exp9w08

exp9w08 - Experiment 9: An Unusual SN2 Displacement of a...

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Experiment 9: An Unusual S N 2 Displacement of a primary alcohol: Synthesis of n-Butyl Bromide Mohrig, Chapter 20 As was demonstrated by the previous phenacetin synthesis, substitutions reactions are useful. The synthesis of n -butyl bromide, which is carried out in this experiment is an un- usual and rarely used example among this class of reactions. Unlike 95% of most S N 2 reactions, this synthesis of n-butyl bromide will be carried-out under acidic conditions. In most cases, acidic conditions favor a S N 1 mechanism and proceed through a carbocation intermediate. For example, because of its S N 1 mecahnism, racemization occurs during the conversion of 2-butanol into 2-bromo-butane. NaBr H 2 SO 4 OH Br nonracemic racemic CH 3 H Br makes anhydrous HBr S N 1 mechanism In the preceding example, and the example below, bromide is a good nucleophile. How- ever, hydroxide (HO-) is a poor leaving group. Indeed, when sodium bromide is com- bined with 1-butanol or 2-butanol without other additives, no substitution occurs. Once protonated, however, the leaving group can leave as water (H 2 O). In the preceding ex- ample, this occurs to give a secondary cation (the rate determining step, in a S N 1 reac- tion). In the process below, however, departure would result in a very unstable primary carbocation. The protonated leaving group instead departs with the help of the nucleo- phile (bromide ion) [rate depends on concentrations of both components, S N 2]. OH NaBr H 2 SO 4 Br S N 2 mechanism please regenerate this completed table in part 1 of your notebook reagents and products butyl alcohol Water sodium bromide 18 M sulfuric acid butyl bromide formula equiv 1 1.19 2 1 MW 74.12 mg/mmol 102.89 mg/mmol 98.08 mg/mmol 137.02 mg/mmol den 1280 mg / mL vol 1.2 mL 1.2 mL mass 800 mg 1330 mg 2000 mg 1478 mg mmol. 10.79 mmol
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This note was uploaded on 04/07/2008 for the course CHEM 6a taught by Professor Pettus during the Winter '07 term at UCSB.

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exp9w08 - Experiment 9: An Unusual SN2 Displacement of a...

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