To identify a functional group, scan the structure for the part that is not just carbon and hydrogen joined by single bonds. A C=C double bond means an alkene, an -OH group means an alcohol, and a -COOH group means a carboxylic acid. If none of these appears, the molecule is an alkane.
This skill opens organic families and naming and is used again in every later organic question, including organic reactions.
What are you actually looking for?
Every organic molecule has a carbon chain, but the chain is rarely what decides the chemistry. The functional group does. Two molecules with different chains but the same group react in a similar way.
| Family | Functional group | Name ending | Example |
|---|---|---|---|
| Alkane | none (only C-C and C-H single bonds) | -ane | ethane, CH₃CH₃ |
| Alkene | C=C double bond | -ene | ethene, CH₂=CH₂ |
| Alcohol | -OH | -ol | ethanol, CH₃CH₂OH |
| Carboxylic acid | -COOH | -oic acid | ethanoic acid, CH₃COOH |
Esters, which contain -COO-, are met later in interpreting an ester-forming relationship. Check the current 0620 syllabus on the Cambridge page for the families your exam year expects.
How do you find the group, step by step?
- Look for a C=C double bond. If there is one, the molecule is an alkene (when nothing else is present).
- Look for oxygen. If there is no oxygen and no C=C, it is an alkane.
- If there is oxygen, check what it is attached to. A single -OH on a carbon with no C=O is an alcohol.
- If the carbon holding the -OH also has a C=O, read it as -COOH. That is a carboxylic acid.
- Count the carbons to choose the stem: meth (1), eth (2), prop (3), but (4), then add the ending from the table.
Worked example
Classify these three structures and name them:
P: CH₃CH₂CH₂OH Q: CH₃CH=CH₂ R: CH₃CH₂COOH
P. There is no C=C. There is an oxygen, present as a single -OH at the end of the chain, and no C=O. So P is an alcohol. It has three carbons, so the stem is prop and the name is propanol (propan-1-ol if the position is written).
Q. The double bond between the last two carbons is a C=C, so Q is an alkene. Three carbons gives propene.
R. The end of the chain is -COOH, which is the carboxyl group. So R is a carboxylic acid. The carbon of the -COOH counts as part of the chain: CH₃, CH₂ and the carboxyl carbon make three carbons, so the name is propanoic acid.
Notice that P, Q and R all have three carbons. The chain length is the same, but the family, and so the chemistry, is different.
The mistake to watch for
A very common slip is to see “OH” at the end of CH₃COOH and call it an alcohol.
Mistaken answer: CH₃COOH is an alcohol because it contains -OH.
The student stopped reading at the OH and ignored the C=O on the same carbon.
The correction is to ask “what is the OH attached to?”. In an alcohol it sits on a carbon that has only single bonds. In CH₃COOH the carbon also carries a C=O, so the group is -COOH and the molecule is ethanoic acid.
Check yourself
1. Name the family of CH₂=CHCH₂CH₃ and say how many carbons it has.
Show answer
It contains a C=C double bond, so it is an alkene. There are four carbons, so the stem is but and the name is butene.
2. Name the functional group and the family of CH₃CH₂CH₂CH₂COOH.
Show answer
The group is -COOH, so it is a carboxylic acid. Counting the carbon of the -COOH, there are five carbons, so it is pentanoic acid.
3. Both CH₃CH₂OH and CH₃COOH contain -OH. Explain why only one of them is a carboxylic acid.
Show answer
In CH₃COOH the -OH is on a carbon that also has a C=O double bond, which makes the -COOH group, so it is a carboxylic acid. In CH₃CH₂OH the -OH is on a carbon with only single bonds, so it is an alcohol.
Where this leads next
With the family identified, the next step is to see how members of one family relate, in distinguishing a homologous family from an isomer. The equation balance reasoning trainer and the mole and equation-ratio tutor become useful once you start writing equations for these molecules.
Some students can name a family from a list but freeze when the structure is drawn in an unfamiliar way. A teacher in online one-to-one Chemistry tuition can give you fresh structures until the scanning habit is automatic.