alkyl part | Alcohol | Named first, often ending in -yl | methyl |
| Carboxylate part | Carboxylic acid | Named second, ending in -oate | ethanoate |
In methyl ethanoate, methyl is the alkyl part from the alcohol and ethanoate is the carboxylate part from the acid.
Numbering the Acid Part
In the carboxylate part of an ester name, the carbon chain is numbered from the carbonyl carbon. This carbon is carbon 1 of the acid-derived part.
No position number is usually needed for the ester group because the ester functional group is at the end of the carboxylate chain. The name focuses on the alkyl group followed by the acid-derived -oate ending.
Exam focus: Work backwards from the name. The first word is attached to the oxygen atom. The second word tells you the carbon chain containing the carbonyl carbon.
Methyl propanoate has a methyl group attached to the ester oxygen and a three-carbon propanoate chain containing the carbonyl carbon.
Ester Priority in IUPAC Names
Esters have lower priority than carboxylic acids. If a -COOH group is present in the same molecule, the compound is named as a carboxylic acid rather than using the ester suffix as the main functional group.
In this situation, the ester group is named as an alkoxycarbonyl substituent. This reflects the lower naming priority of esters compared with carboxylic acids.
No higher-priority acid group
Use the ester naming pattern: alkyl name first, then carboxylate name ending in -oate.
Carboxylic acid group present
The -COOH group takes priority, so the ester is treated as a substituent rather than the main suffix.
Common Ester Examples
The names methyl ethanoate and methyl propanoate show how ester names combine an alkyl group with a carboxylate part.
Changing the carboxylate chain changes the second part of the name. Ethanoate contains two carbon atoms in the acid-derived chain, while propanoate contains three carbon atoms in the acid-derived chain.
Methyl ethanoate contains a methyl group and an ethanoate chain.
Methyl propanoate contains the same alkyl group but a longer carboxylate chain.
Check Your Understanding
Use these short activities to check ester functional groups, naming patterns, acid-derived chains and priority before moving on.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.