Methyl ketones, Haloform reaction

Hagemann s-ester a compound prepared by treating a mixture of formaldehyde and ethylacetoacetate with base and then with acid and heat, half-chair conformation the least stable conformation of cyclohexane, haloform reaction the reaction of a halogen and HO — with a methyl ketone, halogenation reaction with halogen (Br, Cl, Ij). 

The haloform reaction is sometimes used for the preparation of carboxylic acids from methyl ketones... 

The methyl ketone shown in the example can enohze in only one direction and typifies the kind of reactant that can be converted to a carboxylic acid in synthetically accept able yield by the haloform reaction When C 3 of a methyl ketone bears enolizable hydro O... 

Haloform reaction (Section 18 7) The formation of CHX3 (X = Br Cl or I) brought about by cleavage of a methyl ketone on treatment with Br2 CI2 or I2 in aqueous base... 

The haloform reaction of unsymmetrical perfluoroalkyl and co-hydroper-fluoroalkyl trifluororaethyl ketones gives the alkane corresponding to the longer alkyl chain [54] (equation 53) If the methyl group contains chlorine, the reaction can take different pathways, leading to loss of chlorine (equation 54), because of the variable stability of the chlorine-substituted methyl carbanions in alkali. 

Methyl ketones are cleaved on reaction with excess halogen in the presence of base. The products are a trihalomethane (haloform) and a carboxylate salt. 

In addition to the applications indicated on p. 858. hypohalous acids are useful halogenating agents for Ixjth aromatic and aliphatic compounds. HOBr and HOI are usually generated in silii. The ease of aromatic halogenation increa.ses in the sequence OCl < OBr < Ol and is facilitated by salts of Pb or Ag. Another well-known reaction of hypohalites is their cleavage of methyl ketones to form carboxylates and haloform ... 

Methyl ketones 1, as well as acetaldehyde, are cleaved into a carboxylate anion 2 and a trihalomethane 3 (a haloform) by the Haloform reaction The respective halogen can be chlorine, bromine or iodine. 

Haloform reaction (Section 22.6) The reaction of a methyl ketone with halogen and base to yield a haloform (CHX3) and a carboxylic acid. 

Fig. 30. Haloform reaction of methyl ketones with BTMA Bt3...

In the haloform reaction, methyl ketones (and the only methyl aldehyde, acetaldehyde) are cleaved with halogen and a base. The halogen can be bromine, chlorine, or iodine. What takes place is actually a combination of two reactions. The first is an example of 12-4, in which, under the basic conditions employed, the methyl group is trihalogenated. Then the resulting trihalo ketone is attacked by hydroxide ion ... 

While the haloform reaction normally only cleaves methyl ketones because of the structural requirements for the a,a,a-tribromomethyl ketone to induce fragmentation, the strain release that accompanies cleavage of a cyclobutanone permits extension... 

Hi) Oxidation of methyl ketones by haloform reaction Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of O corresponding carboxylic... 

Any compound that oxidizes to a methyl ketone also gives a haloform reaction, because halogens are also oxidizing agents. For example, the compound shown in Figure 11-8 reacts. 

In general, ketones don t undergo oxidation however, methyl ketones undergo a haloform reaction. In a haloform reaction, the oxidation converts the methyl group to a haloform molecule (usually iodoform (CHI3)), which leaves the Ccirbon backbone one carbon atom shorter. The oxidant in a haloform reaction is sodium hypohalite (NaOX), which forms by the reaction of sodium hydroxide (NaOH) with a halogen (X, where X = Cl, Br, or 1). 

Apparently the only previous case of the isolation of a chlorinated acid from a haloform reaction is that reported by Arnold et al. [8], who acidified the reaction mixture before decomposition of the excess hypochlorite. (Even though we decomposed the excess hypochlorite before acidification, the unchlorinated acid was not obtained.) These authors [8] also obtained an ester as the product of the haloform reaction. It seems likely that conditions for performing the haloform reaction can be found which will make the one-step conversion of a methyl ketone to the ester of the corresponding acid a general reaction. ... 

Hi) Carboxylic acids and derivatives. The haloform reaction on thienyl methyl ketones leads to thiophenecarboxylic acids, which can be treated with Raney nickel to produce the open-chain acids. This method has been used for the five-carbon homologation of carboxylic acids and to produce dicarboxylic acids (Scheme 51). Long-chain carboxylic acids may also be prepared by using dicarboxylic acid derivatives for the acylation of thiophenes (Scheme 52). In the synthesis of Queen substance, the precursor (213) has been generated by Raney nickel desulfurization of the appropriate thiophene-2-acetic acid derivative (79T329). 

The haloform reaction of unsymmctrical perfluoroalkyl and m-hydroperfluoroalkyl trifluoro-methyl ketones 6 gives the alkanes 7 corresponding to the longer alkyl chain.174... 

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