JEE Chemistry 2023 Question with Solution

Given: The compounds differ by substituents on the benzene ring.

Find: The decreasing order of reactivity towards electrophilic substitution.

Reactivity towards electrophilic aromatic substitution depends on the electron density on the benzene ring. Electron-donating groups activate the ring, while electron-withdrawing groups deactivate it.

Compound (e) contains $\mathrm{-NMe_2}$, a very strong electron-donating group due to its $+M$ effect. Hence it is the most activated.

Compound (d) contains $\mathrm{-OCH_3}$, which also donates by $+M$ effect, but less strongly than $\mathrm{-NMe_2}$.

Compound (a) contains $\mathrm{-CH_3}$, which activates the ring by $+I$ effect and hyperconjugation, so it is moderately activating.

Compound (b) is benzene itself, so it shows baseline reactivity.

Compound (c) contains $\mathrm{-CF_3}$, a strong electron-withdrawing group due to $-I$ effect, making it least reactive.

Therefore, the decreasing order is

$$e > d > a > b > c$$

So, the correct option is B.

Quick Tip: For electrophilic substitution, the activating strength follows

$$\mathrm{-NR_2 > -OR > -R > H > -CF_3}$$

Given: Substituted benzene rings.

Find: The fastest to slowest ring for electrophilic substitution.

Use the standard activating order of substituents:

$$\mathrm{-NR_2 > -OR > -R > H > -CF_3}$$

Now match the compounds:

• (e) is $\mathrm{-NMe_2}$
• (d) is $\mathrm{-OCH_3}$
• (a) is $\mathrm{-CH_3}$
• (b) is $\mathrm{H}$
• (c) is $\mathrm{-CF_3}$

Hence the order is

$$e > d > a > b > c$$

So, the correct option is B.