Functional groups, homologous series and naming
A functional group decides how a compound reacts. Members of a homologous series share a group and differ by -CH₂-. Names use carbon count plus a suffix or prefix.
Functional groups
| Group | Formula | Name ending | Example with 3 carbons |
|---|---|---|---|
| Halogen | -Cl, -Br | Prefix chloro- or bromo- | Chloropropane, CH₃CH₂CH₂Cl |
| Alcohol | -OH | -ol | Propanol, CH₃CH₂CH₂OH |
| Aldehyde | -CHO (at the end of the chain) | -al | Propanal, CH₃CH₂CHO |
| Ketone | C=O (inside the chain) | -one | Propanone, CH₃COCH₃ |
| Carboxylic acid | -COOH | -oic acid | Propanoic acid, CH₃CH₂COOH |
| Double bond (alkene) | C=C | -ene | Propene, CH₃CH=CH₂ |
| Triple bond (alkyne) | C≡C | -yne | Propyne, CH₃C≡CH |
How to name a compound
- 1Count the carbon atoms in the chain and write the root: meth (1), eth (2), prop (3), but (4), pent (5), hex (6).
- 2★Find the functional group and choose its prefix or suffix.Halogens go in front (chloro-, bromo-); the other groups go at the end
- 3If the suffix begins with a vowel, drop the final "e" of the alkane name.Propane + one = propanone; propane + ol = propanol
- 4For a double bond, use -ene; for a triple bond, use -yne.Propane becomes propene or propyne
- 5Write the full name and check it against the structure.
★ key step
Worked example: name these compounds
Name: (i) CH₃CH₂CH₂CH₂Cl (ii) CH₃CHO (iii) CH₃CH₂CH₂COOH (iv) HC≡CH
- (i) 4 carbons, so the root is but. The group is -Cl, so the prefix is chloro-. Name: chlorobutane.
- (ii) 2 carbons (count the C in CHO too), so the root is eth. The group is -CHO, so the suffix is -al. Drop the "e" of ethane. Name: ethanal.
- (iii) 4 carbons (count the C in COOH too), so the root is but. The group is -COOH, so the suffix is -oic acid. Name: butanoic acid.
- (iv) 2 carbons, so the root is eth. There is a triple bond, so the ending is -yne. Name: ethyne.
Chlorobutane, ethanal, butanoic acid, ethyne
A homologous series: the alcohols
| Name | Formula | Change from the one before |
|---|---|---|
| Methanol | CH₃OH | First member |
| Ethanol | C₂H₅OH | + CH₂ (14 u) |
| Propanol | C₃H₇OH | + CH₂ (14 u) |
| Butanol | C₄H₉OH | + CH₂ (14 u) |
Features of a homologous series
- The same functional group and the same general formula.
- Each member differs from the next by -CH₂-, that is, by 14 u in molecular mass (12 + 2 × 1).
- Similar chemical properties, because the functional group is the same.
- A gradual change in physical properties: melting point and boiling point rise as the molecular mass rises.
- Example: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
The vinegar in pickles contains ethanoic acid, and acetone in nail polish remover is propanone, a ketone. Knowing the functional group tells you how each one behaves.
1 marks: “What is the difference in molecular mass between two successive members of a homologous series?” 14 u, the mass of one -CH₂- unit.
2 marks: “Write the names of: (i) CH₃CH₂CHO (ii) CH₃COCH₃.” (i) Propanal (an aldehyde with 3 carbons). (ii) Propanone (a ketone with 3 carbons).
3 marks: “Methanol, ethanol and propanol belong to one family of compounds. What is such a family called? Give three of its features, using these compounds.” A homologous series: the same functional group (-OH here), and each member differs from the next by -CH₂- (14 u). Similar chemical properties; gradual change in physical properties. CH₃OH, C₂H₅OH, C₃H₇OH.
Roots: "My Elephant Prefers Bananas": meth, eth, prop, but (1 to 4 carbons). Endings: -ol alcohol, -al aldehyde, -one ketone, -oic acid acid.