In SNAr reactions, a carban ion is formed as an intermediate,so any substituent that increases the stability of carban ion and hence the transition state leading to its formation will enhance the SNAr reactions. To compare the rates of substitution in chloro benzene, chloro benzene having electron-withdrawing group, and chloro benzene having electron-releasing group, we compare the structures carban ion I (from chloro benzene), II (from chloro benzene containing electron-withdrawing group) and III (from chloro benzene containing electron-releasing group)

Because of high electronegativities of the halogen atom,the carbon halogen (C – X) is highly polarised covalent bond. Thus, the carbon atom of the C – X bond becomes a good site for attack by nucleophiles (electron rich species). Nucleophilic substitution reactions are the most common reactions of alkyl halides.

Due to + M effect of – OH group and hyper conjugation of -CH₃ group.

The amount of s-character in various hybrid orbitals is as follows.
sp= 50%, sp²= 33% and sp³ = 25%
Therefore s character of the C – H bond in acetylene(sp) is greater than that of the C–H bond in alkene (sp² hybridized) which in turn has greater s character of the C – H bond than in alkanes. Thus owing to a high s character of the C – H bond in alkynes, the electrons constituting this bond are more strongly held by the carbon nucleus with the result the hydrogen present on such a carbon atom can be easily removed as proton.The acidic nature of three types of C – H bonds follows the following order -C = C - H > -C = C-H > -C -C -H
Further, as we know that conjugate base of a strong acid is a weak base, hence the correct order of basicity is \(H-C \equiv \bar{C} < CH_{2} = \bar{C}H < C\bar{H_{2}} - CH_{3}\)

In given structures –After releasing H⁺ from (ii) group it forms a more stable resonating structure of carboxylate ion. In case of (i)the anion formed after release of H⁺ is more stable due to present of e^-withdrawing NO₂ in benzene ring. In(iii) phenoxide ion formed after release of H⁺ is more stable in comparis onto Acetylide ion of group (iv)which is formed after releasing of (iv) H⁺ ion so acidic structure of K_a is
(ii)> (i) > (iii)> (iv)

Lone pair present at X can enter in the ring. This gives rise to double bond character in C – X bond attached to the ring .

Since diazo compounds may lose nitrogen in the form of nitrogen gas, they sometimes do not respond Lassaigne’s test for nitrogen.

Order of stability of carbocation 3° > 2° >1° and I is the best leaving group among halogens.