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Question

The first member in the homologous series of alkane is:


A

Ethene

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B

Ethyne

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C

Methane

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D

Propyne

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Solution

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 CnH2n+2, alkenes with the general formula CnH2n and alkynes with a general formula CnH2n-2form 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 CnH2n+2,where n equals any integer greater than one.
  • The chemical formula of methane is CH4, 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 C2H2, which belongs to the alkene family has a general formula CnH2n-2.
  • The chemical formula of ethene is C2H4, which belongs to the alkene family has a general formula CnH2n.
  • The chemical formula of propyne is C3H4, 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(CH4).


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