Thiocyanates reactions

Base-treatment of the compound proposed to be l-(2-bromoethyl)-2-thiourea (m.p. 173.6-174.2°), formed by the reaction of 2-bromo-ethylamine hydrobromide with potassium thiocyanate, was reported to yield 2-amino-2-thiazoline 116 however, Klayman119 has shown that the product of the thiocyanate reaction is actually 2-amino-2-thiazoline hydrobromide (m.p. 176-177°). 3-(2,3-Dibromopropyl)-2-thioureas... 

Numerous studies on bead bio chips were therefore based on magnetic [16,17], glass or silica [18-20], and polystyrene beads [21]. The DNA immobilization chemistry of those beads could be very different, from the classical avidin/biotin affinity reaction [17,22] (Fig. 2B) to the disulfide bridging onto thiol modified silica [19] (Fig. 5A), the thiocyanate reaction onto amino-terminated latex beads [21] (Fig. 5B), and finally the hybridization-based immobilization of poly(A)-tagged probes onto poly(T)-bearing magnetic beads [16] (Fig. 5C). 

DOT CLASSIFICATION 5.1 Label Oxidizer SAFETY PROFILE A strong oxidizer. Ignites on contact with potassium thiocyanate. Reaction with CI2 yields explosive CIO2. When heated to decomposition it emits toxic fumes of CT. See also CHLORITES and CALCIUM COMPOUNDS. 

When allowance is made for the equilibrium constants for formation of ONX " , the rate coefficients for the chloride and bromide-catalysed reaction are almost equal (at 0 °C) and differ by only one power of ten from those for the thiocyanate reaction, viz. 

The Ferric Thiocyanate Reaction. This test is carried out on a solution of the material to be examined if the sample is gaseous, a solution may be obtained by bubbling it through an alkaline... 

In addition to the DCP determinations, other analytical techniques were employed as necessary. One of these was the colorimetric determination of ferric ion by the thiocyanate method (2). This was used to calculate the ferrous iron content which is the difference between the total iron, determined by DCP, and the trivalent iron obtained by the thiocyanate reaction. 

As with most complexation and drug solubility situations, pH b a critical variable. Cocaine base b not soluble in water, and if the drug b in thb form rather than a soluble salt, no reaction occurs. Acid b needed to ensure that the cocaine b in the water-soluble ionic form to allow for the formation of a complex. The color b the result of an ion-pair compound formed from the cationic cocaine and the anionic cobalt complex. As with all amine bases, such as ammonia, the base becomes protonated in acidic solution. The pKg of the base determines the ratio of the protonated, ionized form to the neutral form. It is possible to add too much HQ, because cobalt forms a water-soluble pink complex with chloride [CoCy . The pH can also influence the type of complex and ion pair formed. Under acidic conditions, the ion pair favored b [Co(cocaine)2l(SCN)2 (which b pinkbh and soluble in water), while in the neutral-to-basic ranges, the ion pair b assigned the structure [cocaine-H ]2 [Co(SCN)4] (which b a blue solid and soluble in chloroform). The important points of the cobalt thiocyanate reaction with cocaine are summarized in Figures 7.24r-7.26. 

Figure 7.26 Additional pH effects that can occur in the cobalt thiocyanate reaction. 

Phosphates, arsenates, oxalates, tartrates, and other organic hydroxyl compounds, fluorides, and many compounds which give stable complex salts with iron III, can, according to the proportion present, so reduce the concentration of ferric ions that the ionic product necessary for the color reaction is not reached. Even considerable amounts of iron may then fail to be detected by the sensitive thiocyanate reaction. The same interference occurs when great amounts of mercury salts are present because they form slightly dissociated mercuric thiocyanate or double thiocyanates, e.g. Ka[Hg(CNS)J, and thus consume the thiocyanate ions. 

Iron salts yield a red-violet color with thioglycolic add and ammonia probably because of the formation of inner complex anions containing iron. This color reaction is more sensitive than the thiocyanate reaction with iron it is not impaired by materials which form complexes with iron If thioglycolic add containing a,a -dipyridyl or a,a -phenanthroline is used, the sensitivity is still better. The reaction described on page 263 occurs. 

The thiocyanate test for iron fails or is unreliable when applied to concentrated nitric acid (or after the latter has been diluted). The thiocyanic acid is oxidized and yields orange ether-soluble decomposition products. The usual procedure for testing concentrated nitric acid for traces of iron for their colorimetric determination has been to evaporate 7 ml of the acid, take up the evaporation residue in dilute hydrochloric acid, oxidize with persulfate, and then apply the thiocyanate reaction. The rapid procedure given here is based on the production and fixing of hydrous ferric... 

In the thiocyanate reaction, however, both linkage isomers [CrNCS] + and the less stable [CrSCN] + appear to be produced and it may be that an ion pair of the type [TaaBrial +NCS- is formed where the thiocyanate ion could act as a bridging ligand, leading to the formation of the S-bonded Cr product. 

The kinetics of the reaction between peroxomonosulphate and thiocyanate were earlier interpreted in terms of the intermediate of species such as HS(0)CN, (SCN)a, and HO(SO)CN. No definite proof of this is available, but the detection of similar intermediates, RSO2CN and R(SO)CN, in reactions between alkyl thiocyanates and peroxomonosulphate lends support to the intermediacy of HS(0)CN and HO(SO)CN in the peroxomonosulphate-thiocyanate reaction.The kinetics of decomposition of the iV-nitroso-hydroxylamine-A -sulphonate anion to sulphate and nitrous oxide have already been described under nitrogen. The mechanism of substitution at sulphur(iv) in sulphinyl compounds, RS(0)Y, is still a topic of unresolved discussion, despite much effort in this area. The question awaiting answer is whether the mechanism is simple Ssl or whether there is a four-co-ordinate intermediate of significant lifetime. > Though the determination of Bronsted coefficients did not provide definite mechanistic evidence relating to substitution at... 

The use of this model leads to derived rate constants which exceed the diffusion-controlled limit. Further n.m.r. studies and a reconsideration of earlier published experimental data lead to a new proposal of an associative mechanism for iodide exchange, in which iodide attacks at one end of the tri-iodide. It is possible, from the observed kinetic pattern, that the I so generated has a sufficient lifetime to be considered an intermediate rather than a transition state. A transfer diffusion investigation of the same reaction also culminates in the proposal of an associative mechanism, with a linear transition state. Allowing for the non-spherical nature of the tri-iodide, it is possible to calculate a diffusion-controlled rate constant, which turns out to be the same as the experimentally determined (by this method or from n.m.r.) second-order rate constant. Some calculations on the transition state have been made in connection with this transfer diffusion study of the iodide-tri-iodide exchange reaction. Further study of the iodide-thiocyanate reaction has resulted in an estimate of the association constant for the initial rapid association of the reactants to give the intermediate charge-transfer complex la.SCN-. ... 

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