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In cresol both $+I$ effect as well as hyperconjugation work together to determine its acidity. When the methyl group is present in the ortho- and para-position both of them can act upon phenol.

Hyperconjugation as a rule dominates which increases the electron density of the aryl. This makes it less acidic because the conjugate base is now less stable. But when its meta-cresol, hyperconjugation no longer works.

Why is it so? It happens even though an empty $\mathrm{p}$-orbital is present near the $\alpha$-hydrogen. What else does it need?

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Above are the resonance forms of m-cresol after deprotonation. As you can see, the negative charge resonates around the aryl ring, but never resides on the carbon directly bonded to the methyl group at the meta position.

Thus, the hyperconjugative effect from the meta methyl group does not affect the acidity of cresol in this isomer. Notice how the destabilising hyperconjugation effect would destabilise the resonating negative charge, if the methyl group was at the ortho- or para- position.

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  • $\begingroup$ Would you please elaborate on the destabilising effect of hyperconjugation. How does it decrease the stability when the methyl group is at the ortho/para position? Thanks. $\endgroup$
    – Parth Chauhan
    Commented Aug 6, 2018 at 14:48
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    $\begingroup$ @ParthChauhan Methyl groups are electron donating (from the electron density of the C-H sigma bonds donating into adjacent parallel(ish) orbitals i.e. hyperconjugation). When the adjacent parallel(ish) orbital is a lone pair of electrons (which it would be in this case), the interaction between the lone pair and C-H sigma electrons is destabilising as one would expect through simple unfavourable Coulombic interactions. $\endgroup$
    – Ava
    Commented Aug 7, 2018 at 15:17
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It is known that cresol containing methyl group destabilises the conjugate base ion by releasing electrons which results in decreasing of acidic strength.

But in meta-cresol, this destabilisation is only due to $+I$ effect. While in ortho- and para-cresols, the destabilisation is also due to hyperconjugation with $+I$ effect as the methyl group is attached in the para- and ortho-positions (since the hyperconjugation is only shown in para- and ortho-substituted).

Means the factors which are trying to decrease the acidic strength is 2 in para- and ortho-cresols, while in meta-isomer, only one $+I$ is decreasing the acidic strength.

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