Preparation and SN1 Reactivity of 2-Bromobutane
Introduction
The overall goal of this experiment is to understand and be familiar with SN1 reactivity. We also learned how to prepare 2-Bromobutane by learning how to distill and extract this product from its organic layer. Finally, another goal was to specifically understand the relative reactivity of alkyl halides under SN1 conditions by reacting the alkyl halide and silver nitrate in ethanol.
Results and Discussion
The Overall Reaction
The proposed mechanism experimental procedure to prepare the 2-Bromobutane you must fill a 100 mL round bottom flask with 20 mL of 12M sulfuric acid and 7. 4 mL of 2-butanol. Then add 8. 0 g of ammonium bromide along with a magnetic stir bar. Then attach the flask to the apparatus, which is mounted on a Thermowell over a stirrer and a condenser attached to it, along with the thermometer in the flask. Then the mixture was heated at 90oC then kept at the range of 90oC-100oC for 30 minutes.
When that is completed add 20 mL of water into the flask to perform a simple distillation to collect the distillate in a graduated cylinder, keep doing this until there is no more 2-Bromobutane collected. Use a Pasteur pipette to draw off the organic layer to another container. Add the potassium carbonate to allow the liquid to dry by swirling it. Once this is completed the reactivity of alkyl halides under SN1 conditions was tested. Adding two drops of our product and the other different compounds into different test tubes along with 1mL of ethanolic silver nitrate reagent.
We are comparing these by seeing how they react and if there is no reaction after 5 minutes we will place the test tubes in a beaker of water that is heated at a temperature of 70oC-80oC and observe what happens.
Experimental Stoichiometry
| Compound | Molecular Weight | Quantity | Moles |
| 2-Butanol | 74. 122 g/mol | 7. 41 mL (6. 01 g) | 0. 081 |
| Sulfuric Acid | 98. 079 g/mol | 20. 1 mL | 0. 242 |
| Ammonium Bromide | 97. 94 g/mol | 8. 02 g | 0. 082 |
The limiting reagent is the 2-Butanol.
Yield Data
| Product Name | 2-Bromobutane |
| Molecular Weight | 137. 02 g/mol |
| Boiling Point Range | 89°C-91°C |
| Theoretical Yield (Moles) | 0. 081 moles |
| Theoretical Yield (grams) | 11. 09 grams |
| Actual Yield (grams) | 6. 84 grams |
| Actual Yield (moles) | 0. 05 moles |
| Percent Yield | 62% |
NMR Table Signal
| * | Chemical Shift (ppm) | Multiplicity | Integration |
| A | 4. 1 ppm | Sextet | 1 |
| B | 1. 85 ppm | Pentet | 2 |
| C | 1. 7 ppm | Doublet | 3 |
| D | 1. 05 ppm | Triplet | 3 |
Reactivity of Alkyl Halides
| Compound | Room Temperature | Heating |
| 1-Bromobutane | After 30 seconds, the mixture was a cloudy whitish color but no precipitate formed | After 5 minutes, it changed into a yellow color with precipitate formed. |
| 2-Bromobutane | After 30 seconds, the mixture was a light yellowish color and a precipitate formed instantly. | Not Heated |
| 2-Bromo-2-methylpropane | After 30 seconds, the mixture was yellow and a precipitate formed instantly. | Not Heated |
| Compound | Room Temperature | Heating |
| 1-Chlorobutane | After 30 seconds, the mixture was clear. | After 5 minutes, the color changed slightly to white and lightly cloudy. |
| 2-Bromobutane | After 30 seconds, the mixture was a white cloudy color and formed a precipitate instantly. | Not Heated |
| 2-Iodobutane | After 30 seconds, the mixture was yellow and precipitated instantly. | Not Heated |
Conclusion
In conclusion, the SN1 reaction of 2-Bromobutane was performed. According to the Yield Data table, when the actual yield and theoretical yield are calculated we can get the percent yield, which was 62%. At least this number was higher than 50%; I can understand how this could have happened. When I was drying the reagent with potassium carbonate, prior to that I had put the other pellets, which were calcium chloride on accident.
This probably affected my percent yield also with the fact that the temperature was not well maintained it kept fluctuating under 90OC. According to the reactivity of alkyl halides, the results of these tables can conclude the theories of the conditions of SN1 reactions. The first table states that the tertiary carbocation is more stable which allows this reaction to perform faster than second and primary structured carbocations. Then the second table can conclude that iodine is the best leaving group because it is a much weaker base than the bromide ion and chloride ion causing it to react faster.
