Arrange the following compounds in the increasing order of acidity and give a suitable explanation.
Phenol, o-nitrophenol,o-cresol.

Explanation

Nitro group shows-I-and-R-effect as follows:

Due to the-I-and-R-effect of o-nitrophenol, it is a stronger acid than phenol thus, making the release of `H^(+)` ion from O-H bond easier. -I- and -R-effect increases the acidic strength by increasing the polarity of -OH bond. On the other hand, `-CH_3` group in o-cresol produces +I-effect which 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.