Acetone, general acid catalysis

Hydride transfer from [(bipy)2(CO)RuH]+ occurs in the hydrogenation of acetone when the reaction is carried out in buffered aqueous solutions (Eq. (21)) [39]. The kinetics of the reaction showed that it was a first-order in [(bipy)2(CO)RuH]+ and also first-order in acetone. The reaction proceeds faster at lower pH. The proposed mechanism involved general acid catalysis, with a fast pre-equilibrium protonation of the ketone followed by hydride transfer from [(biPy)2(CO)RuH]+. 

It has been proposed (79) that eaq reacts with Bronsted acids following the Bronsted general acid catalysis law. The work was based on the competition of four acids with acetone and with I- for e aq. The acids... 

A prerequisite that the mechanism in a series should be the same is a linear free-energy correlation if a member of the series takes a different pathway then the alternative mechanism will be more favourable, will predominate over the normal and will be diagnosed as a positive deviant point. An example of this behaviour is the demonstration that general acid catalysis by primary amines of the iodination of acetone differs from that by carboxylic acids (Fig. 24) [3a] simple proton transfer is identified with the carboxylic acid catalysis and it is thought that amines utihse Schiff s base formation (Eqn. 116) leading to a more favourable path. 

The addition of thiols to ketones shows very interesting kinetics. One finds specific-base catalysis of the addition step, whereas there is also general-acid catalysis for the reaction. Given this, propose a mechanism for the addition of ethane thiol to acetone in aqueous media. Explain why thiols may be expected to show specific-base catalysis in their addition, but that the nature of a thiol also leads to general-acid catalysis of the reaction. 

The reaction between acetone and iodine was one of the first reactions for which general acid catalysis was established. 

The reactions are accompanied by a considerable volume change, and a dilatometric method was employed by Bell and Higginson (1949), who added acetaldehyde-water mixtures (containing about equal quantities of MeCHO and MeCH(OH)2) to an excess of acetone, and thus measured kj, in presence of a large number of acid catalysts. The direct hydration of acetaldehyde in aqueous buffer solutions is inconveniently fast at room temperatures, but ( (j + A ) was measured dilatometrically at 0°C by Bell and Darwent (1950), who established the existence of general acid-base catalysis. 

One of the very first kinetic examples of this catalyzed prototropy was found in the halogenation of acetone. In polar solvents, it is found that the rate of halogenation of acetone is first-order in acetone, zero-order in halogen, X2, and subject to general acid-base catalysis ... 

The aldol condensation can also be brought about with acid catalysis. There are many aldol-like reactions which involve an aldehyde or ketone. However, the word aldol is a common name for the product of these reactions. Generally, the word aldol is used to refer to any P-hydroxyaldehyde or (3-hydroxyketone. For example, treatment of acetone with a base results in the aldol reaction. The product, -hydroxyketone 3.14, is separated from the reaction mixture as it is formed. 

This occurs by acid catalysis, but generally the immediate product, a furfuryl alcohol, reacts further- 2-(3,3,3-trichloro-l-hydroxy)ethylfuran can however be isolated. A macrocycle can be obtained by condensation with acetone via a sequence exactly comparable to that described for pyrrole (section 13.1.7). 

We should not expect that a given reaction will exhibit only general acid (or base) catalysis or specific acid (or base) catalysis. In principle, reactions may be subject to more than one kind of catalysis. For example, the catalytic rate expression for the reaction of iodine with acetone in buffer solutions was determined to be... 

Aldol reactions can occur under acid catalysis, in which case the reaction generally leads to the a,/3-unsaturated product by direct dehydration of the j8-hydroxy aldol intermediate. This is one way by which ketones can successfiiUy be utilized in an aldol reaction. The following is an example, in which acetone forms its aldol condensation product, 4-methylpent-3-en-2-one, on treatment with hydrogen chloride. 

Since keto-enol tautomerism is accelerated by both acids and bases (the case of general acid-base catalysis), the reaction rate increases with both pH increase and decrease (V-shaped dependence). For example, the rate of acetone enolization in an acetate buffer is related to concentrations of die reactants by the following expression (H20,298 K) ... 

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