Question

Which of the following pairs of compounds are positional isomers?

Options

• \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH2 - CH2 - C - CH3}\\
  \phantom{..............}||\\
  \phantom{..............}\ce{O}
  \end{array}\]

  and

  \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH2 - C - CH2 - CH3}\\
  \phantom{}||\\
  \phantom{}\ce{O}
  \end{array}\]

• \[\ce{CH3 - CH2 - CH2 - CH2 - CHO}\]

  and 

  \[\begin{array}{cc}
  \phantom{..............}\ce{O}\\
  \phantom{..............}||\\
  \phantom{}\ce{CH3 - CH2 - CH2 - C - CH3}
  \end{array}\]

• \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH2 - CH2 - C - CH3}\\
  \phantom{..............}||\\
  \phantom{..............}\ce{O}
  \end{array}\]

  and

  \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH - CH2 - CHO}\\
  |\phantom{..........}\\
  \ce{CH3}\phantom{.......}
  \end{array}\]

• \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH2 - C - CH2 - CH3}\\
  \phantom{}||\\
  \phantom{}\ce{O}
  \end{array}\]

  and

  \[\begin{array}{cc}
  \ce{CH3}\phantom{.................}\\
  \backslash\phantom{.............}\\
  \phantom{......}\ce{CH - CH2 - CHO}\\
  /\phantom{..............}\\
  \ce{CH3}\phantom{.................}
  \end{array}\]

Answer 1

\[\begin{array}{cc}
\phantom{}\ce{CH3 - CH2 - CH2 - C - CH3}\\
\phantom{..............}||\\
\phantom{..............}\ce{O}
\end{array}\]

and

\[\begin{array}{cc}
\phantom{}\ce{CH3 - CH2 - C - CH2 - CH3}\\
\phantom{}||\\
\phantom{}\ce{O}
\end{array}\]

Explanation:

Option (a) Pentan-2-one and pentan-3-one are positional isomers.

Options (b), (c) and (d) contain different compounds aldehydes and ketones.

These exhibit functional group isomerism.