Sn1 Sn2 Reactions

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Experiment 15 & 16:Preparation of 1-bromobutane, an SN2 reaction Preparation of 2-chloro-2-methylbutane, an SN1 reaction

Introduction

The purpose of this experiment is to synthesize 1-bromobutane from 1-butanol and sodium bromide. In order for this reaction to reach completion there are four major operations that need to be performed. The four major operations include refluxing, simple distillation, separation, and drying.

To begin, in order for the compounds to react they will be dissolved in water and sulfuric acid will be added. The addition of sulfuric acid will then generate hydrobromic acid, an important product in the reaction mixture. The hydrobromic acid will react with the 1-butanol when heat is added to the flask during refluxing. Refluxing is the heating of a flask to boiling and then allowing the vapors to condense and run back into the reaction flask. Refluxing is a good way of keeping a reaction at a constant temperature.

After refluxing is complete the reaction is distilled by the method of simple distillation. The simple distillation process is used to help remove and purify a particular substance, in this case the 1-bromobutane, from other components in the reaction flask. However, since the boiling point of 1-bromobutane is 101°C and the boiling point of water is 100°C it would be expected that there would be a large amount of water that will distill out of the flask along with 1-bromobutane. Other remaining reactants such as a small amount of alcohol and/or acid may be distilled out along with the water of 1-bromobutane.

Since there is such a high probability of other components in the distillate, separation processes will need to be carried out. Water is first added to the 1-bromobutane layer to remove any leftover starting material and after the 1-bromobutane layer is isolated sodium hydroxide, water, and sodium chloride solutions are added to remove any remaining acid. Drying using anhydrous sodium sulfate is then used to remove any remaining water in the solution and then the simple distillation process is performed again on the dried 1-brombutane. It would be hypothesized that the amount of recovered 1-bromobutane will be relatively low since there are many techniques involved which allow for many places for error and product loss. Also, the theoretical yield for the experiment is 38.06%, which is relatively low. After the recovered 1-bromobutane is weighted, infrared spectroscopy will be performed. It would be hypothesized that the 1-bromobutane spectrum would produce peaks at the characteristic alkane and halogen peaks. SN2: An SN2 reaction mechanism occurs when a nucleophile directly attacks the substrate at an angle of 180 degrees to the C-L bond. The C-L bond is then broken with the formation of the C-Nu bond. The substrate, R-L, and C-Nu are all involved in the transition state which is the rate determining step. With two reactants being involved in the transition state it is bimolecular making is an SN2. The larger the substrate and how the more condensed the skeletal structure of the more difficult it is for the nucleophile to attack the carbon, making the reaction take longer. Nucleophilicty parameters are components in a second order rate law reaction that is a constant for a given nucleophile. In order for the desired reaction to occur the leaving groups in nucleophilic substitution must be weakly basic, if a strong acid isn’t present the reverse reaction will occur. SN1: An SN1 reaction mechanism occurs in two successive steps. The first is the ionization of the C-L bond. The second step is bond formation of the C-Nu bond; this is very quick making the first step the rate determining step. Since the reaction rate depends only on the concentration of the substrate it is uni-molecular making it SN1. The substrates that tend toward SN1 reactions are ones that have their carbon mostly blocked off by bulky R groups rather than just hydrogen’s.

Materials and Methods...
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