Benzidine value

Figure 2-15 Autoxidation of Lard. (A) peroxide value, (B) benzidine value, (C) acid value. Source From J. Pokomy, Stabilization of Fats by Phenolic Antioxidants, Can. Inst. Food Sci. Tech-nol. J., Vol. 4, pp. 68-74, 1971.

Traditionally, the extent of oxidation is determined by the peroxide value and some of the carbonyl compound assays. Most involve the formation of color products that result from the reaction of the carbonyl group with the amine group of a cyclic leuco-compound. The most popular old method involves determination of carbonyl compounds in reaction with 2,4-dinitrophenylhydrazine (carbonyl value). The reaction with benzidine (benzidine value) was used for the first time by Wode in 1957 Soviet authors referred to it as the Lyubavina technique. This technique was abandoned due to carcinogenicity of benzidine. 

Lipid oxidation during frozen storage of fat and medium-fat fish is an important quality problem, primarily due to effects on sensory properties such as rancid off-odor (appearing earlier than off-flavor), orange-brown discoloration, and texture changes. Changes in peroxide value (PV) and carbonyl compounds such as anisidine value (AsV), benzidine value, carbonyl value, and thiobarbituric acid (TEA) proceed... 

This is not the only method of forming tetrazo-compounds. Each amino-group of a diamine may be diazotised and coupled. Benzidine and its homologues, which ha e been utilised in this way, have a special value for the cotton dyer, as the shades produced are not only ery biilliant, but, unlike the maiorily of... 

In addition the role played by the sorbent on which the chromatography is carried out must not be neglected. For instance, it is only on aliuninium oxide layers and not on silica gel that it is possible to detect caffeine and codeine by exposure to chlorine gas and treatment with potassium iodide — benzidine [37]. The detection limits can also depend on the sorbent used. The detection limit is also a fimction of the h/ f value. The concentration of substance per chromatogram zone is greater when the migration distance is short than it is for components with high h/ f values. Hence, compounds with low h/ f values are more sensitively detected. 

Electrophoretic separation of methemalbumin HpHb, and Hb after addition of excess amounts of Hb, staining with benzidine (L4) or o-dianisidine (J2), and finally scanning of the spots are other possibilities available for routine work. At low Hp concentration these methods do not give exact values since HBG is not taken into account, but the results are accurate enough for routine clinical work. Hommes (H10) eluates the HpHb spots and estimates the peroxidase activity. In this method HbBC is not taken into account. 

Limit values for primary aromatic amines, aromatic aminosulfonic acids, certain cancerogenic amines (e.g., benzidine, 2-naphthylamine, 4-aminobiphenyl and 2-methyl-4-chloroaniline) and for polychlorinated biphenyls (PCB) should be viewed in the listed references of the specific country. 

The benzidine rearrangements can also be brought about thermally, but very few mechanistic studies have been carried out. One set of heavy-atom KIE measurements has been made in the reaction of 2,2 -hydrazonaphthalene (18)21. Substantial nitrogen (1.0611 for the [15N, 15N ]) and carbon (1.0182 for the [1,1 -13C2]) KIE values were obtained showing that, just as for the acid catalysed reaction, this is a [3,3]-sigmatropic rearrangement, this time presumably of the non-protonated reactant. 

No evidence has been forthcoming on the nature of the intramolecular shift of the NO group to the 4-position (generally) in the aromatic ring. It would be helpful to have KIE values such as those obtained for the benzidine rearrangement as a first step in order to see if rearrangement is a concerted process. 

It was concluded that non-protonated 3,3 -dichlorobenzidine is subject to hydrophobic bonding to some extent (Boyd et al. 1984). It is clear from these studies that adsorption constants for 3,3 -dichloro-benzidine cannot be accurately predicted for a given soil based only on a K, value. 

TLVs are revised by the ACGIH annually as new information becomes available. Each year, additional substances of interest are added to the TLV list. Certain compounds that are proven or suspected carcinogens in humans such as benzidine, 4-aminodiphenyl, and 4-nitrodiphenyl have no TLV value, and human exposure to these agents should be avoided. Note For a detailed discussion of carcinogenic risks to humans, the publications of the lARC should be consulted. 

The ACGIH has classified benzidine as an A1-confirmed human carcinogen with no assigned threshold limit value and a notation for skin absorption. 

The bimolecular reduction of nitro compounds is believed to involve reduction of some of the starting material to a nitroso compound and another portion to either a substituted hydroxylamine or an amine. These intermediates, in turn, condense to form the azo compound. The exact mechanism of the reaction requires critical study. On the one hand, reducing conditions are always on the alkaline side to prevent the benzidine rearrangement of an intermediate hydrazo compound under acidic conditions, yet it is difficult to visualize the formation of hydrazo compounds by the indicated condensation. As a practical matter, this method is of value only if symmetrically substituted azo compounds are desired. 

The blue complex of 34 has a formation constant of 1044 dm mok while the corresponding value for the complex of 35 is 140 dm mok. In both cases the NMR spectra confirm that these species are true inclusion complexes. The com-plexation of 34 was also investigated by voltammetry the half-wave potential for the single electron oxidation of benzidine in acetonitrile is shifted anodically after the addition of excess host to the guest. 

However, in 1978, Hendrickson and co-workers reported a value of -9 cm" for a binuclear copper(II) complex bridged by benzidine (Rmm = 12.2 A). The interaction was supposed to be propagated mainly by a orbitals, with a possible role of k orbitals [88a]. Measurements on this complex in solution, to avoid intermolecular couplings, later gave a maximum value of 3.5 cm" for the / parameter [88b]. 

The benzidine or anisidine value method uses the reaction of nonvolatile a- and p-unsaturated aldehydes with these reagents (42, 43). Absorption readings are made at 350 nm in 1-cm cells. Originally, benzidine was used as a reagent. Later, because of the carcinogenicity of benzidine, anisidine was recommended. This method of determining the degree of oxidation is extensively used in Europe and... 

With anilines and AT-alkylanilines the way of dimerization is influenced by the reaction conditions and the substitution of the aryl ring [70]. The benzidines predominate in strongly acidic aqueous media. The formation of benzidine is further favored by a low substrate concentration and a high current density. The benzidine derivatives may be reversibly oxidized to their dications. Diphenylamines are the major products under neutral conditions and higher pH values, a low current density, a high substrate concentration, a small alkyl group in monoalkylanilines, and a non-aqueous solvent [Eq. (7)]. Under weakly acidic conditions in aqueous medium, an additional electron discharge followed by hydrolysis produces benzoquinones in almost quantitative yield. 

Also highly nitrated compounds can be detected and identified by means of donors, such as hydrocarbons. Their identification was suggested by the method of thin-layer chromatography [103]. A method of rapid detection, of explosives such as TNT, TNB, Picryl chloride, wi-DNB, Tetryl in the form of charge-transfer complexes with aromatic amines, such as aniline, dimethylaniline, toluidines, anisidines, naphthylamines, benzidine etc. was developed by Dwivedy et al. [104]. The authors used for identification thin-layer chromatography establishing Rf values for model complexes. 

The reaction enthalpy for monoacetylation of benzidine, ArNH2 = p-IENQJUCfilU NH2-jt> is the likewise exothermic —33.1 kJmol-1. However, per NH2 group, the acetylation of benzidine to diacetyl derivative is —73 kJ mol-1, nearly twice the expected value. In support of this implausible result, the acetylation enthalpy of the monoacetyl to the diacetylbenzidine is — 114 kJ mol 1. The enthalpy of formation of A,A -diacetylbenzidine is likely erroneous. 

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