Nucleophilic Substitution Reactions:
The purpose of this lab is to compare the nucleophilic abilities of the addition of chloride and bromide ions to the compounds of n-butyl alcohol and tertiary butyl alcohol, in SN1 and SN2 environments. This will determine which of the anions is the better nucleophile, in each mechanism.
To better understand the reactivity of competing nucleophiles, the mechanisms of their reactions must be observed. In the lab, both of the basic nucleophilic mechanisms of unimolecular and bimolecular substitution reactions take place. In both of these reactions, the reactivity is based on the substrates structure, the nucleophileâ€™s basicity, and the reaction conditions, such as the solvent used and the temperature (Smith 389). In the substitution bimolecular reaction (SN2), the reaction is bimolecular and takes place in a one-step process. The rate is dependent on both the concentration of the substrate and the nucleophile. This makes the mechanism a second-order reaction. The rate law is described as Rate = k [substrate][nucleophile] (Carey 331). If a large excess of the nucleophile is present then the law would be a pseudo-first order reaction (false-first order) (Smith 390). But in this experiment, this will not be observed. SN2 reactions occur mostly with the substrates being a methyl structure or a primary and sometimes a secondary structure. These structures allow for a backside attack of the nucleophile to take place. The mechanism rarely proceeds with a tertiary structure. This is due to the steric hindrance that does not allow the back attack of the nucleophile, because of the decreased availability to attack the carbon atom (390). Usually a good leaving group on the substrate, such as a halogen, is present for substitution, but in this experiment the halogens will be acting as the nucleophiles. The nucleophiles of SN2 reactions must act as a Lewis base. The stronger the Lewis base, the fas