Q.What happens when 1-phenylethanol is treated with acidified KMnO4?

PCC (pyridinium chlorochromate) is a mild, anhydrous chromium(VI) oxidant that stops the oxidation of a primary alcohol exactly at the aldehyde stage, whereas $KMnO_4$/$Na_2Cr_2O_7$/$K_2Cr_2O_7$ in aqueous acidic medium drive the oxidation all the way to the carboxylic acid.

Oxidation of a primary alcohol, $R-CH_2OH$, can in principle proceed in two steps: first to the aldehyde ($R-CHO$), and then, if water/excess strong oxidant is present, further to the carboxylic acid ($R-COOH$). Strong aqueous oxidants such as acidified $KMnO_4$, $K_2Cr_2O_7$, or $Na_2Cr_2O_7$ readily drive the reaction all the way to the carboxylic acid, because water is required for/present during the aldehyde-to-acid oxidation step, and these reagents are vigorous oxidants. Pyridinium chlorochromate (PCC, $C_5H_5NH^+CrO_3Cl^-$), by contrast, is used in an anhydrous, non-aqueous medium (typically dichloromethane) and is a comparatively mild chromium(VI) oxidant — since no water is present to hydrate the aldehyde intermediate (a step needed b …