why alkyl halides are more reactive towards neucleophilic substitution than vinyl halides?
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Hi friend,
Aryl halides are less reactive than alkyl halides towards nucleophilic substitution reactions because-
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(i) Resonance effect-
The electron pairs on halogen atom are in conjugation with pi-electrons of the ring. This causes delocalisation of electrons on C-X bond (X= halogen) which acquires partical double bond character. As a result the bond cleavage in aryl halides is difficult than alkyl halides in which carbon is attached to halogen by a pure single bond.
For resonating structures please refer the image posted by Miss Poetess.
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(ii) Difference in hybridisation states of carbon in C-X bond-
In alkyl halides carbon atom of C-X bond is sp3 hybridised while in aryl halides, the carbon atom attached to halogen is sp2 hybridised . The sp2 hybridised carbon atom with greater s-character is more electronegative . It can hold the electron pair of the bond more tightly than sp3 hybridised carbon atom in alkyl halides with less s-character. This causes shortening of bond length in aryl halides than in alkyl halides. The shortening of length imparts to aryl halides and as result, the bond cleavage becomes rather difficult.
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(iii) Instability of phenyl cation-
The phenyl cation formed as a result of self ionisation will not be stabilized by resonance and therefore SN1 mechanism cannot occur at all.
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HOPE THIS HELPS YOU:-))
Aryl halides are less reactive than alkyl halides towards nucleophilic substitution reactions because-
_______________________________
(i) Resonance effect-
The electron pairs on halogen atom are in conjugation with pi-electrons of the ring. This causes delocalisation of electrons on C-X bond (X= halogen) which acquires partical double bond character. As a result the bond cleavage in aryl halides is difficult than alkyl halides in which carbon is attached to halogen by a pure single bond.
For resonating structures please refer the image posted by Miss Poetess.
_______________________________
(ii) Difference in hybridisation states of carbon in C-X bond-
In alkyl halides carbon atom of C-X bond is sp3 hybridised while in aryl halides, the carbon atom attached to halogen is sp2 hybridised . The sp2 hybridised carbon atom with greater s-character is more electronegative . It can hold the electron pair of the bond more tightly than sp3 hybridised carbon atom in alkyl halides with less s-character. This causes shortening of bond length in aryl halides than in alkyl halides. The shortening of length imparts to aryl halides and as result, the bond cleavage becomes rather difficult.
_______________________________
(iii) Instability of phenyl cation-
The phenyl cation formed as a result of self ionisation will not be stabilized by resonance and therefore SN1 mechanism cannot occur at all.
______________________________
HOPE THIS HELPS YOU:-))
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