Q. 5

CnH2n+2 is the general formula of a homologous series of hydrocarbons.

i) Is this series saturated or unsaturated?

ii) Name the series described above. Give the formula and name of the member with two carbon atoms.

iii) Draw the structural formula of the first member of this series.

iv) Define the homologous series and find the common difference between the successive members of this family.

v) Write the formula of n-butane and n-pentane.

Answer :

i) The series is saturated.


Explanation: CnH2n+2 is the general formula of a homologous series of alkane which is a saturated hydrocarbon.


Note: Saturated hydrocarbons are those organic compounds which contain carbon–carbon single bond. These were earlier named as paraffins.


ii) The given is homologues series of alkane hydrocarbons.


The formula and name of the member with two carbon atoms:


n=2


C2H2×2+2 = C2H6 (ethane)


Thus, formula is C2H6 and name is ethane


iii) The first member of the series is:


n=1


C1H2×1+2 = CH4 (methane)


The structural formula of methane is:



iv) Homologues series: A homologous series is a group or a class of organic compounds having same general molecular formula and similar chemical properties in which the successive members differ by a CH2 group.


v) In butane, there are four carbon atoms and in pentane there are five carbon atoms.



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