Question

Q21)

Which three compounds are members of the same homologous series?

• A)

  C₂H₄, C₃H₈, C₄H₁₀

• B)

  CH₃Br, C₂H₅Br, C₃H₇Br

• C)

  CH₄, C₂H₅OH, C₃H₇Cl

• D)

  CH₃OH, C₂H₅Cl, C₃H₇COOH

Answer:BSolution:

A series of organic compounds that contains the same functional group and in which the number of carbon atoms in each consecutive member differs by one is called a homologous series for that functional group. The series CH₃Br, C₂H₅Br, and C₃H₇Br represents the first three members of the homologous series of bromides.

The correct answer is B.

why the answer is B and not A

Answer 1

There are 4 homologous series,

1. alkane whose general formula is C_nH_{2n +2}

Hence members of this series are: CH₄, C₂H₆, C₃H₈, C₄H₁₀ etc.

2. alkene with general formula is C_nH_2n

Hence the general members are: C₂H₄, C₃H₆, C₄H₈, C₅H₁₀ etc.

3. alkynes with general formula is C_nH_2n-2.

Hence member of this group are as follow: C₂H₂, C₃H₄, C₄H₆, etc.

4. Alkyl chain with general formula C_nH_2n+1

Hence the members of this groups are: CH₃, C₂H₅, C₃H₇ etc.

So out of all four only B exist as homologous series and lies in the alkyl group homologous series. whereas in option A, only C₃H₈, C₄H₁₀ belongs to alkane group series but C₂H₄ violates this series.