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  Reactions at the a-Carbon Carboxylic Derivatives Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions at the α-Carbon

Several aldehydes and ketones were found to go through electrophilic substitution at an alpha carbon. These reactions, that included halogenation, the aldol reaction and isotope exchange, occur by way of enol tautomer or enolate anion intermediates, a feature which needs at least one hydrogen on the α-carbon atom. In this section the same reactions of carboxylic acid derivatives will be observed. Formulas for the subsequent enol and enolate anion species that might be generated from these derivatives .

Acid-catalyzed alpha-chlorination and bromination reactions carry on more slowly with the carboxylic acids, esters and nitriles than with ketones. This might reflect the smaller equilibrium enol concentrations found in these carboxylic acid derivatives. However, base and acid catalyzed isotope exchange takes place as expected; some instances are displayed in equations no.1 and 2 below. The chiral alpha-carbon in equation no.2 is racemized in the course of this exchange and small amount of nitrile is hydrolyzed to the subsequent carboxylic acid.

Anhydrides and Acyl halides are more simply halogenated than nitriles and esters, possibly Due to their higher enol concentration. This variation may be employed to facilitate the alpha-halogenation of carboxylic acids. So, conversion of the acid to its acyl chloride derivative is followed by alpha- chlorination or bromination, and the resultant halogenated acyl chloride is then hydrolyzed to the carboxylic acid product. This three-step chain can be reduced to a single step by using a catalytic amount of phosphorus trichloride or phosphorus tribromide, as displayed in equation no.3. This simple alteration works well because acyl chlorides and carboxylic acids swap functionality as the reaction progresses. The final result is the alpha-halogenated acid, accompanied through a trace of the acyl halide. This halogenation method is called the Hell-Volhardt-Zelinski reaction.

 

In an identical way, acetic anhydride serves as a halogenation catalyst for acetic acid (first equation below). The Carboxylic acids which have a higher equilibrium enol concentration do not require to be activated for alpha-halogenation to take place, as illustrated by the substituted malonic acid compound in the second equation. The enol concentration of malonic acid (about 0.01%) is approximately ten thousand times greater than that of acetic acid. This affect of a second activating carbonyl function on equilibrium enol concentrations had been noticed previous in the example of 2,4-pentanedione.

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