Question

What is a homologous series? 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

Homologous series describes the sequence of compounds which have the same general formula. The compounds of homologous series consist of the same functional group and the same physical and chemical properties. The next compound of the homologous series is obtained by adding $C{{H}_{2}}$ to the molecular formula of the previous member.

Complete answer:
Homologous series describes the sequence of compounds which have the same general formula. The compounds of homologous series consist of the same functional group and same physical and chemical properties. In the aforementioned examples, ${{C}_{2}}{{H}_{6}}O$ and $C{{H}_{4}}O$ have the same general formula that is ${{C}_{n}}{{H}_{2n+1}}OH$.
When $n=1$, the formula of organic compounds will be $C{{H}_{3}}OH$ or $C{{H}_{4}}O$.
When $n=2$, the formula of organic compound will be ${{C}_{2}}{{H}_{5}}OH$or ${{C}_{2}}{{H}_{6}}O$.
Therefore, ${{C}_{2}}{{H}_{6}}O$ and $C{{H}_{4}}O$ belong to the same homologous series. These compounds of homologous series differ by $C{{H}_{2}}$ and contain alcohol as a functional group. ${{C}_{2}}{{H}_{6}}O$ represents ethanol and $C{{H}_{4}}O$ represents methanol.
In contrast, the rest of the molecular formulas i.e. ${{C}_{2}}{{H}_{6}}$ and ${{C}_{2}}{{H}_{6}}{{O}_{2}}$doesn’t form in accordance to above general formula i.e. ${{C}_{n}}{{H}_{2n+1}}OH$. Therefore, these organic compounds do not belong to the same homologous series.
Overall, the organic compounds ${{C}_{2}}{{H}_{6}}O$ and $C{{H}_{4}}O$ belong to the same homologous series.
-Methanol is more toxic than ethanol and causes blindness.

Note:
It is important to note that homologous series describe the sequence of compounds which have the same general formula. In the aforementioned examples, ${{C}_{2}}{{H}_{6}}O$and $C{{H}_{4}}O$ have the same general formula that is ${{C}_{n}}{{H}_{2n+1}}OH$. Therefore, ${{C}_{2}}{{H}_{6}}O$ and $C{{H}_{4}}O$ belong to the same homologous series.