.This experiment was performed to determine the structure of alkyl-halides formed as a result of substitution reactions, and whether the reaction used an SN1 or SN2 mechanism. The structure of the starting alcohol determined the mechanistic pathway of the substitution reaction. Reaction 1 involved the substitution of a primary alcohol which produced one primary alkyl-halide via SN2 reaction. Reactions 2 and 3 began with a secondary alcohol, forming two products as the result of direct substitution and/or a hydride shift, via SN1 reaction. Reaction 2 formed two secondary alkyl-halides, and Reaction 3 formed one secondary and one tertiary alkyl-halide.
The overall premise of the various experiments chronicled in this article is the determination of the mechanistic pathways and products formed via substitution reactions. Substitution reactions occur when one atom or functional group replaces another. For the purposes of this experiment, there were two types of substitution reactions: SN1 or unimolecular nucleophilic substitution, 1st order and SN2 or bimolecular nucleophilic substitution, 2nd order. 1
Substitution reactions, whether they are an SN1 or SN2 reaction, must contain molecules known as nucleophiles and electrophiles. The electrophile is a component of the substrate, in this case the starting alcohol, also commonly known as the “leaving group.” Electrophiles are electron deficient, while nucleophiles are “electron donating.” The mechanism of a substitution reaction is as follows: in the presence of the nucleophile, the leaving group separates from the substrate allowing the nucleophile to form a new bond with the substrate in place of the recently departed electrophile. 2
The key difference between the SN1 and SN2 mechanisms is the manner in which the electrophile departs the substrate molecule. SN2 reactions rely on a nucleophilic attack, in which the nucleophile forms a sigma bond with the substrate, forcing the bond between the electrophile and substrate to break. In SN1 reactions, the electrophile breaks its bond with the substrate, leaving behind a carbocation intermediate to which the nucleophile bonds in a step-wise reaction. 2
The determining factor of whether a reaction follows the SN1 pathway or the SN2 pathway is the position of the electrophile on the substrate. An electrophile located in the primary position undergoes an SN2 reaction, resulting in a single product. An electrophile located in the secondary position results in a carbocation intermediate. The presence of the carbocation indicates an SN1 reaction. Carbocation intermediates commonly undergo rearrangement resulting in either a racemic mixture of secondary products, or a major tertiary product and a minor secondary product, due to the favorability and stability of the tertiary position. 2
Scheme 1. Substitution of 3-pheny-1-propanol to form 1-bromo-3-phenylpropane.
Reaction 1 resulted in a total yield of 25.00% of...