Good and Poor Leaving Groups

The better the leaving group, the more can the mechanism tilt towards a mono-molecular (SnI) substitution. With good leaving groups carbocation formation is faster than the Sn2 reaction and becomes the rate determining (slowest) step in the new mechanism. The reaction of the nucleophile with the carbocation is the fastest step in SnI reactions (and much faster than the corresponding Sn2 reaction). 

Acidic reaction conditions can also lead to protonation of some leaving groups, thereby increasing their reactivity in nucleophilic substitutions. Such groups include, for instance, alcohols, ethers, amines, amides, or alkyl fluorides. 

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Scheme 2 A proposed mechanism for Lal+ ( OCH3)2 catalyzed transesterification of carboxylate esters with good and poor leaving groups.

For the purpose of initially identifying good and poor leaving groups, consider development of a practical synthesis of diethyl ether. One route is by way of SN2 displacement using an ethyl compound, CH3CH2X, and ethoxide ion ... 

TABLE 7.4 Some Good and Poor Leaving Groups and Their Conjugate Acids ... 

If tlie 1,2-addition is reversible (the nucleophile is a good leaving group), then we get thermodynamic control and the conjugate addition product predominates. When the 1,2-addition is not reversible (the nucleophile is a poor leaving group), we get kinetic control and simple addition. Stereochemical considerations are also partly responsible, since it will be easier for larger nucleophiles, especially enolate... 

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