1. Account for the differences in the results for both sn1 and sn2 reactions. yo
ID: 902868 • Letter: 1
Question
1. Account for the differences in the results for both sn1 and sn2 reactions. your answer needs to address substrate structure, nucleophiles, reaction conditions, and proposed reaction mechanisms.
2. Explain how the phase trasnfer catalyst works and give a rationale why it was needed. (Diagram is ok)
3. why was sn2 reaction passed through a plug of silica gel to extract?
4. what is the function of the h2SO4/ice mix in the beginning of the sn1 rxn?>
SN1 data:
RBr(%) RCl(%)
1.74ppm 1.56ppm
39.59 60.41 56.18 43.82 33.55 66.44 44.24 55.76 53.76 46.24 19.05 80.95 43.22 56.78 43.47 56.52 44.83 55.16 21.23 78.77 84.70 14.60 27.62 72.36 33.89 66.11Explanation / Answer
1) The differences in the results for both sn1 and sn2 reactions is seen. In SN2 reaction the rate of reaction is dependent upon the nature of substrate and the type of nucleophile used in the reaction. The order of reactivity for the alkyl halde for SN2 reacton is : primary > secondary > tertiary. The reaction mechanism involves a five-membered transition state formation, so as the steric hindrance around the carbon containing the leaving group increases, the energy of transition state increases and so the rate of SN2 reaction decreases. Also in case of SN2 reaction stronger is the nucleophile faster is the rate of reaction, so for halides the nucleophilicity for SN2 reaction follows the order : I- > Br- > Cl- > F-. So iodine has the fastest rate and F- has the slowest rate. a polar aprotic solvent favors SN2 reaction.
In case of SN1 reaction, the rate determinng step involves only the alkyl halide. the halide leaves forming acarbocation. The stabiity of the carbocation determines the rate of reaction that is the order of alky halide reactivity : tertiary > secondary > primary. So tertiary alkyl halide with alkyl group with electron donating ability by inductive effect, shows highest rate of SN1 rection and primary the lowest rate. The type of nucleophile employed for SN1 reaction does not effect the rate of reaction. A polar protic solvent would favor the formation andstabilization of the cabocation generated in SN1 reaction.
2) Phase transfer catayst helps a ionic salt which is insoluble in organic phase containing the substrate to reach the substrate by acting as the surfactant. When a reaction is run say alkyl halide and NaCl are mixed together, the alkyl halide will have greater solubiity in organic phase and NaCl will have very little solubility in this phase, so no reaction or extremely slow reaction would occur. To improve the rate of this reaction a phase transfer catalyst is used, which complexes with the ionic reagent and helps it move in the organicphase. the reaction occurs at the interface of the organic and the aqueous phase. thus we require a phase transfer catalyst.
3) In an SN2 reaction, we form the salt like NaBr as oe of the products of the reaction. To remove this salt we use, a silica plug which acts as the filter and holds the salt while allowing the organic product in the solvent to pass through it.
4) H2SO4/ice mix is employed for the first step of the SN1 reaction so as to generate the carbocation from the alkyl halide. The ice ensures the temperature of the reaction does not go beyond a certain limit.
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