Question

The third member of the methyl ketone homologous series is:
A. acetone
B. 2-butanone
C. 2-pentanone
D. 3-pentanone

Answer 1

Hint: For this problem, we have to write the homologous series of the methyl ketone one by one. Firstly, we will write the first member of ketone methyl and then we will add the methyl group to get the third member of a methyl ketone.

Complete Step-by-step answer:
- In the given question, we have to choose the third member of methyl ketone of the homologous series.
- As we know that homologous series is the series of the same compound which has the same functional group and the members can be found in the branched or unbranched form.
- To form the homologous series of the methyl ketone whose molecular formula is $\text{C}{{\text{H}}_{3}}\text{ - CO - C}{{\text{H}}_{3}}$ which is also considered as the first member of the methyl ketone.
- Now, to form the second member of the methyl ketone we have to add the methyl group to the compound.
- So, the compound formed will be known as butan - 2 - one and the molecular formula will be:
 $\text{C}{{\text{H}}_{3}}\text{ - CO - C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{3}}$
- Similarly, to form the third member of a homologous series of methyl ketone we will add a methyl group to the second member that is butan - 2 - one.
- So, the product formed will be:
$\text{C}{{\text{H}}_{3}}\text{ - CO - C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{3}}$
- The IUPAC name of the compound formed is pentane - 2 - one or 2 - pentanone.

Therefore, option C is the correct answer.

Note: Similarly, to form the fourth, fifth and more homologous series we will continue to add the methyl group in each successive member. The members of the homologous series show similarity in the chemical properties.