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Question
What is the order of reactivity of the following compounds towards nucleophilic substitution?
What is the order of reactivity of the following compounds towards nucleophilic substitution?
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Solution
The correct option is C IV<I<II<III
IV is least reactive because it is an aryl halide.
Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to resonance stabilization.
Due to resonance, C−Cl bond acquires partial double bond character and becomes shorter and stronger and cannot be easily replaced by nucleophiles.
Other reasons for the low reactivity of aryl halides are:
(1) Difference in the hybridisation states of carbon atom in C−X bond. In alkyl halides, the carbon atom is sp3 hybridized whereas in aryl halides, it is sp2 hybridized and hence, more electronegative. Hence, C−X bond in aryl halides is more difficult to break.
(2) Polarity of C−X bond in aryl halides is lower than that in alkyl halides. Lower is the polarity, lower is the reactivity.
III is most reactive as the carbocation formed will be stabilized by the resonance of the positive charge with 2C=C double bonds.
In case of II, the resonance in the carocation is between positive charge and only one C=C double bond. Hence, II is less reactive than III.
In case of I, there is no resonance stabilization. Hence, I is less reactive than II.
The order of reactivity towards nucleophilic substitution is IV<I<II<III
IV is least reactive because it is an aryl halide.
Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to resonance stabilization.
Due to resonance, C−Cl bond acquires partial double bond character and becomes shorter and stronger and cannot be easily replaced by nucleophiles.
Other reasons for the low reactivity of aryl halides are:
(1) Difference in the hybridisation states of carbon atom in C−X bond. In alkyl halides, the carbon atom is sp3 hybridized whereas in aryl halides, it is sp2 hybridized and hence, more electronegative. Hence, C−X bond in aryl halides is more difficult to break.
(2) Polarity of C−X bond in aryl halides is lower than that in alkyl halides. Lower is the polarity, lower is the reactivity.
III is most reactive as the carbocation formed will be stabilized by the resonance of the positive charge with 2C=C double bonds.
(1) Difference in the hybridisation states of carbon atom in C−X bond. In alkyl halides, the carbon atom is sp3 hybridized whereas in aryl halides, it is sp2 hybridized and hence, more electronegative. Hence, C−X bond in aryl halides is more difficult to break.
(2) Polarity of C−X bond in aryl halides is lower than that in alkyl halides. Lower is the polarity, lower is the reactivity.
III is most reactive as the carbocation formed will be stabilized by the resonance of the positive charge with 2C=C double bonds.
In case of II, the resonance in the carocation is between positive charge and only one C=C double bond. Hence, II is less reactive than III.
In case of I, there is no resonance stabilization. Hence, I is less reactive than II.
The order of reactivity towards nucleophilic substitution is IV<I<II<III
The order of reactivity towards nucleophilic substitution is IV<I<II<III
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