Question

The first member of the alkyne homologous series is:

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

Answer 1

The correct option is A

Ethyne

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 option:

Alkynes:

• The alkynes comprise a series of carbon and hydrogen‐based compounds that contain at least one triple bond.
• This group of compounds 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.
• If we put C=1 in the above formula, then the first compound's formula will be ${\mathrm{C}}_1{\mathrm{H}}_{2-2}={\mathrm{CH}}_{0=}\mathrm{C}$, that will be carbon only.
• So, we put C=2 in the above formula to get ${\mathrm{C}}_2{\mathrm{H}}_2$, which is called ethyne, the first member of the alkyne series.

The explanation for the incorrect options:

• 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.
• The chemical formula of methane is ${\mathrm{CH}}_4$, which belongs to the alkane family and is the first member of the family.

Therefore, the correct option is (A),i.e., the first member of the alkyne homologous series is ethyne(${\mathrm{C}}_2{\mathrm{H}}_2$).