Question

The first member in the homologous series of alkane is:

• A. Ethene
• B. Ethyne
• C. Methane
• D. Propyne

Answer 1

The correct option is C

Methane

Homologous Series:

• A homologous series is a series of compounds, which has the same functional group.
• This also contains a similar general formula and chemical properties.
• Since there is a change in the physical properties, we can say that there would be an increase in molecular size and mass.
• Alkanes with the general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$, alkenes with the general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}$ and alkynes with a general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}-2}$form the most basic homologous series in organic chemistry.

The explanation for the correct answer:

• A homologous series is a series of compounds, which has the same functional group.
• This also contains a similar general formula and chemical properties.
• This group of compounds, called alkanes is a homologous series with the general molecular formula of ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$_,where n equals any integer greater than one.
• The chemical formula of methane is ${\mathrm{CH}}_4$, if we put C=1 in the above formula, which belongs to the alkane family and is the first member of the family.

The explanation for the incorrect options:

• The chemical formula of ethyne is ${\mathrm{C}}_2{\mathrm{H}}_2$, which belongs to the alkene family has a general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}-2}$.
• The chemical formula of ethene is ${\mathrm{C}}_2{\mathrm{H}}_4$, which belongs to the alkene family has a general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}$.
• The chemical formula of propyne is ${\mathrm{C}}_3{\mathrm{H}}_4$, which belongs to the alkyne family and is the second member of the family.

Therefore, the correct option is ‘C’,i.e., the first member in the homologous series of an alkane is methane(${\mathrm{CH}}_4$).