Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

This is a long answer type question as classified in NCERT Exemplar

Because of the following reasons, aryl halides are less reactive towards nucleophilic substitution processes than alkyl halides:

(i) The halogen atom's electrons are conjugated to the aryl ring's -electrons, giving the C-X bond a partial double bond character. The whole molecule is then resonance stabilised, allowing for the formation of the following configurations. C-X bond cleavage in haloarenes is more difficult than in haloalkanes due to this bond creation.

ii) The carbon linked to the halogen in haloalkanes is sp³ hybridised, whereas the carbon coupled to the halog

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This is a long answer type question as classified in NCERT Exemplar

Because of the following reasons, aryl halides are less reactive towards nucleophilic substitution processes than alkyl halides:

(i) The halogen atom's electrons are conjugated to the aryl ring's -electrons, giving the C-X bond a partial double bond character. The whole molecule is then resonance stabilised, allowing for the formation of the following configurations. C-X bond cleavage in haloarenes is more difficult than in haloalkanes due to this bond creation.

ii) The carbon linked to the halogen in haloalkanes is sp³ hybridised, whereas the carbon coupled to the halogen in haloarene is sp² In haloalkanes, the sp² hybridised carbon atom binds the halogen atom more firmly than a connection between sp³ carbon atom and halogen atom. Haloarenes have a shorter C-X bond than haloalkanes, making them more difficult to break.

(iii) Because the phenyl cation is not resonance stabilised, the S_N¹ mechanism of substitution cannot be used.

(iv) Because of the potential for repulsion between electron-rich arenes and approaching electron-rich nucleophiles, aryl halides are less reactive than alkyl halides.

The inclusion of an electron withdrawing group at the ortho- and para-positions can enhance the reactivity of aryl halides. At the meta-position, there is no impact. The presence of an electron withdrawing group, such as the nitro group, at ortho- and para- locations pulls electrons away from the benzene ring, making it easier for the nucleophile to attack haloarene. The carbanion is stabilised as a result of resonance. The -NO₂ group stabilises the negative charge at ortho- and para-positions with regard to the halogen substituent, but none of the resonant structures in meta-nitrobenzene contain the negative charge on the carbon atom bearing the -NO₂ group. As a result, the inclusion of a meta-position electron withdrawing group has no influence on the aryl halide's reactivity.

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<p><span data-teams="true">This is a long answer type question as classified in NCERT Exemplar</span></p><p>Because of the following reasons, aryl halides are less reactive towards nucleophilic substitution processes than alkyl halides:</p><p><strong> (i)</strong> The halogen atom's electrons are conjugated to the aryl ring's -electrons, giving the <strong>C-X</strong> bond a partial double bond character. The whole molecule is then resonance stabilised, allowing for the formation of the following configurations. <strong>C-X</strong> bond cleavage in haloarenes is more difficult than in haloalkanes due to this bond creation.</p><div><div><picture><source srcset="https://images.shiksha.com/mediadata/images/articles/1747113247phpu96hoG_480x360.jpeg" media=" (max-width: 500px)"><img src="https://images.shiksha.com/mediadata/images/articles/1747113247phpu96hoG.jpeg" alt="" width="332" height="83"></picture></div></div><p><strong>ii)</strong> The carbon linked to the halogen in haloalkanes is <strong>sp³</strong> hybridised, whereas the carbon coupled to the halogen in haloarene is <strong>sp²</strong> In haloalkanes, the <strong>sp²</strong> hybridised carbon atom binds the halogen atom more firmly than a connection between <strong>sp³</strong> carbon atom and halogen atom. Haloarenes have a shorter <strong>C-X </strong>bond than haloalkanes, making them more difficult to break.</p><p><strong> (iii)</strong> Because the phenyl cation is not resonance stabilised, the <strong>S_N</strong><strong>¹</strong> mechanism of substitution cannot be used.</p><p><strong> (iv)</strong> Because of the potential for repulsion between electron-rich arenes and approaching electron-rich nucleophiles, aryl halides are less reactive than alkyl halides.</p><p>The inclusion of an electron withdrawing group at the ortho- and para-positions can enhance the reactivity of aryl halides. At the meta-position, there is no impact. The presence of an electron withdrawing group, such as the nitro group, at ortho- and para- locations pulls electrons away from the benzene ring, making it easier for the nucleophile to attack haloarene. The carbanion is stabilised as a result of resonance. The <strong>-NO₂</strong> group stabilises the negative charge at ortho- and para-positions with regard to the halogen substituent, but none of the resonant structures in meta-nitrobenzene contain the negative charge on the carbon atom bearing the <strong>-NO₂</strong> group. As a result, the inclusion of a meta-position electron withdrawing group has no influence on the aryl halide's reactivity.</p><div><div><picture><source srcset="https://images.shiksha.com/mediadata/images/articles/1747113247phpu96hoG_480x360.jpeg"1 media=" (max-width: 500px)"><img src="https://images.shiksha.com/mediadata/images/articles/1747113069phptU9S6V.jpeg" alt="" width="576" height="655"></picture></div></div>

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