Electrophilic Addition and Bromohydrin Formation

Introduction

Alkenes are electron rich, and due to the electron density of the π bond above and below the plane of the molecule, they act as nucleophiles in reactions. When 1-methylcyclohexene reacts with NBS in water and THF, NBS decomposes to Br2. The nucleophilic nature of the alkene will force the electron density of Br2 to one side of the molecule, creating a slightly negative and slightly postive Br atom, which is then electrophilically added to 1-methylcyclohexene. Next an alcohol group is electrophilically added to the carbocation that is formed as a result of this first intermediate to produce one of two products. This reaction followed the mechanism of electrophilic addition, referring to the regioselectivity of the reaction and how an unsymmetrical alkene has the potential to produce two different products when reacting with an unsymmetrical electrophilic reagent. The more stable intermediate will be the one that is more highly alkyl group substituted, which is referred to as the Markovnikov’s product. In this specific reaction, the synthesis of a Markovnikov’s product was obtained and then verified from the bromohydration of 1-methylcyclohexene by the addition of an alcohol group to the sp2 carbon attached to the greatest number of carbons.

Methods

450 mg (2.53x10-3 mol) of N-bromosuccinimide (NBS) was added to a 5.0 mL conical vial containing a mixed solution of 1.3 mL of water and 1 mL of THF equipped with a spin vane. Sequentially, 320 μl (2.70x10-3 mol) of 1-methylcyclohexene was added to this mixture with stirring. The reaction went from a clear liquid to a heterogeneous bright orange-yellow liquid after the addition of 1-methycyclohexene. This mixture was stirred for up to ten minutes until the solid white NBS was dissolved and the mixture returned to a clear liquid. 2 mL of water was added to the mixture to dilute it with stirring for another 3 minutes. The mixture was allowed to stand and separate so that the heavier bromohydrin layer could be removed to a small Erlenmeyer flask and dried with anhydrous sodium sulfate. The clear liquid was purified using this column chromatography first by