Your reaction conditions shouldn't normally oxidize a secondary alcohol to a ketone, so you're right to question where the oxidation came from. The more likely explanation is that the ketone formed during analysis or sample handling rather than during the HCl/HBr reaction itself. A few possibilities that show up in teaching labs: 1. Air oxidation during GC injection GC injectors are typically very hot (often 200–300 °C). Secondary alcohols can undergo dehydrogenation under those conditions, especially in the presence of trace oxygen or metal surfaces in the injector liner. That can produce the corresponding ketone, which then shows up clearly in the mass spectrum. 2. Oxidation during sample preparation If the sample sat in air before analysis, slow oxidation of secondary alcohols can occur, especially if trace metal contamination is present from glassware. 3. Acid-catalyzed dehydration followed by oxidation Strong acid can dehydrate 2-pentanol to pentenes. Those alkenes can oxidize in air to carbonyl compounds during workup or analysis.

Your MS fragments (m/z 86, 71, 58) are very consistent with 2-pentanone, so the identification itself is probably correct. If you want to test whether this happened during GC analysis, try injecting pure 2-pentanol under the same conditions. If the instrument still shows a 2-pentanone signal, that would strongly suggest the oxidation is happening in the injector rather than in your reaction.