A family with one difference
Add one carbon to a chain. Two more hydrogens follow.
- One carbon, four hydrogens: CH₄, methane.
- Two carbons, six hydrogens: C₂H₆, ethane.
- Three carbons, eight hydrogens: C₃H₈, propane.
- Four carbons, ten hydrogens: C₄H₁₀, butane.
Pick a family. Slide the carbon count. Watch the mass step.
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The rule
A homologous series is a row of compounds with the same group. Each is one CH₂ above the last. They share chemical properties. Physical properties change step by step.
Take carbon's mass as 12 and hydrogen's as 1. So each CH₂ step adds 12 + 1 + 1 = 14 to the mass.
| Carbons | Formula | Mass |
|---|---|---|
| 1 | CH₄ | 16 |
| 2 | C₂H₆ | 30 |
| 3 | C₃H₈ | 44 |
| 4 | C₄H₁₀ | 58 |
Chains with only single bonds have no group. They form a series too. Let n be the number of carbons. A saturated chain with no ring is CₙH₂ₙ₊₂. This family is the alkanes. A chain with one double bond is CₙH₂ₙ. These are the alkenes. One triple bond gives CₙH₂ₙ₋₂, the alkynes. For n = 2, they are ethane C₂H₆, ethene C₂H₄ and ethyne C₂H₂.
Replace one hydrogen of each alkane with –OH. You get the alcohols. Methanol is CH₃OH. Ethanol is C₂H₅OH. They differ by CH₂ too.
Melting and boiling points rise as the chain grows.
Watch out!
One CH₂ apart is not enough. Methanol, CH₃OH, and dimethyl ether, CH₃OCH₃, differ by CH₂. Their groups differ, so they are not one series.