Chloral purity

By the use of quinine instead of quinidine, the opposite enantiomer (+)-18 can be synthesized with good optical purity (76% ee) (Scheme 17). It is also noteworthy that exceedingly simple catalysts such as 1,2-dimethylpyrrohdine and N,J7-dimethyl-a-phenylethylamine gave good enantiomeric excess (60 and 77% ee s, respectively). Alhough the reaction mechanism and the basis of the stereoinduction are not clear, the reaction is thought to proceed in a stepwise manner via a tertiary amine-ketene complex as initial intermediate and not via a tertiary amine-chloral complex (Scheme 18). 

Chloral hydrate is easily decomposed by alkali into chloroform and alkali formate. This reaction is used stoichiometrically to determine chloral hydrate purity by measuring the amount of an excess of IN sodium hydroxide consumed in the reaction (titration with IN sulfuric acid to phenolphthalein)15,99,127,T28,152 to l66. Alternatively, the formate generated in the reaction may be determined by reaction with an excess of iodine (O.IN), followed by back titration w ith... 

After a mixture of 1.68 g chloral hydrate (10.2 mmol), 2.03 g hydroxylamine hydrochloride (29.2 mmol), 13.4 g anhydrous Na2S04, and 45 mL water was stirred at 45-50°C for 15 min, a 7.0 mL solution prepared from 1.517 g aniline, 0.75 mL cone. HCl, and water was added. The resulting mixture was heated for 1 h each at 45-50°C, 55-60°C and 65-70°C. An additional 0.45 g chloral hydrate and 4.1 g hydroxylamine hydrochloride were added, and the reaction mixture was heated at 75-80°C for another hour. When the mixture was cooled in ice water, a light brown precipitate formed, which was filtered to afford 1.58 g A-(2,3-dimethoxy-4-methylphenyl)-2-(hydroxyimino)-acetamide of sufficient purity, in a yield of 73%. To 20 mL polyphosphoric acid at 100°C was added 2.0 g M-(2,3-dimethoxy-4-methylphenyl)-2-(hydroxyimino)-acetamide (8.39 mmol) over a period of 5 min. After 20 min, the reaction mixture was cooled, and 40 mL water was carefully added to yield an orange precipitate, which was filtered to give 1.30 g 6,7-dimethoxy-5-methyl-lH-indole-2,3-dione, in a yield of 70%. This compound could be further purified by recrystallization in methanol, m.p. 180.5-181.5°C. 

Alkali easily decomposes chloral into chloroform and alkali formate and the method of estimation of the purity of the compound is by the use of this reaction. 

---