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Experiment 8: Markovnikov Addition of HBr to 1-HexeneBy Jason LeeAim:To investigate the regiochemisty of ionic addition of hydrogen halide by reacting HBr with 1-hexene, and subsequently to ascertain whether the addition conforms to Markovnikovs rule.To investigate the presence of 2-bromohexane in the product by noting the relative rates of reaction of the product in AgNO3/Ethanol test and NaI/Acetone Test.Hypothesis:If the distillate is collected from temperature of 80 oC and onwards during simple distillation of crude product, then the distillate will contain 2-bromohexane.

If the product contains 2-bromohexane, then the time taken for a white precipitate to form in a AgNO3/EtOH Test done on the product will be less than 30 seconds*.

If the product contains 2-bromohexane, then the time taken for a white precipitate to form in a NaI/Acetone test done on the product will be more than 60 seconds*.

*: Time range extracted from results of Exp 3:Reactivities of Alkyl halide done on 14/2/2014.Discussion and Conclusion P1:With justification from Table 1 and Table 2, all proposed hypotheses were proven invalid. Referring to Table 2(a) , hypotheses 2 was incorrect for the time taken for white precipitate to form was longer had expected. Referring to Table 2(b) , hypotheses 3 was proven incorrect when the time taken for white precipitate to form was drastically shorter than had expected. Finally, hypothesis 1 was invalid for the product collected from 80oC onwards was not 2-bromohexane. This was clearly proven by the confirmation test which failed as well. In conclusion, 2-bromohexane was not successfully synthesized and this synthesis did not adhere to Markovnikovs rule.Discussion and Conclusion P2:Theoretical Result: 2-bromohexane ( Markovnikovs Rule)Experimental Result: Confirmation test results bore a storng resemblance to that of a 1o alkyl halide. Hence, the possible product formed was 1-bromohexane.Sources of error:Firstly, the distillate vapor was not cooled effectively. When the temperature of the system hit about 90oC, the overwhemling amount of white vapors of distillate was not cooled completely by the condenser, causing them to be lodged in the vacuum adapter. Cooling had to be done poorly by wrapping the adapter with cold cloth. This ineffective cooling might have caused any 2-bromohexane to not be condensed, decreasing any chances of obtaininig the desired product. Secondly, the temperature of the system shot up to 120oC from time to time, causing any unwanted sideproducts and solvents to be distilled as well.

Data and Analysis:

Discussion and Conclusion P3:Markovnikovs RuleThis rule states that the hydrogen will add to the carbon of the C=C bond that already has the most number of hydrogen. This also forms the more stable carbocations(2o/3o) which has the lower activation of free energy. Hence, addition of HBr to 1-hexene will give 2-bromohexane.(8.2A, 8.2B, Organic Chemistry, Solomons 10th ed). However, this was not observed in this experiment for Anti-Markovnikov took charge of synthesis and gave 1-bromohexane. Anti-markovnikov involves formation of radical inermediate instead of carbocation. (10.9,Organic Chemistry, Solomons 10th ed).SN2 reaction of NaI/Acetone TestBeing an SN2 reaction, NaI/acetone test is favored by unhindered substrate with good leaving group that is a weak base, and a weak base nucleophile such as iodide ion. In addition, polar aprotic solvent also favors SN2 reactions. In this experiment, the ease at which the product reacts with NaI in this test indicates that the product is a 1o alkyl halide. If 2-bromohexane were collected, it being a 2o alkyl halide will take a much longer time to react because it is more hindered due to steric effects. (6.13A, 6.13 B, 6.13 C, Organic Chemistry, Solomons 10th ed).

ProcessObservationInterpretationSimple distillation of crude product Distillate was collected from temperature of 80oC onwards when crude product underwent simple distillation. It may seem as if 2-bromohexane has been successfully formed. However, further conformation test proved otherwise.Table 1: The first fraction containing solvent and unreacted 1-hexene was removed over a range of room temperature to 80oC before collecting the product from 80oC onwards. All observation was observed and recorded.Calculations:Mass of 25-ml receiver flask = 28.50gMass of 25ml receiver flask + product = 31.64gMass of product = 31.64-28.50 = 3.14g

TestObservationInterpretation(a)AgNO3/ Ethanol TestA white precipitate(AgBr) was observed 40 seconds after ethanolic AgNO3 solution was added to the product. The product did not contain 2-bromopropane. Ag/NO3 test is an SN1 reaction. The relatively low reaction rate indicated that the product contained 1-bromopropane instead of 2-bromopropane which would show a higher reactivity otherwise.(b) NaI/ Acetone TestA white precipitate was observed almost instantaneously after NaI in acetone solution was added to the product.The product did not contain 2-bromopropane. NaI/Acetone test is an SN2 reaction. The relatively high reaction rate indicated that the product contained 1-bromopropane instead of 2-bromopropane which would have an otherwise lower reactivity.Table 2: The reactivity of the product towards the confirmation test was based on the relative time taken for white precipitate to appear. All observations were recorded and accounted for by interpretation of result.Mass of 1-hexene = volume x density= 6.00 x 0.67= 4.02gNo. moles of cyclohexanol = mass/molar mass= 4.02/84.16= 0.048 mols1 mole of 1-hexene produces 2-bromohexene . Hence, 1:1 ratio leads to 0.192 moles for 2-bromohexene.Mass of cyclohexene= moles x molar mass = 0.048x 165.07= 7.92gMass percentage = (experimental / theoretical)x100% =( 3.14/ 7.92)x100% = 39.63%