Thermodynamic control semicarbazone formation

The C=0 of cyclohexanone is not deactivated by the electron-releasing C6H5 and does not suffer from steric hindrance. The semicarbazone of cyclohexanone is the kinetically controlled product. Conjugation makes PhCH=NNHCONH2 more stable, and its formation is thermodynamically controlled. In such reversible reactions the equilibrium shifts to the more stable product (Fig. 15-1). 

If we react equimolar amounts of acetaldehyde, benzaldehyde, and semicarbazide, the product initially formed will contain 99.5 % of acetaldehyde semicarbazone and only 0.5 % of benzaldehyde semicarbazone. Under these conditions, the reaction is subject to kinetic control, the products being determined by the rates at which they are formed since the backward reactions are much slower. If, however, the reaction mixture is allowed to stand, so that the backward reactions can take place and lead to equilibrium, the product now contains 90% of benzaldehyde semicarbazone and only 10% of acetaldehyde semicarbazone. The reaction is then said to be subject to thermodynamic control, the proportion of products being determined by their relative stabilities rather than their rates of formation. 

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