(6)
(b) বেনাইল হেলাইডবােৰ এৰাইল। | হেলাইডতকৈ
নিউক্লিঅ’ফিলিক প্রতিস্থাপন বিক্রিয়াটো কিয় বেছি
সক্রিয় ? ব্যাখ্যা কৰা।।
Why are benzyl halides more reactive
than aryl halides towards nucleophilic
substitution reaction? Explain.
Answers
Answer:
Explanation:
Aryl halides are less reactive towards nucleophilic substitution reaction due to several reasons. whereas alkyl halides undergo nucleophilic substitution easily. Following factors are responsible for the low reactivity of aryl halides towards nucleophilic substitution.
Resonance concept:—
In case of aryl halides the halogen is attached to SP2 hybridised carbon of the benzene ring .Due to resonance,there is partial double bond characters in C-X bond.And C-X bond length in haloarene is is smaller than C-X bond length in Alkyl halide.
Hybridisation concept:——
In haloarenes,the carbon atom to which halogen is attached is sp2 hybridised .while that in haloalkanes it is sp3 hybridised. Due to the greater percentage of s orbital character in hybridization, sp2 hybridised carbon in haloarenes, tend to hold electron pair of C-X bond . Thus it is difficult to displace halogen in haloarenes by any nucleophile.
Polarity of C-X bond:—-
In haloarenes, difference in electronegativity of carbon and halogen is less. Whereas in Alkyl halides relatively large difference in electronegativity of carbon and halogen is recorded. Thus halogen is easily replaced by nucleophile.
Instability of phenyl cation:—-
In haloarenes,the phenyl cation formed by self ionisation,is not stabilised by resonance. Because sp2 hybridised orbital of carbon bearing positive charge is perpendicular to the p-orbitals of the phenyl ring. Hence substitution by formation of cation (intermediate) is not possible.
Electronic repulsion:——
Arenes being electron rich molecules,due to presence of pi bonds,repel electron rich nucleophilic. Thus attack of nucleophiles on aryl halides becomes less likely.Whereas in Alkyl halides nucleophilic substitution reactions are studied in terms of SN1 and SN2.