4-Methylphenol (p-cresol) is heated with $\mathrm{CHCl}{}_3$ and aqueous $\mathrm{NaOH}$, and the mixture is then acidified. The major product is

The Reimer-Tiemann reaction attacks the phenoxide ring with $\text{:}\mathrm{CCl}{}_2$ at the ortho position. The para position is blocked by $\mathrm{CH}{}_3$, so the $-\mathrm{CHCl}{}_2$ group enters ortho to oxygen, and hydrolysis gives $-\mathrm{CHO}$. Numbering from CHO as C-1 with OH at C-2, the methyl (para to OH) sits at C-5.