p-Cresol (4-methylphenol) is treated with $\mathrm{CHCl}{}_3$ and aqueous $\mathrm{NaOH}$ and then acidified. The main product is

Reimer-Tiemann formylation by dichlorocarbene occurs ortho to the phenoxide oxygen; the para position is blocked by $\mathrm{CH}{}_3$. The $\mathrm{CHO}$ goes to C-2 relative to OH; the methyl, para to OH, is then at C-5 of the benzaldehyde numbering (CHO = C1, OH = C2).