Some important considerations that affect all alkylations
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص584-585
2025-06-25
581
The alkylations in this chapter will each consist of two steps. The first is the formation of a stabilized anion—usually (but not always) an enolate—by deprotonation with base.
The second is a substitution reaction: attack of the nucleophilic anion on an electrophilic alkyl halide. All the factors controlling SN1 and SN2 reactions, are applicable here.
In each case, we shall take one of two approaches to the choice of base.
• A strong base (with a conjugate acid of pKa greater than that of the carbonyl compound) can be chosen to deprotonate the starting material completely. There is complete conversion of the starting material to the anion before addition of the electrophile, which is added in a subsequent step.
• Alternatively, a weaker base may be used in the presence of the electrophile. The weaker base will not deprotonate the starting material completely because its conjugate acid has a lower pKa than the carbonyl compound: only a small amount of anion will be formed, but that small amount will react with the electrophile. More anion is formed as alkylation uses it up. The second approach is easier practically (just mix the starting material, base, and electrophile), but works only if the base and the electrophile are compatible and don’t react together. With the first approach, which is practically more demanding, the electrophile and base never meet each other, so their compatibility is not a concern. We shall start with some com pounds that avoid the problem of competing aldol reactions completely because they are not electrophilic enough to react with their own nucleophilic derivatives.
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