What is the correct increasing order of acidic strength in the following :
(i) Phenol (ii) p-cresol (iii) p-nitrophenol (iv) o-nitrophenol ?

Explanation

Nitro group shows -I and -R effect as follows

Due to this -I and -R effect of o-nitrophenol, it is stronger acid and than phenol. On the other hand, `-CH_(3)` group produces +I effect -I and -R effect increases the acidic strength by increasing the polarity of -OH bond while +I effect decreases the polarity due to increase in electron density on -OH bond. So, o-cresol is a weaker acid than phenol. Thus, the correct order is o-cresol `lt` phenol `lt`o-nitrophenol.