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Which of the following conjugate acid pairs is more acidic, and why? (I) C6H5NH3+ (anilinium ion) (II) p-F-C6H4NH3+ (p-fluoroanilinium ion)

1. (I)
2. (II)
3. Both are equally acidic
4. Cannot be determined

Correct answer: (II)

Solution

The -I effect of fluorine at the para position withdraws electron density from the nitrogen through the sigma framework, reducing the electron density on N in p-fluoroaniline relative to aniline. This makes the conjugate acid p-FC6H4NH3+ more willing to release a proton (more acidic) compared to C6H5NH3+.

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