Question

What is the order of acidic strength of the given molecules?

A.\[I > II > III > IV\]
B.\[IV > III > II > I\]
C.\[IV > II > III > I\]
D.\[I > II > IV > III\]

Answer 1

Hint: We have to remember that the acidity is the tendency to dissociate to protons or anions. Some of the factors will affect the acidic strength. The acidic strength is increased with increasing atomic size. And the inductive effect also depends on the acidic strength. If the compound has a positive inductive group, the acidity of that compound will be decreased. And the pKa value also depends on the acidic strength. If the pKa value is smaller, the acidic strength will be more.

Complete answer:
The correct order of acidic strength is not equal to\[I > II > III > IV\]. Because, among the following compounds, phenol is more acidic. Hence, the option (A) is incorrect.
Among the following compounds, phenol is more acidic. The remaining three compounds have a methyl group which is an electron donating group. And this will increase the electron density of the oxygen and also increase the electron density of the hydroxyl bond. And the methyl group is showing a positive inductive effect. It will reduce the acidic strength of a compound. Hence, phenol is more acidic.
Consider, remaining three compounds, m-cresol is more acidic than -ortho and –para-cresol. Because, the inductive effect will depend on the distance. The ortho-cresol will show more +I effect. And it will be less acidic and meta- cresol will show less +I effect. Hence, it is more acidic than para-cresol. Thus, the order of acidic strength is, \[IV > III > II > I\]
Hence, the option (B) is correct.
The correct order of acidic strength is not equal to\[IV > II > III > I\]. Because, meta-cresol is more acidic than para-cresol. Hence, the option (C) is incorrect.
The correct order of acidic strength is not equal to \[I > II > IV > III\]. Because, ortho-cresol is the least acidic compound. Hence, the option (D) is incorrect.

Note:
We have to know that the acidic strength mainly depends upon the inductive effect. If a +I group is present in a compound, its acidic strength will be lesser than other compounds. Among the following compounds, phenol is more acidic. The remaining three compounds have a methyl group which is an electron donating group and it also contains +I group.