Bromine titration

Bromine titration has been applied to several heteroaromatic compounds, but it is not a reliable method. The assumption is often made that oxo structures react very slowly with bromine whereas the hydroxy forms react rapidly. Thus, 3,4-diphenylisoxazol-5-one when freshly dissolved in ethanol was found to react with 0.5 mole of bromine, but after standing it reacted with almost 1 mole. These observations led to the conclusion that the solid was in the CH... 

Isoxazol-5-ones can exist in three different types of structures, cf. 45- 7 (R = H). Early investigators assigned structures to these compounds on the basis of unreliable chemical evidence thus the NH structure 47 was favored because the silver salt of 3-phenyl-isoxazol-5-one reacts with methyl iodide to give a product which was incorrectly (see reference 44) formulated as the iV-incthyl derivatives (cf. also reference 46). Bromine titration data led to assignment of an incorrect structure to 3,4-diphcnylisoxazol-5-one cf. article I (Volume 1), Section II,A. Comparison of the dipole moments of 3-phenyIisoxazol-5-one with those of the methyl derivatives 45 (R = Me) and 46... 

Thermal decomposition gives olefins, probably by rearrangement of intermediate carbenes. For example, the decomposition of 3,3-penta-methylenediazirine (68) in nitrobenzene above 160°C gives cyclohexene [Eq, (58)]. The yield as determined by bromine titration... 

In a 3-I. round-bottom flask, a hydrobromic add solution is made by the sulfur dioxide reduction of 480 g. of bromine in the presence of 510 g. of ice water or a mixture is made of 1000 g. of aqueous 48 per cent hydrobromic acid and 300 g. of concentrated su. furic acid. To this are added 385 cc. of aqueous allyl alcohol, which, according to bromine titration, contain 233 g. of pure allyl alcohol. The 3-I. round-bottom flask is fitted with a mechanical stirrer (Fig. 1, p. 4, see also Notes), separatory funnel, and an efficient condenser set for downward distillation. Stirring is started and 300 g. of concentrated sulfuric add are added gradually through the separatory funnel to the warm solution. The allyl bromide distils over completely in about one-half to one hour. The crude allyl bromide is washed with dilute sodium carbonate solution, is dried over calcium chloride and is distilled. The yield of product boiling at 60-72° from a number of experiments varies from 443 to 465 g. (92-96 per cent theory). A small high-boiling fraction is also obtained and examination has shown this to consist of propylene bromide. 

The 195-260° fractions of the distillates are treated with potassium carbonate to salt out the allyl alcohol and to neutralize the little formic acid present. This allyl alcohol is then distilled and the fraction boiling up to about 103° is collected, or if a column is used, up to 98°. In this way, 845 g. of an allyl alcohol are obtained, which by a bromine titration shows a purity of about 68-70 per cent. This is equivalent to 5 70 to 590 g. of pure allyl alcohol (45-47 per cent theory). 

To determine the purity of any sample of allyl alcohol, 1 cc. is run into 15 to 25 cc. of carbon tetrachloride and this solution is then treated in the cold with a carbon tetrachloride solution of bromine (standardized with potassium iodide and sodium thiosulfate) until a permanent bromine coloration is obtained. The amount of allyl alcohol present in any solution may also be determined roughly by conversion to allyl bromide. From several experiments it was found that the allyl bromide obtained was equivalent to the amount of allyl alcohol as determined by bromine titration. 

This technique, called bromine titration method , was extensively used by K.H. Meyer in the early twentieth century.18 It was later extended to determine the enol content of simple ketones using faster flow methods combined with more sensitive potentiometric measurements of bromine uptake, but this technique sometimes produced apparent enol contents that were far too high, such as the enol content of acetone of 2.5 x 10 4% that is frequently quoted in older textbooks of organic chemistry. The excessive values so obtained have been attributed to the presence of small amounts of impurities reacting with bromine. 

The thermal decomposition of this diazirine in nitrobenzene at 160°C. has been reported to yield cyclohexene in about 80% yield (determined by bromine titration) as the only hydrocarbon product, and it is suggested that this proceeds via a carbene, viz.. 

In an attempt to investigate the assumed keto-enol tautomerism of these compounds, a series of 2-oxoaIkylphosphonates were synthesized from triethyl phosphite and oc-bromoketones, and the amount of enol was determined by the bromine titration melhod. - These results, however, did not support the presence of the enol form because diethyl l,l-dimethyl-2-oxopropyIphosphonate, in which enolization is unlikely, had an enol content of 23% according to the consumption of bromine. 

Although the participation of the phosphoryl group in the tautomeric equilibria of certain esters has been alluded to already (Section II), early determination (by B. A. Arbuzov and co-workers, and reviewed by Mastryukova and Kabachnik ) of the enol content of tautomeric mixtures by bromine titration or by UV spectroscopy, indicated, at most, only low percentages (0-10%) of enol content for many compounds and it was felt that such low figures could easily represent the presence of unsaturated compounds as impurities. A relatively high pX value, e.g. 11.89 for triethyl phosphonoacetate, is now recognized as characteristic of a compound as a CH acid, and an indication of lack of enolization equally, a relatively low is associated with an OH acid to be found as the... 

Chemical evidence (bromine titration) and spectroscopic evidence for a mechanism involving the enolic intermediate (7.2) was obtained. Visual observations of the Fe(III)-catalysed reaction provided confirmatory evidence for the reaction scheme. 

For example, a dilute aqueous solution of ethyl acetoacetate contains 0.4% of enol (determined by bromine titration) and the measured dissociation constant is 2 x 10 This is therefore effectively the dissociation constant of the keto form, and the constant for the enol form is 5 x 10 . Since the keto form usually predominates in aqueous solution, the enol form is commonly the stronger acid, but this is not always the case. For example, dimedone (5,5,-dimethyIcyclohexane-l,3-dione) exists in aqueous solution as 95% enol and 5% keto, and its apparent dissociation constant is 5.9 X 10 hence in this case the dissociation constants of the keto and enol forms are respectively 1.2 x lO and 5.6 x 10 . ... 

Surface areas of hydrophilic (hydroxylated) silicas have been measured by adsorption of nitrophenol from water or benzene (79) and phenol from decane or carbon tetrachloride (80, 81). The system must be anhydrous and the solvents dried by molecular sieves. The phenol may be determined by bromine titration or interferometer. Stearic acid is adsorbed from anhydrous methanol and determined by difference by potentiometric titration with alkali. Ttie data given indicate that each stearic acid molecule covers only 20.6 A (82). 

Polyesters or copolyesters derived from 1,4-CHDM have lower heat resistance than PET. By unsaturated bromination titration, GC/MS and H-NMR it was possible to determine the type and amount of end groups present in PECT and PCT samples that were heated at 290 °C (102) and to propose a mechanism for the thermal degradation of these polyesters and other related 1,4-CHDM polyesters (Figure 6.23). 

Biuret acyl-, 194 alkyl-, 197 formyl-, 194, 195 preparation of, 194 reactions of, 195, 200 Bromine titration, as method to study tautomerism, 1... 

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