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प्रश्न
Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides?
Why are aryl halides less reactive than alkyl halides towards nucleophilic substitution reactions?
Why are haloarenes less reactive towards nucleophilic substitution reactions as compared to haloalkanes?
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उत्तर
Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons.
- In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring. So, C – Cl bond acquires partial double bond character which strengthen C – Cl bond and difficult to be substituted by nucleophile.
Therefore, they are less reactive towards nucleophilic substitution reaction.
- In haloarenes, the carbon atom attached to halogen is sp2 hybridised. The sp2 hybridised carbon is more electronegative than sp3 hybridised carbon. This sp2-hybridised carbon in haloarenes can hold the electron pair of \[\ce{C - X}\] bond more tightly and make this \[\ce{C - Cl}\] bond shorter than \[\ce{C Cl}\] bond of haloalkanes.
Since, it is difficult to break a shorter bond than a longer bond, therefore, halorenes are less reactive than haloalkanes. - In haloarenesm the phenyl cation is not stabilised by resonance therefore SN1 mechanism cannot be followed.
- Because of the repulsion between the nucleophile and electron-rich arenas, aryl halides are less reactive than alkyl halides.
संबंधित प्रश्न
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|
Nucleophilic Substitution: Influences of solvent polarity: The reaction rate (SN2) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has an influence on both SN1 and SN2 reactions but with different results. Generally speaking, a weak polar solvent is favourable for SN2 reaction, while a strong polar solvent is favourable for SN1. Generally speaking, the substitution reaction of tertiary haloalkane is based on SN1 mechanism in solvents with a strong polarity (for example ethanol containing water). |
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(ii)
OR
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