You can recognise a carbon compound in two steps. Check whether the formula contains an -OH or -COOH group, and if not, compare the hydrogen count with 2n + 2 and 2n.
This lesson is part of carbon compounds. The next lesson, comparing saturated and unsaturated compounds, uses the same test.
How do I tell the groups apart?
Start with the functional group, because it overrides the hydrogen count. An -OH group means alcohol and a -COOH group means carboxylic acid.
For compounds with only carbon and hydrogen, count the carbon atoms as n and compare the hydrogen atoms with two patterns.
| Group | General formula | Example |
|---|---|---|
| Alkane | CnH2n+2 | C3H8 |
| Alkene | CnH2n | C3H6 |
| Alcohol | contains -OH | C2H5OH |
| Carboxylic acid | contains -COOH | CH3COOH |
Worked example: classify four formulae
Classify C4H10, C5H10, C3H7OH and C6H14.
- C4H10. No -OH. n = 4, so 2n + 2 = 10. The hydrogen count matches, so it is an alkane.
- C5H10. n = 5, so 2n = 10. It matches the alkene pattern, so it is an alkene.
- C3H7OH. It contains -OH, so it is an alcohol.
- C6H14. n = 6, so 2n + 2 = 14. It is an alkane.
Check the last: if it were an alkene, the formula would be C6H12, which has 12 hydrogen atoms, not 14.
The mistake that costs marks
Students classify by name sound rather than by formula. Propene and propane look alike, and “pro” in both leads students to call C3H6 an alkane.
| Compound | Formula | Wrong | Right |
|---|---|---|---|
| Propane | C3H8 | alkene | alkane, 8 = 2(3) + 2 |
| Propene | C3H6 | alkane | alkene, 6 = 2(3) |
A second slip is forgetting that the functional group wins. CH3OH has one carbon and four hydrogen atoms, which looks like methane. The -OH shows it is methanol, an alcohol.
Check yourself
Classify C7H16, C8H16 and C2H5COOH, and give the reason for each.
Answer
C7H16: n = 7, 2n + 2 = 16, so it is an alkane.
C8H16: n = 8, 2n = 16, so it is an alkene.
C2H5COOH contains -COOH, so it is a carboxylic acid.
What to study next
Go on to comparing saturated and unsaturated compounds to see how the double bond changes behaviour. Then test yourself with the carbon compounds practice set. The mistake log and paper error review helps you track formula slips.
For a teacher to drill the pattern with you, see online one-to-one Science tuition.