Question
Which is a stronger acid-phenol or cresol? Explain.
✓
Answer
All the cresols are weaker acids than phenols. Methyl group has +I effect (positive inductive effect) as well as hyperconjugation effect but the hyperconjugation effect predominates over the +I effect. Since both these effects increase the electron density in the O-H bond and hence all the cresols are weaker acids than phenols
As hyperconjugation effect can operate only through ortho and para positions and not through meta positions, therefore, meta-cresol is stronger acid than ortho and para-cresols. However, due to stronger +I effect at ortho position than at para position (+I effect decreases with distance), ortho-cresol is a weaker acid than para-cresol. Thus, the order of acidic strength in increasing order is:
ortho-cresol < para-cresol < meta-cresol < phenol
As hyperconjugation effect can operate only through ortho and para positions and not through meta positions, therefore, meta-cresol is stronger acid than ortho and para-cresols. However, due to stronger +I effect at ortho position than at para position (+I effect decreases with distance), ortho-cresol is a weaker acid than para-cresol. Thus, the order of acidic strength in increasing order is:
ortho-cresol < para-cresol < meta-cresol < phenol
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