Explain nucleophilic substitution in aryl halides is difficult then alkyl halides
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Aryl halides are the compounds in which halogen atom is attached to sp² hybridised carbon atom of benzene ring whereas in alkyl halides halogen is attached to sp³ hybridised carbon atom . Reactivity of nucleophilic substitution reactions depends upon several factors like nature of leaving group (Group which depart from substrate ),bond dissociation enthalpy of C-X bond , solvent used .
In aryl halides non bonding electron pair of halogen atom are in conjugation with the benzene ring which confers partial double bond character to C-X bond .Another important consideration is hybridisation of carbon atom to which halogen atom is attached ,in case of aryl halides it is sp² hybridised which reduces the polarity of C-X bond and hence increases bond dissociation enthalpy of C-X bond .This is the chemistry of aryl halides .
Now come to the alkyl halides.Carbon atom to which halogen is attached is sp³ hybridised which confers greater polarity to C-X bond and hence lower bond dissociation enthalpy .In this case non bonding electron on halogen atom is not in conjugation with alkyl group so C-X bond is purely single bond which requires less energy to break than a partial double bond .
Another factor which affects reactivity is stability of carbocation formed .Aryl carbocation is not stabilized by resonance .
This is the story of aryl halides and alkyl halides which greatly affect the reactivity towards nucleophile.
In aryl halides non bonding electron pair of halogen atom are in conjugation with the benzene ring which confers partial double bond character to C-X bond .Another important consideration is hybridisation of carbon atom to which halogen atom is attached ,in case of aryl halides it is sp² hybridised which reduces the polarity of C-X bond and hence increases bond dissociation enthalpy of C-X bond .This is the chemistry of aryl halides .
Now come to the alkyl halides.Carbon atom to which halogen is attached is sp³ hybridised which confers greater polarity to C-X bond and hence lower bond dissociation enthalpy .In this case non bonding electron on halogen atom is not in conjugation with alkyl group so C-X bond is purely single bond which requires less energy to break than a partial double bond .
Another factor which affects reactivity is stability of carbocation formed .Aryl carbocation is not stabilized by resonance .
This is the story of aryl halides and alkyl halides which greatly affect the reactivity towards nucleophile.
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