Sn1 reactions take place in more than one step. One of these steps is slower tha

Question

Sn1 reactions take place in more than one step. One of these steps is slower than the others. The slowest step is the formation of a carbocation. Give the overall stabilities of carbocations. (see page 256) How does an adjacent sigma bond help stabilize the positive charge of a carbocation (page 257) Can stereochemistry be retained in an Sn1 reaction explain (page 258) What are the factors that affect the rate of Sn1 and Sn2 reactions. Don't give specifics just looking for the four basic factors.

Explanation / Answer

13. Carbocations are electron deficient species and thus any group/bond that may stabilize the electron-deficient carbocation would stabilize it. Thus a tertiary carbocation is more stable than a secondary carbocation which in turn is more stable than a primary carbocation species. An allylic and benzylic carbocations have more stablity due to resonance stabilization of the +ve charge of the carbocation.

14. An adjacent sigma bond stabilizes the carbocation by inductive effect. The shared pair of electrons are shifted more towards the carbocation center and are thus stabilized in the process.

15. No stereochemistry cannot be retained in an SN1 reaction. The carbocation intermediate formed is symmetrical and thus the attack of nucleophile may occur from either side of it. The steric bulk of the groups may however, dictate the selectivity ratio of the products formed and may help in retaining some degree of stereochemical outcome of the SN1 reaction.

16. The rate of an Sn1 reaction are enhaced by increasing solvent polarity (polar protic), increasing electron donating groups around the carbocation center, increasing steric bulk of the molecule, having a very good leaving group. All these factors stabilize the resulting carbocation intermediate and hence enhance rate of SN1 reaction.

For an Sn2 reaction, having a polar aprotic solvent, low or no steric bulk around the carbon at which the reaction occurs, good leaving group all enhance rate of SN2 reaction.

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