Haloforms, determination

The Yorkshire Water Authority has published a method for the determination of haloforms in sewage sludge [24],... 

Yorkshire Water Authority (1974) Method YWA501-01. Determination of Haloforms in Sewage Sludge. 

Nicholson AA, Meresz 0, Lemyk B. 1977. Determination of free and total potential haloforms in drinking water. Anal Chem 49 814-819. 

There has been a growing interest in applying high performance liquid chromatography, to the determination of the not only volatile compounds, such as aliphatic and polyaromatic hydrocarbons, saturated and unsaturated aliphatic and polyaromatic hydrocarbons, saturated and unsaturated aliphatic halogen compounds, haloforms and some esters, phenols and others but also non volatile components of water. 

Other applications of gas chromatography to the determination of haloforms and aliphatic halogen compounds in non saline waters are discussed in Table 15.9. 

Gas chromatography has been applied to the determination of a wide range of organic compounds in trade effluents including the following types of compounds which are reviewed in Table 15.15 aromatic hydrocarbons, carboxylic acids aldehydes, non ionic surfactants (alkyl ethoxylated type) phenols monosaccharides chlorinated aliphatics and haloforms polychlorobiphenyls chlorlignosulphonates aliphatic and aromatic amines benzidine chloroanilines chloronitroanilines nitrocompounds nitrosamines dimethylformamide diethanolamine nitriloacetic acid pyridine pyridazinones substituted pyrrolidones alkyl hydantoins alkyl sulphides dialkyl suphides dithiocaibamate insecticides triazine herbicides and miscellaneous organic compounds. 

To determine the contribution of solvent to the proton shifts of halogen-substituted compounds, the shifts of haloformic protons (CHC13, CHBr3 and CHI3) have been measured in 24 -electron donor solvents1. The effect of hydrogen bonding on the proton shift... 

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

The haloform reaction, using iodine, was once used as an analytical test in which the formation of a yellow precipitate of iodoform was taken as evidence that a substance was a methyl ketone. This application has been superseded by spectroscopic methods of structure determination. Interest in the haloform reaction has returned with the realization that chloroform and bromoform occur naturally and are biosynthesized by an analogous process. (See the boxed essay The Haloform Reaction and the Biosynthesis of Trihalomethanes. )... 

Sodium hypobromite and sodium hypoiodite solutions react in an analogous manner and yield bromoform (CHBrj) and iodoform (CHI,) respectively. The smooth production of the trihalomethanes by the use of the appropriate hypohalides is termed the haloform reaction. It is applicable to all compounds containing the —COCH3 group or which yield a substance containing this group by oxidation (e.gr., acetaldehyde from ethyl alcohol). Iodoform is a stable, crystalline, yellow solid, m.p. 119°, with a characteristic odour it is only sparingly soluble in water and hence will separate, even in very minute quantity, from an aqueous solution and can easUy be identified by m.p. and mixed m.p. determinations. 

The yield of nitration was determined by a ring opening reaction ( ) in which the dinitro salt is converted to 1,1,4,4-tet-rabromo-1,4-dinitrobutane (eq 3). The ring opening which is essentially a haloform reaction has been established to proceed in a yield of about 85%. 

Having established that there is a pre-equilibrium between haloforms and trihalocarbanions, the rate-determining step of the over-all hydrolysis remains to be located among the subsequent reactions of the trihalocarbanions. Two mechanisms are compatible with the observed kinetics viz. 

The classical organic reaction for the synthesis of THMs, the so-called haloform reaction, is actually a series of reactions of enolizable compounds whose rate is usually determined by the rate of enolization of a precursor molecule. It is outlined in Figure 5.5. 

Nicholson, A. A., Meresz, O., Lemyk, B. Determination of Free and Total Potential Haloforms in Drinking Water, Anal, Chem., 814 (1977)... 

Problem 9.3. The following experimental observations have been made with respect to the representation of the h oform reaction shown in Scheme 9.9 and other such base-catalyzed haloform processes (a) the rate of disappearance of starting ketone is independent of both the concentration and nature (Cl, Br, or I) of the hypohalite used, and (b) as long as the solution remains basic and there is sufficient halogen present, the reaction will continue until all of the starting ketone is consumed. Formulate a rate expression for what you believe to be the rate-determining step in Scheme 9.9 and that is consonant with the observations. 

The production of carbenes from haloforms (Eq. 10.54) is an interesting reaction. The reaction sequence displays second-order kinetics, first order in both base and haloform. This supports a mechanism involving an equilibrium deprotonation prior to rate-determining a-halogen departure. The loss of an equivalent of HCl from HCCI3 constitutes an elimination reaction, and is specifically called a 1,1-elimination or an a-elimination. 

Humic and fiilvic acids commonly present in natural waters have the required functional groups and structural arrangement for the haloform reaction. The extent of replacement of H atom on the CH3 group by Cl atom determines the type of chloromethane formed. 

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