Which of the following pair is more acidic and why?

1.	Which of the following pair is more acidic and why?
Answer» (a). Due to high electronegativity of sp² - hybridized carbon, electron density on oxygen in phenol (I) decreases. This increases the polarity of O – H bond and results in more ionization of phenol than that of cyclohexyl alcohol (II). Therefore, phenol is more acidic than cyclohexyl alcohol. (b). In p-nitrophenol, nitro group (NO₂) is an electron-withdrawing group present at ortho position which enhances the acidic strength (-I effect). The O – H bond is under strain and release of proton (H⁺) becomes easily. Hence, o-nitrophenol is more acidic than phenol.

Answer»

(a). Due to high electronegativity of sp² - hybridized carbon, electron density on oxygen in phenol (I) decreases.

This increases the polarity of O – H bond and results in more ionization of phenol than that of cyclohexyl alcohol (II).

Therefore,

phenol is more acidic than cyclohexyl alcohol.

(b). In p-nitrophenol, nitro group (NO₂) is an electron-withdrawing group present at ortho position which enhances the acidic strength (-I effect).

The O – H bond is under strain and release of proton (H⁺) becomes easily.

Hence,

o-nitrophenol is more acidic than phenol.

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