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o-hydroxytoluene - 10.29–10.37 (Average = 10.33)p-hydroxytoluene - 10.26–10.30 (Average = 10.28)
Since the smaller the pKa the more acidic the compound is, it looks like p-hydroxytoluene is more acidic than o-hydroxytoluene. Perhaps the question should be why is p-hydroxytoluene more acidic than o-hydroxytoluene?
First, consider that the difference is really small (average 0.05 pKa units) and without knowing the experimental errors on the pKas, the pKas for these two compounds maybe, experimentally, identical.
Second, let’s assume the 0.05 pKa unit difference is real. In the o- or p-position, substituents interact by resonance and inductive effects. There is not much in the way of resonance possible, though they are not distant dependent and will be the same in the o-/p-positions, so any differences are likely due to inductive effects. Inductive effects fall rapidly with distance. A methyl group tends to be electron donating and this will reduce the acidity of a phenol. In the ortho case, the methyl group is nearer the OH than in the para case so the prediction of acidity is in agreement with what is observed.
Finally, note that the pKa of phenol is 9.9–10. So, the effect of the methyl group on acidity supports the argument, above, as simply placing a methyl group on the ring reduces the acidity by 0.3 pKa units.
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