Chapter 4 · Question 8
What is a homologous series? Explain using the example of alcohols from methanol to butanol. How do melting points, boiling points, and chemical properties change as we move up a homologous series?
Q8
What is a homologous series? Explain using the example of alcohols from methanol to butanol. How do melting points, boiling points, and chemical properties change as we move up a homologous series?
Answer Revealed
Direct Answer:
A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, and successive members differ by a unit. The alcohol series: methanol (), ethanol (), propanol (), butanol (). Each successive member differs by one group (molecular mass difference of ). Physical properties: melting and boiling points increase with increasing molecular mass due to stronger intermolecular forces. Solubility in a given solvent shows a similar gradation. Chemical properties remain very similar because they are determined by the same functional group ( in this case).
Simple Explanation
A homologous series is a family of organic compounds that share the same functional group but get one bigger each step — like a staircase. The alcohol family starts with methanol (), then ethanol (), propanol (), butanol (), and so on. As you go up, the molecules get heavier, so they boil at higher temperatures and melt less easily. But chemically, they all behave the same way because the alcohol group () is the part doing the reacting.
Exam-Ready Structure
The concept of a homologous series arises from carbon's catenation property combined with a fixed functional group: 1. Definition: A homologous series is a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain. Successive members of a homologous series differ from each other by a (methylene) unit. 2. Example — Alcohol homologous series: (a) Methanol: (molecular mass = ). (b) Ethanol: (molecular mass = ). (c) Propanol: (molecular mass = ). (d) Butanol: (molecular mass = ). Each successive pair differs by (atomic mass of one C + two H = ). 3. General formula: For alkanes — . For alkenes — (where ). For alkynes — . 4. Trend in physical properties: (a) Melting points and boiling points increase with increasing molecular mass because the molecules are larger and the van der Waals (intermolecular) forces become stronger. (b) Solubility in a particular solvent also shows a similar gradation. 5. Chemical properties: The chemical properties of all members of a homologous series remain very similar because they are determined solely by the functional group present in the molecule, which is the same across the series. The carbon chain length primarily affects physical properties, not chemical reactivity.
Key Points
- Homologous series: same functional group, successive members differ by (mass difference )
- Alcohol series: → → →
- Alkanes: ; Alkenes: ; Alkynes:
- Physical properties: melting and boiling points increase with molecular mass
- Chemical properties remain similar — determined by the functional group, not chain length
Related Questions
Q7
What is a functional group? List the main functional groups in organic chemistry, giving the formula and the class of compounds for each. Explain why compounds with the same functional group show similar chemical properties.
Q9