Synthesis of Bromopentane According to the Scheme - 3-Bromopentane Only 2 Peaks

Synthesis, Pages 3 (653 words)

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The synthesis of scheme 2 led to a 54.4% yield when producing 2.8g of 2-bromopentane. Both sodium bromide and sulfuric acid are used in excess in order for the reaction to be pushed to form the product. The sulfuric acid allows for the hydroxyl group to become water, forming a better leaving group in this SN1 reaction. Due to this reactant 2-pentanol being a secondary substrate it can follow either an SN1 mechanism or an SN2 mechanism. Due to the formation of two products, it follows the SN1 mechanism by forming a carbocation intermediate and allowing for rearrangement.

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Due to 2-bromopentane being less sterically hindered it is considered the major product while 3-bromopentane with more steric hindrance is considered minor. 2-bromopentane is formed by the following procedure.

A mixture of starting alcohol, sulfuric acid, and sodium bromide was added to a round bottom flask with. The round bottom flask was refluxed in a hot water bath at the start of boiling for 40 minutes allowing for the activation energy to be overcome and allowing for the reaction between the reactants to occur.

The mixture was allowed to cool completely to room temperature for a week before separating to avoid the formation of crystals when added to the separatory funnel. The separation included two washes of water and one wash of sodium bicarbonate. This showed two distinct layers, verifying the presence of both products and reactants. Once separated, the mixture was dried with sodium sulfate removing any traces of leftover water or impurities to form the purest form of the product.

The product was pipetted out from the sodium sulfate and was determined to be 2-bromopentane, which was then analyzed. The final product was run through IR, TLC, and 1HNMR in order to be analyzed further. The IR indicated an sp3 C-C bond by a broad peak at 2964.58 cm-1. The lack of an OH peak indicated that there was no starting material left in the sample. The 1HNMR verified the presence of both products, 2-bromopentane and 3-bromopentane. Due to 3-bromopentane having symmetry, only 2 signals are expected when run through 1HNMR while 2-bromopentane is expected to have 5 signals when ran.

The results of 3-bromopentane had only 2 peaks while 2-bromopentane only had 3 peaks. The peak at 0.5ppm indicated a triplet at the terminal CH carbon of 2-bromopentane. The next peak was found at 1.15ppm, indicating a triplet at the two symmetrical terminal CH carbons on 3-bromopentane. The peak at 1.7ppm was identified as a doublet found on 2-bromopentane at the terminal CH carbon near the bromine attachment. The last two peaks were very low intensity, one at 3.95ppm and the other at 4.15ppm. The peak at 3.95ppm indicated a pentet at the CH carbon bearing the bromine bond in 3-bromopentane. The peak at 4.15ppm indicated a sextet at the CH carbon bearing the bromine atom in 2-bromopentane. The gas chromatography results showed a peak with a retention time of 3.2 minutes, being the product formed. There was also a peak with a retention time of 2.4 minutes and that was determined to be the DCM that was added to the sample before running. There were also two peaks that had retention times of 4.1 minutes and 6.1 minutes that potentially could have been starting material.

Due to neither the IR and 1HMR verifying no starting material remaining in the sample, these peaks were inconclusive. Several errors occurred through the process of this reaction allowing the adjustment to create the purest, highest yield product. If the starting materials and the reagents were heated too quickly for too long the solution immediately turned a dark brown/black color allowing for little to no yield from the reaction. Along with that, if the starting material was not completely removed from the product, it would become impure. This was avoided by adding enough sulfuric acid in order to push the reaction to products, while also adding sodium sulfate in order to remove any remaining aqueous starting material.