Four anilines are shown beside the options (the drawn compound is toluene). Which structure is the weakest base?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

Despite the +I effect of CH3, ortho methyl groups lower basicity: o-toluidine has pKaH about 4.45 (aniline 4.60, m-toluidine 4.69, p-toluidine 5.08) and 2,6-dimethylaniline about 3.9. Two ortho groups hinder solvation of the NH3+ group and crowd the nitrogen, so the effect is cumulative (ortho effect in anilines).

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