An organic name has three parts: the prefix that counts the carbons in the main chain, a position number, and an ending that shows the family. Build the name in that order.
This lesson is part of SPM Chemistry carbon compounds. It follows identifying homologous series and functional groups.
How do the prefixes and endings work?
The carbon count gives the stem: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). The family gives the ending.
| Family | Ending | Example |
|---|---|---|
| Alkane | -ane | butane |
| Alkene | -ene | but-1-ene |
| Alcohol | -ol | propan-1-ol |
| Carboxylic acid | -oic acid | ethanoic acid |
Worked example: numbering from the correct end
Name CH₃–CH=CH–CH₂–CH₃. The chain has five carbons, so the stem is pent and the family is alkene.
Number from the end that gives the double bond the lower number. From the left the double bond starts at carbon 2. From the right it starts at carbon 3, so use the left.
The name is pent-2-ene.
Worked example: a branched alkane
Draw 2-methylbutane. The main chain has four carbons (butane), and a methyl group, CH₃, hangs from carbon 2.
The full structural formula has carbons 1 to 4 in a line, and a CH₃ joined to carbon 2. Carbon 2 has bonds to carbon 1, carbon 3, the methyl carbon and one hydrogen, which makes four.
The mistake that loses marks
A student numbers 2-methylbutane from the wrong end and writes 3-methylbutane. The methyl group is on the second carbon from one end and the third from the other.
The rule is to choose the direction that gives the branch the lowest number. Always check both ends before you write the name.
For a drawing, count the bonds on each carbon. A carbon with only three bonds is missing a hydrogen, and one with five is wrong.
Check yourself
Name CH₃CH₂CH(OH)CH₃, and draw the full structural formula of ethanoic acid.
Answer
The main chain has four carbons with –OH on carbon 2 counting from the right end (the lower number), so the name is butan-2-ol.
Ethanoic acid: a carbon with three hydrogens, joined to a second carbon that has a double bond to O and a single bond to –OH. Each carbon has four bonds. Written as H₃C–C(=O)–OH.
What to study next
The next lesson asks what happens when one formula gives two names: distinguishing isomers. Use the mistake log and paper-error review to record which naming step you slip on.
For a teacher to check your drawings with you, see online one-to-one Chemistry tuition.