Acid and base catalysts act to increase the rate at which the common intermediate is formed, and their concentration also influences the overall rate of substitution.įrom previous knowledge and experience, we surmise that the common intermediate is an enol tautomer of the carbonyl reactant. This suggests the existence of a common reaction intermediate, formed in a slow (rate-determining step) prior to the final substitution. first order) to aldehyde or ketone concentration, but independent of halogen concentration. At low to moderate halogen concentrations, the rate of halogen substitution is proportional (i.e. The rates of halogenation and isotope exchange are essentially the same (assuming similar catalysts and concentrations), and are identical to the rate of racemization for those reactants having chiral alpha-carbon units. Kinetic studies of these reactions provide additional information. Mechanism of Electrophilic α-Substitution This is demonstrated convincingly by the third ketone, which is structurally similar to the second but has no alpha-hydrogen. Second, the substitutions are limited to hydrogen atoms. The second ketone confirms this fact, only the alpha-carbon undergoing substitution, despite the presence of many other sites. Depending on the reaction conditions, one or all four of these hydrogens may be substituted, but none of the remaining six hydrogens on the ring react. Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens). First, these substitutions are limited to carbon atoms alpha to the carbonyl group. Two important conclusions may be drawn from these examples. Not all carbonyl compounds exhibit these characteristics, the third ketone being an example.įigure 1: Reactions at the \(\alpha\) carbon Indeed, treatment of this ketone reactant with acid or base alone serves to racemize it. If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic. These reactions are acid or base catalyzed, but in the case of halogenation the reaction generates an acid as one of the products, and is therefore autocatalytic. Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |