Deuterated compounds Benzene

At the present stage we cannot specify precisely the part of the molecule from which the H atom is being split off, i.e., whether it is from the benzene ring or from the substituent group. The fact that the same process occurs with benzene itself indicates that the H atom of the ring may be dissociated in the derivatives as well. For the methyl and the amino derivatives it is more likely that the H atom comes from the substituent. Experiments with deuterated compounds will settle this point. 

Carbon-13 shift values of a small selection of monosubstituted benzenes [383] are collected in Table 4.53. Signal assignments are based on conventional techniques such as proton off-resonance and gated decoupling as well as comparative measurements of specifically deuterated compounds [384],... 

For example, in the analysis of chlorophenol in soil by accelerated solvent extraction followed by GC-MS, deuterated benzene may be used as the matrix spike. The deuterated compound will not be present in the original sample and can easily be identified by GC-MS. At the same time, it has chemical and physical properties that closely match those of the analyte of interest. 

The kinetic isotope effect was introduced in Chapter 19. If a bond to deuterium is formed or broken in the rate-determining step of a reaction, the deuterated compound will react more slowly, usually by a factor of about 2-7. This effect is particularly valuable when C-H bonds are being formed or broken. In Chapter 22 we told you that the rate-determining step in the nitration of benzene was the attack of the electrophile on the benzene ring. This is easily verified by replacing the hydrogen atoms round the benzene ring with deuteriums. The rate of the reaction stays the same. 

Temperature dependent lineshape variation of the Pake doublet represents then an experimental evidence of a motion occurring in the solid state. 2H NMR investigations on selectively deuterated compounds, therefore, are important tools for the recognition of molecular motions in solids. This method has been applied to the characterization of molecular motions in molecular solids [8] and inclusion compounds [9]. As an example in Fig. 3.2.4 is reported the effect of the motion about the molecular symmetry axis, either by two-fold or n-fold (n>3) flips, on the deuterium spectrum of a deuterated para-substituted benzene [10]. 

In accordance with the constitution LV, the mild (70°) treatment of macralstonidine with deuterium chloride in deuterium oxide gives a mixture of the nonadeutero ketone (XXXIc), 9,11,12-trideutero-ilIa-methylsarpagine, and 9,10,11,12,18,18,18,20,11, 12 -decadeutero-macralstonidine (LV-dio)- The first of these products was unequivocally identified with the fission product obtained from macralstonine under similar reaction conditions (see above). The deuterium substitution pattern in the trideutero-V,j-methylsarpagine becomes clear from its mass spectrum since all fragment ions which contain the intact benzene ring (e.g., LVIII) are observed three mass units higher than in the un-deuterated compound. 

Heavier isotopes have smaller atomic volume deuterated compounds have smaller molar volumes than the corresponding unlabeled compounds. The molar volume differences between, e.g., deuterated and ordinary benzene are due to a molecular size effect caused by differences in zero point intermo-lecular motion of the molecules. 

Since measurable concentrations of BTEX are present in the blank matrix, detection limits were determined using the corresponding deuterated compounds. Iso-topically labeled compounds are also useful to improve specificity and to ensure precision in peak assignment. Limits of detection were 5 ng/L for benzene and toluene, and 10 ng/L for ethylbenzene and xylenes. Detection limits of 10 to 20 ng/L were also obtained for halogenated anaesthetics. The sensitivity of SPME depends on Kfs values and is not enhanced by larger sample volumes, especially for compounds with Kf < 500 [12]. 

The microbial oxidation of benzene and its derivatives using Pseudomonas putida have been used in several cyclitol syntheses. These include the preparation of conduritol D and the deuterated compound 99", D-cA/ro-inositol and D-cAiro-3-inosose for which the key step involves a one pot oxidation-protection to an epoxy-acetonide derivative," " and racemic quebrachitol." Also reported are the syntheses of (+)-D-chrro-inositol, a//o-inositol, muco-inositol and neo-inositol from a halogenated conduritol epoxide." ... 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

A kinetic isotope effect, kH/kD = 1.4, has been observed in the bromination of 3-bromo-l,2,4,5-tetramethylbenzene and its 6-deuterated isomer by bromine in nitromethane at 30 °C, and this has been attributed to steric hindrance to the electrophile causing kLx to become significant relative to k 2 (see p. 8)268. A more extensive subsequent investigation304 of the isotope effects obtained for reaction in acetic acid and in nitromethane (in parentheses) revealed the following values mesitylene, 1.1 pentamethylbenzene 1.2 3-methoxy-1,2,4,5-tetramethyl-benzene 1.5 5-t-butyl-1,2,3-trimethylbenzene 1.6 (2.7) 3-bromo-1,2,4,5-tetra-methylbenzene 1.4 and for 1,3,5-tri-f-butylbenzene in acetic acid-dioxan, with silver ion catalyst, kH/kD = 3.6. All of these isotope effects are obtained with hindered compounds, and the larger the steric hindrance, the greater the isotope... 

Nesmeyanov et a/.545 used a mixture of ferrocene, deuterated trifluoroacetic acid and benzene in the molar ratios 1 2 20 in a preliminary investigation of the reactivity of ferrocene and its derivatives. At 25 °C, rate coefficients were 1,620 x 10-7 (ferrocene) and 19.3 xlO-7 (acetylferrocene). In a subsequent publication by Alikhanov and Shatenshtein543 these values were altered to 1,600 x 10-7 and 1.5 x 10 7, respectively, and a value of 0.77 x 10"7 added for 1,1-diacetylferrocene. Under the same conditions, toluene gave a value of 0.3 x 10-7 so that the activating effects of these compounds relative to benzene can be approximately determined. 

Blackley548 measured the rates of deuteration of biphenylene, fluorene, tri-phenylene, and phenanthrene relative to o-xylene as 6.15 5.85 1.08 1.32, which is in very good agreement with the values of 8.80 7.00 - 1.14 which may be deduced from the detritiation data in Table 159, obtained using anhydrous trifluoroacetic acid. Aqueous trifluoroacetic acid (with the addition in some cases of benzene to assist solubility) was used by Rice550, who found that triptycene was 0.1 times as reactive per aromatic ring as o-xylene (cf. 0.13 derivable from Table 159) whereas the compound (XXXI) was 0.9 times as reactive as o-xylene. An exactly comparable measure is not available from Table 158, but dihydroanthracene (XXXII), which is similar, was 0.51 times as reactive as o-xylene and... 

Most of these values differ from those given in the original papers and which have been derived by the unjustifiable comparison of the benzene deuteration data with the dedeuteration data for the substituted compound this same error has been made elsewhere by these Russian workers (seep. 265). In some cases there is no alternative to this approximation and the data so derived is marked with an asterisk. Some of the values differ very markedly from those given in the original papers and which seem to the reviewer to have been obtained by methods which defy the laws of simple arithmetic. b E, - 15.8. 

There is one further piece of kinetic evidence which throws light on an aspect of the benzidine rearrangement mechanism, and this is comparison of the rates of reaction of ring-deuterated substrates with the normal H compounds. If the final proton-loss from the benzene rings is in any way rate-determining then substitution of D for H would result in a primary isotope effect with kD < kH. This aspect has been examined in detail42 for two substrates, hydrazobenzene itself where second-order acid dependence is found and l,l -hydrazonaphthalene where the acid dependence is first-order. The results are given in Tables 2 and 3. 

Fifolt [ 130] reported this chemical shift additivity method for fluorobenzenes in two deuterated solvents d6 acetone and d6 dimethyl sulfoxide (DMSO) Close correlations between experimental and calculated fluorine chemical shifts were seen for 50 compounds Data presented in Table 18 result from measurements in deuterochloroform as (he solvent [56] Fluorine chemical shifts calculated by this additivity method can be used to predict approximate values for any substituted benzene with one or more fluorines and any combination of the substituents, to differentiate structural isomers of multisubstituted fluorobenzenes [fluoromtrotoluenes (6, 7, and 8) in example 1, Table 19], and to assign chemical shifts of multiple fluorines in the same compound [2,5 difluoroamline (9) in example 2, Table 19] Calculated chemical shifts can be in error by more than 5 ppm (upfield) in some highly fluonnated systems, especially when one fluonne is ortho to two other fluorines Still, the calculated values can be informative even in these cases [2,3,4,6-tetrafluorobromobenzene (10) in example 3, Table 19]... 

We have shown that there is a very large number of organic compounds which can be fully deuterated provided the exchange reactions are catalysed by potassium amide. The deuterio-derivatives of benzene, naphthalene, phenanthrene, toluene, xylene, mesitylene, durene, hexamethylbenzene, and other hydrocarbons prepared in this way have been used in studies of the vibrational and electronic spectra in the laboratories of Academician G. S. Landsberg (Landsberg et al., 1954, 1956 Shatenshtein et al., 1958b) (Physical Institute, Academy of Sciences, U.S.S.R., Moscow), and of the Corresponding Member of the Academy of Sciences A. F. Prikhot ko (Broude et al., 1958) (Physics Institute, Academy of Sciences, Ukrainian S.S.R., Kiev). 

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