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Question
Which of the following compounds is most reactive towards nucleophilic addition reactions?
Options
\[\begin{array}{cc}
\phantom{...}\ce{O}\\
\phantom{...}||\\
\phantom{}\ce{CH3 - C - H}
\end{array}\]\[\begin{array}{cc}
\phantom{.}\ce{O}\\
\phantom{.}||\\
\phantom{}\ce{CH3 - C - CH3}
\end{array}\]
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Solution
\[\begin{array}{cc}
\phantom{...}\ce{O}\\
\phantom{...}||\\
\phantom{}\ce{CH3 - C - H}
\end{array}\]
Explanation:
\[\ce{CH3CHO}\] is most reactive towards nucleophilic addition reactions. Carbonyl compounds are polar with positive charge on carbon atom which is attacked by nucleophiles. Two electron releasing alkyl groups in ketones make carbonless electron deficient than aldehydes. Benzene ring exhibits + R-effect which thereby decreases the ease of nucleophilic addition reaction in benzaldehyde and acetophenone. Hence the reactivity order is
\[\begin{array}{cc}
\phantom{...}\ce{H}\phantom{..........}\ce{H}\phantom{.........}\ce{R}\phantom{.......}\\
\phantom{......}\backslash\phantom{...........}\backslash\phantom{.........}\backslash\phantom{........}\\
\phantom{.......}{\overset{δ+}{\ce{C}} = \overset{δ-}{\ce{O}} > \phantom{..}\overset{δ+}{\ce{C}} = \overset{δ-}{\ce{O}} > \phantom{..}\overset{δ+}{\ce{C}} = \overset{δ-}{\ce{O}}}\phantom{}\\
\phantom{.......}/\phantom{...........}/\phantom{...........}/\phantom{........}\\
\phantom{.....}\ce{H}\phantom{..........}\ce{R}\phantom{..........}\ce{R}\phantom{.........}\\
\end{array}\]
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\phantom{....}||\\
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