Question

What is the molecular formula of:
 $ (i){\text{ }} $ Fourth higher homologue of ethyl alcohol
 $ (ii){\text{ }} $ Second lower homologue of butyl alcohol

Answer 1

Hint: Here we have to find the random member of the homologous series of the alcohols. Homologous series is a continuous series of hydrocarbons. We will write the general molecular formula for alcohol and then find the respective members of the alcohols. Each consecutive member of a homologous series is differ by $ - C{H_2} $ group.

Complete answer:
Homologous series is a series of compounds having similar physical and chemical properties, but they only differ in a $ - C{H_2} $ group. They contain the same functional group. Each member of this series is called homologous and the series is also known as homology. Thus we can say that every consecutive member of the homologous series differs by molecular weight of $ 16{\text{ }}g $ which is the weight of a $ - C{H_2} $ group . In the case of alcohols, every member of the homologous series contains hydroxyl groups $ \left( { - OH} \right) $ . Each consecutive member of the series will be differ by $ - C{H_2} $ group. Also we know the general formula for alcohol which is $ {C_n}{H_{2n + 1}} - OH $ . Here n is a positive integer value and it cannot be zero. Therefore the first member of the series will be $ C{H_3} - OH $ , methyl-alcohol.
 $ (i){\text{ }} $ When we are asking for a fourth higher homologue of ethyl alcohol it means we have to add four $ - C{H_2} $ to the ethyl alcohol. We know the formula for ethyl alcohol is $ C{H_3} - C{H_2} - OH $ . Now we will add four $ - C{H_2} $ groups to get the fourth higher member of the series. Thus we get,
 $ C{H_3} - C{H_2} - {\left( {C{H_2}} \right)_4} - OH $ ,which is hexanol.
 $ (ii){\text{ }} $ When we are asking for the second lower member of butyl alcohol, it means we have to subtract two $ - C{H_2} $ from the butyl alcohol. We know that butyl alcohol can be written as $ C{H_3} - C{H_2} - C{H_2} - C{H_2} - OH $ . Now we will subtract two $ - C{H_2} $ groups to get a second lower homologue of butyl alcohol. Thus we get,
 $ C{H_3} - C{H_2} - OH $

Note:
Every member of the homologous series differs by $ - C{H_2} $ group only. For finding a higher member it means we have to increase the $ - C{H_2} $ group and for finding a lower homologue we have to subtract $ - C{H_2} $ . Also the molecular weight of each consecutive member differs by the weight of $ - C{H_2} $ group only.