Thioureas natural waters

Place a mixture of 0-5 g. of finely powdered thiourea, 0-5 g. of the alkyl halide and 5 ml. of alcohol in a test-tube or small flask equipped with a reflux condenser. Reflux the mixture for a j)eriod depending upon the nature of the halide primary alkyl bromides and iodides, 10-20 minutes (according to the molecular weight) secondary alkyl bromides or iodides, 2-3 hours alkyl chlorides, 3-5 hours polymethy lene dibromides or di-iodides, 20-50 minutes. Then add 0 5 g. of picric acid, boil until a clear solution is obtained, and cool. If no precipitate is obtained, add a few drops of water. RecrystaUise the resulting S-alkyl-iso-thiuronium picrate from alcohol. 

Scheme 6.8 Epoxide recognition for epoxide hydrolase that detoxify living cells by catalyzing alcoholysis to water soluble diols. The working model involves the phenolic H-atoms of two tyrosines activating the epoxide for nucleophilic attack. This principle is realized analogously by double hydrogenbonding thiourea catalyst 9 in the natural medium water.

Nitrogen (ca. 5 at. %) occurs as carbon-nitrogen bonds, probably mainly cyanamide (NCN ), although other C-N bonded compounds were also believed to be present. If cyanamide is present as the Cd salt, this would tie up 5% of the Cd. The Cd S ratio was found to be only slightly higher than unity (ca. 1.02), and some of the Cd may be bound to carbonate. Therefore other C-N species are likely also to be present, e.g., cyanide, several of which could adsorb to one Cd or even to a CdS moiety. By reducing the concentration of thiourea in the bath, C-N impurities in the CdS film could be reduced almost to zero [77]. Whatever the nature of the C-N impurity, much of it could be removed simply by dissolution in water at 60°C [78]... 

The critical feature of the Edman degradation is that it allows the N-terminal amino acid to be removed without cleaving any of the other peptide bonds. Let s see how this occurs. The mechanism of the reaction is shown in Figure 26.3. First the nucleophilic nitrogen of the N-terminal amino acid attacks the electrophilic carbon of phenyl isothiocyanate. When anhydrous HF is added in the next step, the sulfur of the thiourea acts as an intramolecular nucleophile and attacks the carbonyl carbon of the closest peptide bond. II is the intramolecular nature of this step and the formation of a five-membered ring that result in the selective cleavage of only the N-terminal amino acid. The mechanism for this part of the reaction is very similar to that for acid-catalyzed hydrolysis of an amide (see Section 19.5). However, because no water is present, only the sulfur is available to act as a nucleophile. The sulfur is ideally positioned for intramolecular attack at the carbonyl carbon of the N-terminal amino acid, so only this amide bond is broken. 

Reaction of primary and secondary alkyl halides with thiourea is usually carried out in the following manner thiourea (2 mmole) is boiled with the corresponding alkyl halide (1 mmole) in ethanol (3 ml) or acetone. Reaction times vary appreciably with the nature of the halide. Primary alkyl bromides may be heated only 20—30 min, and the reaction of primary alkyl iodides requires 10—20 min for primary alkyl chlorides the reaction time has to be prolonged to 120—180 min while adding potassium iodide (0.1 g of KI is first dissolved in a few drops of water). 8econdary alkyl bromides require practically the same time for their reaction as primary alkyl chlorides. 8econdary alkyl chlorides require 3 —10 hr of boiling. 

Thiourea (Tu) and its derivatives (RR NCSNR R ) form strongly bound, water soluble complexes with Ag and the noble metals in acidic media. The Ag complex, for example, is [AgTua] , so that thiourea under these conditions serves as an excellent masking agent for Ag. As the pH is increased, however, the nature of the reaction changes, with the thiourea reacting similarly to dithizone or rhodanine, with release of a proton and forming a precipitate. 

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