Question

Which two of the following organic compounds belong to the same homologous series?
${C_2}{H_6},{C_2}{H_6}O,{C_2}{H_6}{O_2},C{H_4}O$

Answer 1

Hint : In the above question, we are asked about the homologous series. A homologous series is a series of organic compounds. The hydrocarbons, which are compounds that are made from pure carbon and hydrogen and may include some other elements, can form a homologous series.

Complete Step By Step Answer:
A homologous series is the order of organic compounds which belong to the same functional group but gradually increase by a methylene group which is $C{H_2}$ . The successor and predecessor of a compound can be found by adding or subtracting a methylene group respectively.
Compounds belonging to the same homologous only by the number of methylene groups and their functional groups are the same. This classification helps in studying the compounds in a systematic manner as experiments have shown that compounds with the same functional group have similar chemical properties and undergo the same reactions.
Now we are given with the compounds ${C_2}{H_6},{C_2}{H_6}O,{C_2}{H_6}{O_2},C{H_4}O$ . The functional groups of the compounds can be identified by looking at the number of atoms of each element. ${C_2}{H_6}$ is of the form ${C_n}{H_{2n + 2}}$ and so it is an alkane. ${C_2}{H_6}O$ can be re-written as ${C_2}{H_5}OH$ and has the functional group $- OH$ . So, it is alcohol. Also, ${C_2}{H_6}{O_2}$ is of the form ${C_n}{H_{2n + 2}}{O_2}$ and so it belongs to the glycol family. $C{H_4}O$ can be re-written as $C{H_3}OH$ . We can easily see from their functional group that they are alkane, alcohol, glycol and alcohol. Hence the organic compounds belonging to the same homologous series are ${C_2}{H_6}O$ and $C{H_4}O$ .

Note :
An easier method is to find the difference in the number of atoms of different elements in these compounds and those that differ by $- {C_n}{H_{2n}}$ belong to the same homologous series. For example, ${C_2}{H_6}{O_2}$ and $C{H_4}O$ differ by $C{H_2}{\text{ and }}O$ and so they do not belong to the same homologous series.