Nucleophilic Substition 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 nucleophile’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

Starting the lab begins with an assembly of a reflux apparatus. This is needed to carry out a reaction at high temperatures, constantly for long periods of time. The apparatus does not allow any solvent to escape into the atmosphere while boiling (Garner 123). The use of an acid trap is also needed due to the gas created by the heated sulfuric acid. Next, the solvent-nucleophile medium is made. It is composed of concentrated sulfuric acid, ice water, and equal molar amounts of ammonium chloride and ammonium bromide. Since ammonium chloride is only 28.3% soluble at twenty-five degrees Celsius, the solid salts are heated to dissolve, then half of the medium is transferred to a separatory funnel.

The following tables are from the experimentation done in the laboratory:

The data of the lab showed that the bromide ions in SN2 reactions have greater nucleophilicity than that of the chloride ions. Adversely, the chloride ions have greater nucleophilicity than bromide ions in SN1 reactions. In the SN2 reactions, bromide is a better nucleophile because it is a stronger Lewis base than chloride. This is due to the increased distance of the bromine’s electrons from its nucleus (Carey 313). Since the bromide is a larger molecule and there are no side chains on the n-butyl alcohol to cause steric hindrance, the bromide attacks the carbocation at a faster rate than the chloride. These occurrences are evident from the data found from the refractometer reading.

The refractive indices for the SN2 mechanism products of pure 1-chlorobutane and pure 1-bromobutane are 1.4280 and 1.4398 respectively at twenty degrees Celsius. The SN1 mechanism’s pure products of 2-chloro-2-methyl propane and 2-bromo-2-methylpropane have the refractive indices of 1.3877 and 1.4280 at twenty degrees Celsius. Since each of the mechanisms products retain a percentage of each product and no amount of pure product, the refractive index of the SN 2 reactions should read between the pure readings of the 1-chlorobutane and the 1-bromobutane. Also the index of the SN 1 reactions should read between the pure readings of 2-chloro-2-methylpropane and 2-bromo-2-methylpropane (handout). Since this is true, the use of the following graphs could be used to estimate the percentages of each product:

Some topics in this essay:
SN1 SN2, Snell’s Law, Nu-----C------X Nu, CH CH, Celsius SN1, Opposite SN2, SN1 Weight, Theoretical Yield, Competing Nucleophiles, Nu C, bromide ions, n-butyl alcohol, separatory funnel, alkyl halide, refractometer reading, sn1 sn2, sulfuric acid, percent yield, sn2 reactions, refractive indices, chloride bromide ions, tertiary butyl alcohol, ammonium chloride ammonium, concentrated sulfuric acid, chloride ammonium bromide,