13.2.1 General Characteristics of a Homologous Series

Each homologous series has some general characteristics:

(i) Same functional group: The family members of a homologous series have the same functional group.

(ii) Same general formula: The family members have the same general formula.

(iii) Differ by –CH₂– unit: The adjacent family members differ by a –CH₂– unit.

(iv) Trend in physical properties: The family members display a trend in their physical properties.

(v) Similar chemical properties: The family members possess similar chemical properties.

Simple explanation: Members of a homologous series share the same functional group and general formula, differ by CH₂, and show a trend in physical properties but similar chemical properties.

General Formulae of Some Homologous Series

Table 13.1 from the book:

Simple explanation: Each homologous series has its own general formula.

Using the General Formula

From the general formula, you can easily determine the molecular formula of any member of the series by putting the number of carbon atoms in the general formula.

Examples:

Alkanes (CₙH₂ₙ₊₂):

Alkenes (CₙH₂ₙ):

Alcohols (CₙH₂ₙ₊₁OH):

Simple explanation: Plug in the number of carbon atoms (n) to get the molecular formula.

Concept Assessment Exercise 13.1 (from the Book)

Write the molecular formulae of the following compounds using general formulae:

1. Alkane containing (i) 4 carbon atoms (ii) 6 carbon atoms

• (i) n = 4 → C₄H₂₍₄₎₊₂ = **C₄H₁₀**
• (ii) n = 6 → C₆H₂₍₆₎₊₂ = **C₆H₁₄**

2. Alkene containing (i) 3 carbon atoms (ii) 4 carbon atoms

• (i) n = 3 → C₃H₂₍₃₎ = **C₃H₆**
• (ii) n = 4 → C₄H₂₍₄₎ = **C₄H₈**

3. Alkyne containing (i) 3 carbon atoms (ii) 4 carbon atoms

• (i) n = 3 → C₃H₂₍₃₎₋₂ = **C₃H₄**
• (ii) n = 4 → C₄H₂₍₄₎₋₂ = **C₄H₆**

4. Alcohol containing (i) 2 carbon atoms (ii) 3 carbon atoms

• (i) n = 2 → C₂H₂₍₂₎₊₁OH = **C₂H₅OH**
• (ii) n = 3 → C₃H₂₍₃₎₊₁OH = **C₃H₇OH**

Intermediate Level

Why Homologous Series Are Useful

• Millions of compounds can be grouped into a few families.
• Once you know the properties of one member, you can predict the properties of others.
• The general formula allows you to write the molecular formula of any member.
• The trend in physical properties allows you to predict melting/boiling points.

Simple explanation: Homologous series help us organize and understand millions of organic compounds.

General Formulae in Detail

Simple explanation: Each series has its own general formula and functional group.

Physical Properties Trend

• Molecular mass increases.
• Melting point increases.
• Boiling point increases.
• Density increases.
• Solubility in water decreases (for most series).

Simple explanation: Bigger molecules → stronger intermolecular forces → higher melting/boiling points.

Chemical Properties Similarity

All members of a homologous series have the same functional group, so they undergo similar chemical reactions.

Example: All alcohols react with sodium to produce hydrogen gas.

Example: All alkanes undergo combustion.

Simple explanation: Same functional group → similar chemical behaviour.

Examples of Homologous Series

Important Points

• - Homologous series: a series of related compounds in which adjacent members differ by a –CH₂– unit.
• - General characteristics:
• 1. Same functional group.
• 2. Same general formula.
• 3. Differ by –CH₂– unit.
• 4. Trend in physical properties.
• 5. Similar chemical properties.
• - General formulae:
• - Alkanes: CₙH₂ₙ₊₂
• - Alkenes: CₙH₂ₙ
• - Alkynes: CₙH₂ₙ₋₂
• - Alcohols: CₙH₂ₙ₊₁OH
• - Carboxylic acids: CₙH₂ₙ₊₁COOH
• - Examples:
• - Alkanes: CH₄, C₂H₆, C₃H₈, C₄H₁₀
• - Alcohols: CH₃OH, C₂H₅OH, C₃H₇OH

Remember

• - Homologous series = family of related compounds.
• - Differ by –CH₂– unit.
• - Same functional group.
• - Same general formula.
• - Trend in physical properties.
• - Similar chemical properties.
• - General formulae:
• - Alkanes: CₙH₂ₙ₊₂
• - Alkenes: CₙH₂ₙ
• - Alkynes: CₙH₂ₙ₋₂
• - Alcohols: CₙH₂ₙ₊₁OH

Common Mistakes

• 1. Thinking all organic compounds belong to one homologous series. There are many homologous series — one for each functional group.
• 2. Forgetting the –CH₂– difference. Adjacent members differ by –CH₂–, not just any group.
• 3. Confusing the general formulae. Each series has its own general formula.
• 4. Thinking physical properties are the same within a series. Physical properties show a trend (gradual change).
• 5. Thinking chemical properties are different within a series. Chemical properties are similar because the functional group is the same.
• 6. Misusing the general formula. Make sure to substitute the correct n value and calculate correctly.

Quick Revision

• Homologous series: a series of related compounds in which any two adjacent molecules differ by a –CH₂– unit.
• Examples: Alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids.
• General characteristics:
• General formulae:
• Alkanes: CₙH₂ₙ₊₂
• Alkenes: CₙH₂ₙ
• Alkynes: CₙH₂ₙ₋₂
• Alcohols: CₙH₂ₙ₊₁OH
• Carboxylic acids: CₙH₂ₙ₊₁COOH
• Examples:
• Alkanes: CH₄ (n=1), C₂H₆ (n=2), C₃H₈ (n=3)
• Alcohols: CH₃OH (n=1), C₂H₅OH (n=2)

60-Second Revision

• Homologous series = family of related compounds.
• Differ by –CH₂– unit.
• Same functional group; same general formula.
• Trend in physical properties; similar chemical properties.
• General formulae: alkanes CₙH₂ₙ₊₂; alkenes CₙH₂ₙ; alcohols CₙH₂ₙ₊₁OH.
• Warning: each homologous series has its own general formula — don't mix them up.