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Lab 7 - Nucleophilic Substitution Reactions of Alkyl...

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Nucleophilic Substitution Reactions of Alkyl Halides Experiment #8 Dan Guerra 11/13/11 TA: Yi and Luke Section: 6
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Purpose: The purpose of this experiment is to assess the factors that influence the rate of reaction for S N 1 and S N 2 reactions which include: the structure of the alkyl halide (primary and secondary), the nature of the leaving group and the polarity of the solvent being used. Results and Discussion: Primary Structure : For this part of the experiment, the effect of primary structure on the reactivity in nucleophilic substitution of alkyl halides in compounds (1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane and bromobenzene) was tested using two different solvents, NaI in Acetone (S N 2 reaction) and AgNO 3 in Ethanol (S N 1 reaction). In the case of S N 2 reactions, a nucleophile displaces the leaving group through a Compound (NaI in Acetone: SN2) Room Temp. 50˚C No reaction Time (seconds) 1-bromobutane X 50 2-bromobutane X N/A 2-bromo-2- methylpropane X 800 Bromobenzene X N/A Compound (AgNO3 in Ethanol: SN1) Room Temp. 50˚C No reaction Time (seconds) 1-bromobutane X 180 2-bromobutane X 20 2-bromo-2- methylpropane X Immediately Bromobenzene X N/A
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“backside attack” on the molecule. Therefore, the less sterically hindered the nucleophile by substrates attached to the carbon being targeted, the faster the reaction occurs. The order of reactivity for S N 2 reactions from slowest to fastest is: tertiary< secondary< primary< methyl. The expected results in order of reactivity for the tested compounds in increasing order was: Bromobenzene < 2-bromo-2-methylpropane < 2-bromobutane < 1- bromobutane. In the case of S N 1 reactions, a carbocation intermediate is formed by the detachment of the leaving group, therefore the more stable the carbocation intermediate, the faster the rate of reaction; the more substrates the carbon has the better it can stabilize the positive charge through induction and hyperconjugation. The order of reactivity for S N 1 reactions from slowest to fastest is: methyl< primary< secondary<tertiary. The expected results in order of reactivity for the tested compounds in increasing order were: bromobenzene< 1-bromobutane< 2-bromobutane< 2-bromo-2-methylpropane.
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