Ketones halogen-atom substituted

Table V-H-1. Rate coefficients (k, cm molecule s ) for reaction of OH radicals with halogen-atom-substituted ketones...

Figure 5 (a)-(c) shows the absorption spectra of some halogenated ketones in the near ultraviolet. In the case of acetone itself the absorption may be attributed to an n - tt transition associated with the carbonyl group. The substitution of halogen atoms leads to an increase in the molecular extinction coefficient and a shift of the absorption maximum toward the red. The ketones containing both chlorine and fluorine atoms show absorption curves with some fine structure but it is not possible to find any correlation between the structure and the shape of the absorption curve. 

The synthetic utility of the Favorskii rearrangement is somewhat reduced by side-reactions including substitution of the halogen atom, elimination to form unsaturated ketones and epoxide formation. However, if the starting ketone is polyhalogenated, then... 

The mechanism of the Hantzsch synthesis has been established and is shown in Scheme 165. Substitution of the halogen atom of the a-halo ketone by the sulfur atom of the thioamide occurs first to give an open-chain a-thioketone (232), which under transprotonation proceeds to give a 4-hydroxy-A2-thiazoline (233) in aprotic solvents, or a thiazole (234) by acid-catalyzed dehydration of the intermediate thiazoline in protic solvents. 

Steps [3] and [4] result in a nucleophilic substitution reaction of a ketone. Because ketones normally undergo nucleophilic addition, this two-step sequence makes the haloform reaction unique. Substitution occurs because the three electronegative halogen atoms make CX3 (CI3 in the example) a good leaving group. 

Halogen atoms are well-known initiating species (33) and can be readily generated by photolysis of a wide range of suitably substituted phenyl ketones. Examples of those which have been reported to be efficient photoinitiators are given below (7). 

The Hantzsch synthesis involves three intermediate steps. In the first, the halogen atom of the or-halo aldehyde or a-halo ketone is nucleophilically substituted. The resulting iS-alkyliminium salt 2 undergoes a proton transfer (2 -> 3) cyclization produces a salt of a 4-hydroxy-4,5-dihydrothiazole 4 which is converted into a 2,5-disubstituted thiazole 1 in protic solvents by an acid-catalysed elimination of water. 

This carbanion is capable of leaving because its negative charge is stabilized by the combined inductive effects of the three halogen atoms. It is still a moderately strong base with a pA., in the mid-teens. Proton transfer between this anion and the carboxylic acid gives the final products of the reaction, the salt of the acid and the haloform. bromoform (CHBrs). The same process occurs with trichloro- and triiodo-substituted methyl ketones, to give chloroform and iodoform, respectively. 

Substitution of the halogen atom of Nbd-Cl by a hydrazine residue yields a reagent analogous to Dns-H [404] which is also suited to the determination of aldehydes and ketones. The hydrazones are formed in 99% yield. 

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