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Introduction to Mechanism of Electrophilic α-Substitution

Kinetic studies of these reactions give additional information. The halogenation and isotope exchange's rates are must be similar (assuming similar concentrations and catalysts), and are similar to the rate of racemisation for those reactants having chiral alpha-carbon units. The rate of halogen substitution is proportional (that is first order) to aldehyde or ketone concentration, but the independent of halogen concentration, at low to moderate halogen concentrations. This proposes the presence of a general reaction intermediate, produced in a slow (rate-determining step) prior to the final substitution. Acid and base catalysts act to increment the rate at which the general intermediate is formed, and their concentration also affects the entire rate of substitution.
From preceding experience and knowledge, we can assume that the general intermediate is an enol tautomer of the carbonyl reactant. Various facts support this proposal:

(i)   Compounds which do not have any α-hydrogen atoms cannot enolize and do not go through any of the reactions explained above.
(ii)   The carbon-carbon double bond of an enol is planar, therefore any chirality that presented at the α carbon is lost on enolization. If chiral results are got from enol intermediates they will essentially be racemic.
(iii)   In simple aldehydes and ketones enol tautomers are exist in very low concentration. Reactions which involve enol reactants will therefore be limited in rate by the enol concentration. Increasing the amounts of other reactants will have small influence on the reaction rate.
(iv)   Enolization is catalyzed by acids and bases. So, these catalysts will catalyze reactions proceeding through enol intermediates.

The reactions displayed above, and others to be explained, may be considered as an electrophilic attack on the electron rich double bond of an enol tautomer. This looks like closely the first step in the addition of acids and other electrophiles to alkenes. So, if electrophilic substitution reactions of this type are to occur it is essential that nucleophilic character be established at the alpha-carbon. A full explanation of the acid and base-catalyzed keto-enol tautomerization process (displayed below) discloses that only two intermediate species assure this need. These are the enol tautomer itself and its conjugate base (common with that of the keto tautomer) generally considered as an enolate anion.

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Obviously, the proportion of enol tautomer present at equilibrium is a significant issue in alpha substitution reactions. In the example of simple aldehydes and ketones this is extremely small, as noticed above. A complementary property, acidity of the carbonyl compounds is also significant, because this effects the concentration of the more nucleophilic enolate anion in a reaction system. Ketones like cyclohexanone are much more acidic than their parent hydrocarbons (by at least 25 powers of ten); however they are still very weak acids (pKa = 17 to 21) contrast with water. Together with some related acidities, this is listed in the table. Although enol tautomers are about a million times more acidic than their keto isomers, their low concentration makes this characterstic comparatively not important for many simple aldehydes and ketones.

Acidity of α-Hydrogens in Some Activated Compounds

Compound

RCH2-NO2

RCH2-COR

RCH2-C≡N

RCH2-SO2R

pKa

9

20

25

25

In cases where more than one activating function affects a given set of alpha-hydrogens, the enol concentration and acidity is increased.

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