The Stereochemistry of SN1 Reactions

Although many SN1 reactions proceed with racemization, many others result in more inversion of configuration in the product than retention. This is a result of the extremely short lifetime of the carbocation. When the carbocation is first formed, the leav- 

O The carbocation formed in this Sn I reaction is sp2 hybridized and planar. It is not chiral, so the products formed from it must be racemic The nucleophile—water in this case—can approach equally well from either side, resulting in the formation of equal amounts of the R and S enantiomers of the product. Note that it is water, not hydroxide ion, that acts as a nucleophile because the concentration of hydroxide ion in neutral water is extremely low. 

0 In the final step of the reaction, a proton is removed from the protonated alcohol by a base in the solution. This base is often a molecule of the solvent, water in this case. 

16% of the ion pairs react with the nucleophile, acetate ion. Because the chloride is blocking one side of the ion pair, acetate can approach only from the opposite side. Therefore, only the product with inverted configuration is formed when the nucleophile reacts at this stage- 

Stereochemistry of the Sn I reaction of (S)-1 -chloro- I -phenylethane in acetic acid 

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First-Order Nucleophilic Substitution The S j1 Reaction 246 Key Mechanism 6-4 The Sn1 Reaction 247 6-14 Stereochemistry of the Sn1 Reaction 250 6-15 Rearrangements in the Sn1 Reactions 252... 

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