Ketones reaction with halogens

Instead of direct halogenation of ketones, reactions with more reactive derivatives such as silyl enol ethers and enamines have advantages in certain cases. 

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). 

This section examines a reaction of the carbonyl gronp that can proceed through the intermediacy of either enols or enolate ions—halogenation. Aldehydes and ketones react with halogens at the a-carbon. In contrast with deuteration, which proceeds to completion with either acid or base, the extent of halogenation depends on whether acid or base catalysis has been nsed. 

Aldehydes and ketones react with halogens at the a-position via the enol or enolate, depending on the solution pH (Figure 17.14). For the acid-catalyzed reaction, the acid is usually HBr or HOAc, and a typical reaction mechanism is shown in Figure 17.15. In acid, the enol reacts with bromine in base, the enolate is the reacting species. The enolate, as you might expect (remember that the phenolate anion is a better nucleophile than neutral phenol), is the better nucleophile. However, irrespective of pH, the rate of bromination is not dependent on the concentration of molecular bromine. The RDS is enolization, and subsequent reaction of the enol/enolate with bromine is fast. 

Reaction with Organic Compounds. Aluminum is not attacked by saturated or unsaturated, aUphatic or aromatic hydrocarbons. Halogenated derivatives of hydrocarbons do not generally react with aluminum except in the presence of water, which leads to the forma tion of halogen acids. The chemical stabiUty of aluminum in the presence of alcohols is very good and stabiUty is excellent in the presence of aldehydes, ketones, and quinones. 

A commonly used alternative to the direct bromination of ketones is the halogenation of enol acetates. This can be carried out under basic conditions if necessary. Sodium acetate, pyridine or an epoxide is usually added to buffer the reaction mixture. The direction of enolization is again dependent upon considerations of thermodynamic and kinetic control therefore, the proportion of enol acetates formed can vary markedly with the reaction conditions. Furthermore, halogenation via enol acetates does not necessarily give the same products as direct halogenation of ketones 3. 23... 

Codeposition of silver vapor with perfluoroalkyl iodides at -196 °C provides an alternative route to nonsolvated primary perfluoroalkylsilvers [272] Phosphine complexes of trifluaromethylsilver are formed from the reaction of trimethyl-phosphme, silver acetate, and bis(trifluoromethyl)cadmium glyme [755] The per-fluoroalkylsilver compounds react with halogens [270], carbon dioxide [274], allyl halides [270, 274], mineral acids and water [275], and nitrosyl chloride [276] to give the expected products Oxidation with dioxygen gives ketones [270] or acyl halides [270] Sulfur reacts via insertion of sulfur into the carbon-silver bond [270] (equation 188)... 

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

The methyl group of a methyl ketone is converted into an a ,a ,a -trihalomethyl group by three subsequent analogous halogenation steps, that involve formation of an intermediate enolate anion (4-6) by deprotonation in alkaline solution, and introduction of one halogen atom in each step by reaction with the halogen. A... 

The classical Reformatsky reaction consists of the treatment of an a-halo ester 1 with zinc metal and subsequent reaction with an aldehyde or ketone 3. Nowadays the name is used generally for reactions that involve insertion of a metal into a carbon-halogen bond and subsequent reaction with an electrophile. Formally the Reformatsky reaction is similar to the Grignard reaction. 

A particularly common cr-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their a- positions by reaction with Cl2, Br2, or I2 in acidic solution. Bromine in acetic acid solvent is often used. 

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

Reactions 11-22-11-26 involve the introduction of a CH2Z group, where Z is halogen, hydroxyl, amino, or alkylthio. They are all FriedeI-Crafts reactions of aldehydes and ketones and, with respect to the carbonyl compound, additions to the C=0 double bond. They follow mechanisms discussed in Chapter 16. 

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 ... 

The same products are obtained (though in different proportions) when Na or K is omitted but the solution is irradiated with near-UV light.In either case other leaving groups can be used instead of halogens (e.g., NR3, SAr) and the mechanism is the SrnI mechanism. Iron(II) salts have also been used to initiate this reaction. The reaction can also take place without an added initiator Enolate ions of ketones react with Phi in the dark. " In this case, it has been suggested that initiation... 

Reaction of halophosphoranes with aldehydes or ketones reaction of certain ylids with halogen compounds (scoopy reactions)... 

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