Specific base catalysis

The other side of the coin is specifi c base catalysis (SBC). SBC usually involves the removal of a proton from the substrate in a fast pre-equilibrium step followed by a rate-determining reaction of the anion. Most of the base-catalysed reactions you are familiar with work by SBC. Examples include opening of epoxides with thiols.

As with SAC, the rate of the reaction depends on the pH of the solution. If it is around or higher than the pKa of the thiol, thiolate anion will be formed and this opens the epoxide much faster than does the unionized thiol. The nucleophile is then regenerated by the oxy anion produced in the rate-determining step. It is quite common for specifi c acid and specifi c base catalysis to operate on the same reaction, depending on the pH at which the reaction is carried out. In fact.

The pH–rate profile for the hydrolysis of a simple ester such as ethyl acetate shows just two straight lines meeting each other (and zero rate) at about neutrality. Ethyl acetate hydrolysis occurs by SAC or SBC only.

Removal of a proton from heteroatoms by heteroatom bases is never rate determining because it is always fast, but removal of a proton from carbon can be the rate-determining step. A remarkably large inverse solvent deuterium isotope effect was found with this elimination of a tertiary amine in basic solution.

The detailed mechanism cannot be E2 or the isotope effect, if any, would be the other way round. With SBC, however, the mechanism can be E1cB having a carbanion as an intermediate.

The isotope effect observed is certainly inverse (the reaction is faster with H₂O than D₂O) but the magnitude of the effect is too large to be a solvent isotope effect and looks much more like an inverse kinetic isotope effect. And so it is. The tertiary amine is not a very good leaving group in spite of its positive charge (pKa of R3NH+ is about 10) so the carbanion mostly reverts to starting materials. The isotope effect is a kinetic isotope effect on this reverse step—the protonation of the carbanion. This reaction involves a proton transfer from H₂O or D₂O and will be much faster (7.7 times in fact) in H₂O due to an ordinary kinetic isotope effect. The elimination reaction goes faster in D₂O because the back reaction goes more slowly and more of the carbanion goes on to product.

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عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

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مواضيع ذات صلة

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

The 18-electron rule

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation

Specific acid catalysis

Entropy of activation

The kinetic isotope effect

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values