74 Effect of Solvent Leaving Group and Nucleophile on S N 1 Effect of Solvent x

74 effect of solvent leaving group and nucleophile on

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7.4 Effect of Solvent, Leaving Group, and Nucleophile on SN1 Effect of Solvent: xThe rate-determining step of an SN1 reaction involves separating charges. The amount of energy required to separate the charges represents the activation energy (EA) for the step.
CHEM 140A 223 ±Any factor that stabilizes the transition state relative to the reactants will lower the activation energy, and therefore speed up the reaction. xThe choice of solvent can dramatically affect reaction rates. Two properties of solvents, their polarity and proticity, are especially important in affecting SN1 reaction rates: 1.Polar solvents help stabilize charged (or ionic) structures via polar interactions. Hence the more polar the solvent, the faster the SN1 reaction. 2.Protic solvents also stabilize the SN1 transition state, especially the leaving group. These solvents stabilize the leaving group via H-bonding and create a solvation shell around it once it has separated. xNitromethane (CH3NO2) is a polar aprotic solvent that has strong ionizing power. Hence it is a good solvent for SN1 reactions. xIn contrast to SN1 reactions, the SN2 transition state is less polar than the reactants. Therefore polar protic solvents will interact with and stabilize the reactants more than the transition state. This will increase the activation energy and slow down the reaction. Practice Problems:
CHEM 140A 224 5.Arrange the solvents in order of fastest to slowest for SN1 reactions: (a) fluoromethane, iodomethane, bromomethane; (b) CH3OCH3, hexane, CH3OH 6.Determine whether each of the following reactions will occur faster in a protic or aprotic solvent. Effect of Leaving Group: xSince leaving group departure occurs during the first (or rate-limiting) step of an SN1 reaction, the rate of reaction increases with increasing leaving group ability. In other words, the better the leaving group, the faster the SN1 reaction.Effect of Nucleophile xSince the nucleophile is not involved in the rate-determining step, the concentration or strength (i.e. nucleophilicity) of the nucleophile has no effect on the reaction rate. xHowever, when two or more nucleophiles are present in a reaction, their relative strength may affect the ratio of the products formed. Hence nucleophilicity is only important when there are two or more nucleophiles competing for the same carbocation. 7.5 Effect of Alkyl Group: Carbocation Stability xSN1 reactions occur readily on substrates that form stable carbocations upon departure of the leaving group. xTwo general features that can help stabilize carbocations are hyperconjugation and resonance. 1.Carbocations surrounded by alkyl groups are stabilized through hyperconjugation. -The order of reactivity for alkyl halides is therefore: Tertiary > Secondary > Primary > Methyl -Methyl and primary halides react so slowly that there is essentially no reaction by an SN1 mechanism.

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