Preparation of 2-bromopentane

Abstract

The recent publication of an article on the use of a Grignard reagent in the synthesis of insect pheromones (1) and the subsequent appearance of this experiment in various undergraduate organic laboratory manuals (2-4) stimulated our interest in the preparation of 2-bromopentane, which is the starting material for the above sequence of reactions. Although the cost (5) of 2-bromopentane in the above experiment is not excessive ($1.48/student based upon $24.35/500 g), the educational value of an extension of this synthesis appeared worthwhile.Replacement of a hydroxyl group with a halide is one of the most fundamental reactions in undergraduate organic chemistry yet the efficacy and scope of the reaction is not extensively discussed in most textbooks. Many texts hint at the potential for rearrangement in reactions of secondary halides but only a few detail the significance of the problem (6). In all but primary alcohols substitution is often accompanied by rearrangement, yet textbook examples are typically c This value represents an average yield tor a reaction period of 0.5 h. The yield increases as the period allowed for reaction increases. The reaction period must be kept near 0.5 h if the preparation and purification (by distillation) of 2-bromopentane is to be comfortably completed within one 3-h laboratory session.d Scrupulously anhydrous conditions must be maintained. 9 No 3-bromopentane could be detected by glpc.

Introducing Aliphatic Substitution with a Discovery Experiment Using Competing Electrophiles

“…Students work individually and require 3 h to complete the experiment. Students dissolve 1 (5.0 mmol) in equal volumes (1 mL) of ethanol and 20% NaOH, bring the solution to reflux, and add a 1:1 mixture of 2 and 3 (11 mmol each) to the flask . After the solution is heated for 40 min, the ether products are isolated by extraction and rotary evaporation.…”

“…Students dissolve 1 (5.0 mmol) in equal volumes (1 mL) of ethanol and 20% NaOH, bring the solution to reflux, and add a 1:1 mixture of 2 and 3 (11 mmol each) to the flask. 1 After the solution is heated for 40 min, the ether products are isolated by extraction and rotary evaporation. The crude product, an oil, is analyzed by 1 H NMR spectroscopy and/or GC−MS.…”

“…Substitution at sp 3 -hybridized carbon is one of the important reactions covered in the introductory undergraduate organic chemistry course, and a number of substitution experiments have been described and used. − Some of these experiments are traditional verification experiments in which students perform a substitution reaction in order to confirm what had been covered in the lecture part of the course. − In contrast, there are several experiments that explore substitution using a guided-inquiry (discovery) approach. − In these experiments, students can discover one or more aspects of substitution, and the student data can be used to initiate discussion of substitution in the classroom. The discovery experiments that have been described and utilized employ different approaches.…”

A Safer, Discovery-Based Nucleophilic Substitution Experiment

“…For their data analysis, students are asked to compare the results of the two reactions and to draw conclusions as to why one nucleophile reacted faster than the other. 2 Students are also asked to hypothesize about what type of results they would have obtained had they used a tertiary substrate, instead of a primary one. Additionally, students are required to briefly research the topic of phase-transfer catalysis and to present an explanation of how their catalyst worked in the reaction and why it was needed.…”

“…A few nucleophilic substitution experiments have been published in the literature (1)(2)(3)(4)(5)(6), including a well-known "Competing Nucleophiles" experiment (7)(8)(9) in which students compare the reactivity of a primary and tertiary alcohol in the presence of a one-to-one mixture of chloride to bromide ion. This experiment allows students to both examine the effect of nucleophilicity on the reaction rate, as well as the effect of substrate structure on the outcome of the reaction.…”