2.2- Dimethylbutane solvent

The rates of these two reactions have been studied for the attack of trifluoromethyl (51) and methyl radicals (52) in isoprene that has been dissolved in 2,3-dimethylbutane and isooctane, respectively. The rate constants for the reactions with isoprene are much greater than those for the attack on the solvent. The ratio between the two rates for the attack of trifluoromethyl radicals varies from 1090 at 65°C to 233 at 180°C. For the corresponding reaction involving methyl radicals, the ratio is 2090 at 65°C. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

As has been noted earlier, the solvent usually has little effect on free-radical substitutions in contrast to ionic ones indeed, reactions in solution are often quite similar in character to those in the gas phase, where there is no solvent at all. However, in certain cases the solvent can make an appreciable difference. Chlorination of 2,3-dimethylbutane in aliphatic solvents gave 60% (CH3)2CHCH-(CH3)CH2C1 and 40% (CH3)2CHCC1(CH3)2, while in aromatic solvents the ratio became 10 90. This result is attributed to complex formation between the... 

The C-H activation chemistry can also be conducted on solid hydrocarbons, such as adamantane (Equation (13)).78 A suitably inert solvent for such a reaction is 2,2-dimethylbutane. Rh2( -DOSP)4-catalyzed decomposition of methyl phenyldiazoacetate in the presence of 2 equiv. of adamantane generated the C-H insertion product in 90% ee. 

One of the most efficient methods for oxidation of primary alcohols to either aldehydes or carboxylic acids is the one, commonly known as the Anelli oxidation. This reaction is carried out in a two-phase (CH2Cl2/aq.buffer) system utilizing TEMPO/NaBr as a catalyst and NaOCl as the terminal oxidant The new system described here is an extension of the Anelli oxidation, but surprisingly, does not require the use of any organic solvents and replaces the KBr co-catalyst with the more benign, Na2B40y (Borax). The use of the new cocatalyst reduces the volume of the buffer solution and eliminates completely the need of a reaction solvent. The new system was successfully applied in the industrial synthesis of the 3,3-Dimethylbutanal, which is a key intermediate in the preparation of the new artificial sweetener Neotame. 

For example, in the photolysis of (30) in toluene solution, the product of insertion of DPC into the benzylic C—H bonds, 1,1,2-triphenylmethane (31), was accompanied by substantial amounts of 1,1,2,2-tetraphenylethane (32) and bibenzyl (33).When solvents such as cyclohexane are used, tetraphenylethane (32) is formed as the major product, indicating that direct C—H insertion in the singlet state is not the main process in most diarylcarbenes (Scheme 9.7). ° In contrast, 9-cyclohexylfluorene (37) is produced by photolysis of diazofluorene (36) in cyclohexane as a main product (65%) along with a small amount of escaped products (38 and 39). One can estimate in this case that at most 14% of 37 arises from free radical processes. Similarly, direct or sensitized photolysis of diazomalonate in 2,3-dimethylbutane gives C—H insertion products, but in the triplet-sensitized... 

Fig. 31. Decrease of the fluorescence intensity of TMPD during photoionisation, AF, in the presence of an electric field of strength E compared with the fluorescence intensity in the absence of an electric field, F°, for two cases (a) as the electric field is varied and (b) compared with the photocurrent in the solvents used. The solvents used were 9,0, tetramethylsilane A, dimethylbutane A, trimethyl pentane , cyclopentane, shown as the curve of smallest slope in (a) and the insert in (b). Other conditions were TMPD concentration 30 /rmol dm-3, photoionisation energy 5.9 eV and temperature 293 K. After Bullot et al. [387],...

Figure 27-4 Proton spectra of 3,3-dibromo-2,2-dimethylbutane in CF2Cla as solvent at various temperatures...

The operating conditions for tests using 2,2,4-trimethylpentane, 2,3-dimethylbutane, 2-methylpentane, and no solvent are presented in Table II. The product yields and properties are presented in Table III. 

A test was made with 2,3-dimethylbutane as the supercritical solvent it has a lower critical temperature than 2,2,4-trimethyl-pentane. Operating at a temperature of 508-512 K, a pressure of 4.10-4.37 MPa, a molecular sieve/oil ratio of 6.39, and a solvent/ oil ratio of 21.3, the molecular sieve capacity attained is 5.73 g/100 g of molecular sieves (as compared to 3.2 g/100 g of molecular sieves with 2,2,4-trimethylpentane at 550 K). The n-paraffin content of the wax distillate was reduced by 88% to a level of 2 wt %, giving a pour point of 266 K. The yield of denormal oil was lower (63%) and the n-paraffin content of the desorbate was lower (44%) at this lower temperature level. This is probably due to increased capillary condensation. Conversely, operation at temperatures greater than 550 K should produce less capillary condensation and purer n-paraffin product. It would be interesting to try supercritical solvents with critical temperatures in the 600-670 K range. 

Russell, G. A. Solvent effects in the reactions of free radicals and atoms. II. Effects of solvents on the position of attack of chlorine atoms upon 2,3-dimethylbutane, isobutane and 2-deuterio-2-methyl-propane. J. Amer. chem. Soc. 80, 4987 (1958). 

The variation in selectivity of radicals in different solvents has been interpreted as being due to radical-solvent interaction which changes the reactivity of the radical. Thus the selectivity of chlorination of 2,3-dimethylbutane which may react at either a tertiary or a primary position increases in aromatic solvents (Table 26 Russell, 1958, 1960). Since the effect appears to be proportional to the basicity of the aromatic substrate, it was concluded that aromatic solvents yield a complexed chlorine atom which is consequently less reactive and therefore more selective in its reactions. Confirmation of this came from the finding that the increased selectivity of the photochlorination of 2,5-dimethylhexane in aromatic solvents was due to an increase in the activation energy of the reaction (Russell,... 

Another example of a solvent-dependent atom-transfer reaction is hydrogen abstraction by chlorine atoms during the photochemical chlorination of hydrocarbons with molecular chlorine for an excellent review, see reference [571]. Russel reported that in the photochlorination of 2,3-dimethylbutane, according to reaction scheme (5-68), certain solvents do not have any effect on the selectivity of the reaction as measured by the rate ratio whereas other solvents increase this ratio significantly (c/. 

Table 5-12. Solvent effect on the selectivity of the photochemical chlorination of 2,3-dimethylbutane at 55 °C (solvent concentration 4.0 mol/L) [221].

Compared with chlorination, hydrogen abstraction reactions of alkoxy radicals are relatively insensitive to solvent effects [160, 222, 223]. The results of the AIBN-initiated radical chain chlorination of 2,3-dimethylbutane with tert-butyl hypochlorite indicate a solvent effect on tert-butoxy radical reactions of much smaller magnitude, but greater selectivity in aromatic solvents [222, 223], The reduced solvent effect for this hydrogen abstraction reaction has been attributed to steric effects. Due to the bulky... 

Using a similar technique Priola and coworkers (S) studied the reaction between t-butyl chloride and EtjAl, Et2AlCl, EtAlClj and AICI3 using methylene chloride and methyl chloride solvents at —78° C, for 2 h. The results of this study can be summarized as follows 1. The major reaction products are isobutane and 2,2-dimethylbutane in reactions where EtsAl is used. 2. Other products such as 23-dimethyl-butane, isooctane and l-chloro-33 -dimethylbutane are also formed in amounts strictly dependent on the molar ratios of t-BuCl/alkylaluminum or the chlorine content of the alkylaluminum. 3. When EtjAlO or EtAlCl2 react with f-BuCl, the product consists of branched Q, Cg, Cg hydrocarbons and a higher alkyl chloride. 4. Interestingly, AlClj/does not react with t-BuCl, however, it yields a crystalline complex at —78° in the absence of an added olefln. 

Selectivity may also be achieved by altering the solvent in certain cases. Thus, chlorination of 2,3-dimethylbutane in aliphatic solvents yields the primary to... 

A solution of 20 mg (0.07 mmol) of 1.1.2a in 1 mL of 2,2-dimethylbutane/n-pentane (8 3, Rigisolve, Merck) was irradiated at -196 °C in a quartz tube with a low-pressure mercury lamp (Vycor-reactor, Grantzel quartz dewar flask, charged with liquid nitrogen). After evaporation of the solvent 12 mg (66%) of 1.1.2b could be obtained. 1.1.2b could be purified by sublimation at 0.01 Torr and 35 °C. 

In 1957, Russell et al.49-sl also demonstrated that the distribution of the products obtained in the photochlorination of 2,3-dimethylbutane in aliphatic solvents differed from that in aromatic solvents and that this difference was attributed to the... 

The conformation isomerism in a wide range (31 examples) of 1-sub-stituted derivatives of 3,3-dimethylbutane has been examined. In some cases, the temperature variation of the vicinal coupling constants was studied. In 1,1,2-trichloroethane the coupling constants of the gauche and anti isomers are deduced from solvent-dependent changes in the observed coupling constants. In some similar systems (1,1,2-trichloro- and 1,1,2-tribromoethane in carbon tetrachloride and in benzene), a correlation of the dipole moments of the compounds and the vicinal coupling constants was found.Rotational isomerism in the phenylalanine anion and dipolar ion has been studied in deuterium oxide solutions. 

The selectivity of chlorination is influenced by solvents. For example, the chlorination of 2,3-dimethylbutane shows increased preference for the tertiary position in benzene. " The complexation with solvent attenuates the reactivity of chlorine atoms. Halogenated solvents also give evidence of complex formation. Brominated solvents lead to greater selectivity. " ... 

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