Halogenation in acid

Problem 22.9 Why do you suppose ketone halogenations in acidic media referred to as being ackl-calalyzed, whereas halogenations in basic media are base-promoted In other words, why is a full equivalent of base required for halogenation ... 

We do, of course, need a way of making tiie mono bromo derivative of a ketone, and it turns out that we can do this by halogenation in acid solution. Take the same ketone, PhCOMe, make the enol in acid solution, and react it with bromine. 

Halogenation in acid medium is acid-catalyzed because hydrogen ions are regenerated ... 

Problem 22.9 Why do you suppose ketone halogenations in acidic media referred to as being acid-... 

The mechanisms of halogenation in acid and base are somewhat different. 

In their speed of reaction with the halogens, in acid or in alkaline solutions, the simple sugars may be divided into two main classes, the aldoses and the ketoses. The oxidation of the former is very rapid compared with that of the latter. Kiliani showed that when D-glucose and D-fructose were each treated with an equal weight of bromine in water, the ketose required 350-500 hours for completion of the oxidation, in contrast to two to three hours for the aldose. The same difference exists in buffered solutions Honig and Ruzicka found that the rates were two hours and five minutes, respectively. In alkaline solution under controlled conditions, the aldoses can be quantitatively oxidized with sodium hypoiodite in the presence of D-fructose or L-sorbose without appreciable attack on the ketoses. Ochi reported a similar but less clear-cut difference with calcium hypochlorite as the oxidant. Chlorous acid attacks only the aldoses, leaving the ketoses unaltered. The same effect was noted with the keto acids Kiliani reported that 2-keto-L-rhamnonic acid was stable to the action of bromine water. A little preliminary work has been done with iodic acid by Williams and Woods who found that D-fructose was oxidized more rapidly than the aldoses. No confirmation of this work has appeared. 

Free halogens in acid media can be reduced to the corresponding halide ions at the mercury electrode or at various solid electrodes. However, this electrode reaction is not convenient for analytical purposes. Therefore halogens are very often determined indirectly after reaction with some reducing agents [3, 16]. Iodine can be oxidized to iodate which gives a well defined cathodic wave. 

In pyrimidines, a 4-alkyl- is deprotonated more readily than a 2-aUcyl-gronp here again one sees the greater stability associated with a y-qninonoid resonating anion. Side-chain radical halogenation selects a pyrimidine-5-methyl over a pyrimidine-4-methyl the reverse selectivity can be achieved by halogenation in acid solntion - presnmably an iV-protonated, side-chain-deprotonated species, i.e. the enamine tantomer, is involved. ... 

Aldehydes or ketones with at least one a-hydrogen react with halogens in acid or base to form a-haloaldehydes and a-haloketones. 

We have seen that ketones will undergo alpha halogenation in acid-catalyzed conditions. A... 

Ketones and aldehydes will undergo alpha halogenation in acidic or basic conditions. 

---