9.32C - Using homologous series to predict reactions
Use a known reaction to predict what another member of the same homologous series will do. The functional group determines the reaction pattern; the carbon chain determines the particular product.
Same reactive group, similar reactions
Members of a homologous series contain the same functional group, which is the part responsible for their characteristic reactions. They therefore react in similar ways, although their products need not be identical.
| Series | Reactive feature | Useful pattern |
|---|---|---|
| Alkenes | C=C | Addition across the double bond |
| Alcohols studied here | OH attached to an end carbon | Oxidation to the corresponding acid |
| Carboxylic acids | COOH | Typical acid reactions |
Use the complete structure as well as the group: the alcohol oxidation pattern here applies to methanol and the straight-chain alcohols with OH on carbon 1. Do not assume every alcohol arrangement oxidises to a carboxylic acid.
A shared prefix is not enough. Propene and propan-1-ol both have three carbons, but C=C and OH give them different characteristic reactions.
Applying a reaction pattern
Identify the functional group, apply the known change and keep track of all atoms. For reactions that preserve the chain, the new product keeps the new reactant's carbon count.
Worked oxidation prediction: ethanol forms ethanoic acid. Butan-1-ol has the same type of end alcohol group, so it forms butanoic acid. Its four-carbon chain stays four carbons long; the COOH carbon is part of that chain.
Worked addition prediction: bromine adds to ethene to form 1,2-dibromoethane. With propene, CH2=CH–CH3, put one Br on each carbon of the double bond and change C=C to C–C:
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