propanoic acid + polyphosphoric acid + bromine __> alpha-bromopropanoic acidIn the course of this study, the possibility of forming an anhydride and brominating in one step became the primary objective of the investigation. Aliphatic carboxylic acids were heated with polyphosphoric acid until solution of the acid was effected. Phosphoric acid was added as a diluent for the very viscous polyphosphoric acid. In lieu of the addition of phosphoric acid, several milliliters of water were added to the polyphosphoric acid prior to heating the reaction mixture. When solution of the carboxylic acid in the polyphosphoric acid was complete, bromine was added slowly to the stirred mixture which, in general, was maintained at a temperature near 100°. Two methods of isolating the products are feasible. The low boiling bromoacids can be distilled directly from the reaction mixture under reduced pressure. The higher boiling products can be isolated by diluting the reaction mixture with water in order to destroy the polyphosphoric acid. The alpha-bromoacid can then be extracted from the phosphoric acid solution with chloroform or other organic solvents which are insoluble in phosphoric acid. The formation of an anhvdride as the intermediate in this reaction may be postulated, The acyl phosphate type anhydride and the carboxylic acid anhydride are both possible intermediates. The former anhydride is considered to be the more probable postulate since bromination of butyric anhydride under the same conditions utilized for butyric acid did not give the dibromoanhydride but alpha-bromobutyric acid as the only product. This would support an assumption that a mixed anhydride was first formed and then destroyed to yield only the brominated acid. The yields of this reaction, in general, are equivalent to thosee obtained by the use of red phosphorus and bromine.
ExperimentalThe procedures described below are representative of those employed in the preparation of the acids listed in Table 1. With the butyric and valeric acids, the reaction mixture was heated to 100-120° during the addition of bromine and with butyric anhydride no diluent was used. A larger excess of bromine might be required if efficient condenser action is not obtained.
incomplete Table 1Yield from: AcOH : 67.6 % Bp: 100-105°C / 30 mmHg
EtCO2H : 76.0 % Bp: 200-203°C / 740 mmHg
(n-)PrCO2H : 74.7 % Bp: 122-126°C / 25 mmHg
Preparation of a-Bromoacetic Acid
In a 50-ml. 3-neck round-bottom flask were placed 6 g. (0.1 mole) of acetic acid, 10 mL of polyphosphoric acid (PPA) and 1 mL. of H2O. The flask was fitted with a condenser, a dropping funnel and a mechanical stirrer. The mixture was heated with stirring for 45 minutes. The acetic acid became miscible with the PPA and the solution turned a light yellow. Eighteen and seven-tenths grams (0.117 mole) of Br2 was then added drop wise with stirring while the reaction mixture was maintained at 80-100°. The addition required 1h and the mixture was allowed to stir for an additional two hours until the evolution of hydrogen bromide from the condenser had ceased. The reaction mixture was transferred to a distilling flask and the volatile material removed in vacuo. The fraction distilling 100-105° (30 mm.) was collected. This material was redistilled and solidified upon cooling.
Ref: Smissman; JACSAT; J.Amer.Chem.Soc.; 76; 1954; 5805
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