Question

Which of the following orders represent decreasing acidity of cresols?
(1) o-cresol > m-cresol > p-cresol
(2) p-cresol > m-cresol > o-cresol
(3) m-cresol > p-cresol > o-cresol
(4) m-cresol > o-cresol > p-cresol

Answer 1

Hint:The given three structures are very much similar to each other but the position of the methyl group with respect to the alcohol group decides its acidity. One can analyze aspects such as inductive effect and hyperconjugation and decide the acidity order.

Complete answer:1) First of all we will learn about the concept of acidity and the factors which can affect the acidity of a compound. The acidity of a compound depends upon its ability to donate a proton. When a proton or hydrogen is donated by a compound it will form an anion and the stability of an anion is directly proportional to the acidic strength of a compound.
2) Now another factor that affects acidity is the inductive effect. The atoms or groups which are electron-donating groups show a positive inductive effect and the atoms or groups which are electron-withdrawing groups show a negative inductive effect. The +I effect decreases the acidity of a structure as it destabilizes the anion formed after the donation of the proton. The electron-donating groups destabilize the anion formed. This happens exactly the opposite in the case of the -I effect as it increases the acidity of the compound.
3) Now let's discuss the given structures, m-cresol is most acidic as the electron-donating group methyl is at meta position which is far from the ortho position. The structure p-cresol shows the positive inductive effect as well as the hyperconjugation effect. The hyperconjugation effect lowers the acidity of the compound. The o-cresol is the least acidic as the electron-donating group is very near to the anion which will be formed after donating hydrogen which destabilizes it.
Therefore, the acidity order will be m-cresol > p-cresol > o-cresol that shows option C as the correct choice.

Note:Anion is an electron-rich species and the nearer the position of the electron-donating group to the anion, the less acidic will be the compound as it destabilizes the anion. The nearer the electron-withdrawing group to the anion the compound will be more acidic.