Homologous Series: Definition and Key Families of Organic Compounds
What Is a Homologous Series?
Definition
Homologous series
A homologous series is a group of chemical compounds with the same functional group and similar chemical properties, where each successive member differs by a methylene ($CH_2$) group.
In other words, a homologous series is a group of organic compounds that:
- Share the same general formula.
- Differ by a single CH₂(methylene) unit between consecutive members.
- Exhibit similar chemical properties due to the presence of the same functional group.
- Show gradual changes in physical properties, such as boiling and melting points, as the chain length increases.
Key Features of a Homologous Series
- General Formula: Each series can be represented by a formula that applies to all its members.
- Incremental Change: Moving from one member to the next adds exactly one CH₂ group.
- Functional Group: All members of a homologous series share the same functional group, such as –OH for alcohols.
- The alkanes follow the formula $C_nH_{2n+2}$.
- Methane ($CH_4$) becomes ethane ($C_2H_6$) by adding CH₂.
Think of a homologous series as a "family" of compounds where each member is like a sibling, sharing a common "parent" functional group but differing slightly in chain length.
Why Are Homologous Series Important?
Homologous series simplify the study of organic chemistry by allowing you to:
- Predict properties and reactions of compounds based on their functional group.
- Identify trends in physical properties like boiling points, which increase with chain length due to stronger intermolecular forces.
- Deduce the structure of unknown compounds using their general formula.
If you know that alkanes burn in oxygen to produce carbon dioxide and water, you can predict that any member of the alkane series will undergo the same combustion reaction.
Key Homologous Series and Their General Formulas
Alkanes ($C_nH_{2n+2}$)
- Structure: Saturated hydrocarbons (only single bonds between carbon atoms).
- Example: Methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$).
- Chemical Properties: Relatively unreactive but can undergo combustion and substitution reactions.
Alkenes ($C_nH_{2n}$)
- Structure: Unsaturated hydrocarbons with at least one carbon-carbon double bond.
- Example: Ethene ($C_2H_4$), propene ($C_3H_6$).
- Chemical Properties: Undergo addition reactions due to the reactivity of the double bond.
Alkynes ($C_nH_{2n-2}$)
- Structure: Unsaturated hydrocarbons with at least one carbon-carbon triple bond.
- Example: Ethyne ($C_2H_2$), propyne ($C_3H_4$).
- Chemical Properties: Similar to alkenes but more reactive due to the triple bond.
Alcohols ($R–OH$)
- Structure: Contain a hydroxyl (–OH) functional group.
- Example: Methanol ($CH_3OH$), ethanol ($C_2H_5OH$).
