New Look at the Competitive Nucleophilic Substitution Reaction in the Pentanol Series

Substitution reactions of alcohols are some of the simplest experiments performed in undergraduate organic chemistry laboratory settings. However, the use of gas chromatography (GC) as an analytical tool for these reactions can be at times laborious and time-consuming. In this work, 1-pentanol, 2-pentanol, 3-pentanol, and 2-methyl-2-butanol were reacted with a 50/50 mixture of ammonium chloride–ammonium bromide in water to yield the corresponding alkyl bromide and alkyl chloride mixtures. Aliquots of the reaction mixture were analyzed using proton nuclear magnetic resonance (1H NMR) spectroscopy and verified by GC. Secondary to secondary carbocation rearrangements were observed in the nucleophilic substitution reactions of 2- and 3-pentanol yielding a mixture of 2- and 3-substituted alkyl halides, while the primary and tertiary alcohols yielded the expected products. Students were assessed on their observations and explanations with the majority of them able to identify the correct reasoning for the alkyl halides mixture formation. 1H NMR spectroscopy provided an inexpensive and quicker method to analyze the mixture and expose the students to the secondary to secondary carbocation rearrangement in the nucleophilic substitution reaction in the pentanol series.