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

The data also shows a small percent yield of product. This implies that there were many places for error to occur. Possible errors include the following: Not dissolving the ammonium salts by not heating the solution enough, pouring the n-butyl alcohol into the condenser and allowing some to run down the outside (actual occurrence), having the heat too high when refluxing; this may cause the solution to be lossed from rapid boiling, not allowing the layers to separate completely in the separatory funnel, not venting the separatory funnel while mixing; too much pressure could build up causing the stopper to pop off (actual occurrence), confusing the organic layer with the water layers, not using enough of the drying agents, or not sealing the storage vial tight enough to allow the alkyl halides to evaporate (actual occurrence).

SN2 SN1

). 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 faster the nucleophile will attack the substrate. For the SN2 reaction to occur, the use of a polar aprotic solvent should be used. Ideally, the substrate should remain depronated or else a front side attack of the nucleophile may take place. This is why the non-polar solvent is used. The temperature of the mechanism is low (Carey 322). Once the environme!

The main reagents used in this experiment are n-butyl alcohol, tertiary butyl alcohol, ammonium chloride, and ammonium bromide. The substrates are the two alcohols and the nucleophiles are equal amounts of the chloride and bromide ions from the ammonium salts. Consequently, alcohols do not readily react by simple nucleophilic substitution as halides do. For this reason, of the need to remove the strong hydroxide ion base of the alcohol, the use of an acidic medium must be used. Such an acid is concentrated sulfuric acid. The acid causes a rapid pronation of the alcohol, creating water, then the stable water molecule dissociates from the nucleophile. This allows the chloride and bromide ions to attack the carbocation. Depending on the structures of normal or tertiary, the ions will react with an SN1 or SN2 mechanism (Handout). Because equal molar concentrations of the ammonium salts are added, the conclusion of which is the better nucleophile, the one that can readily attack the substrate at a faster rate, can be attained. The analysis of the percent of alkyl halide products can be achieved with the use of a refractometer.

Errors on the refractometer reading may also occur. This could be from the ignorance of cleaning the prisms on the refractometer, or by incomplete covering of the prism with the alkyl halide solutions.

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,