Question

Write the name and formula of the second member of homologous series having general formula $Cn{H_2}n.$

Answer 1

In the field of organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the member of the series can be branched or unbranched.

Complete step by step answer:
The second member of a homologous series having general formula $Cn{\text{ }}{H_2}n$ would be propene and not ethane.
We know that the single carbon (meth.) has only one carbon so it can’t form a double bond. This means that there will be one carbon and two hydrogens surrounding it but carbon needs to form four bonds. Hence its valency is not fulfilled and the compound will not be formed.
So, the first member of the homologous series automatically will be ethene. As in this case there will be two carbon atoms and four hydrogen atoms surrounding them. Each carbon atom will form two single bonds with hydrogen and one double bond with another carbon, thus fulfilling the carbon valency of four.
Therefore, the second member of the homologous series would hence be propene with three carbon atoms and six hydrogen atoms.
So propene is the correct answer to this question.It’s formula is ${C_3}{H_6}$ (propene).

Additional Information.
Compounds with him a homologous series typically have a fixed set of functional groups that gives them similar chemical and physical properties.
However, if the member cannot be arranged in a linear order by a single parameter, the collection may be better called a “chemical family” or “class of homologous compounds” than a “series” . Each successive molecule in the alkene homologous series is formed by adding a carbon atom and two hydrogen atoms to the previous molecule. The incremental change in the relative molecular mass will therefore be fourteen.

Note:
The concept of homologous series was proposed in $1843$ by the French chemist Charles Gerhardt. A homologation of one member of a homologous series to the next member.