Hydrogen addition reactions with constants

By this technique these authors have determined the rate constants and collision yields for a number of simple olefins, substituted olefins, and some aromatic hydrocarbons. For a number of years these determinations represented the only extensive set of rate constants of hydrogen atom reactions with olefins. The technique did not differentiate between addition and abstraction by hydrogen atoms from the olefins and the rates were the sum of the two. 

The apparatus required is similar to that described for Diphenylmelhane (Section IV,4). Place a mixture of 200 g. (230 ml.) of dry benzene and 40 g. (26 ml.) of dry chloroform (1) in the flask, and add 35 g. of anhydrous aluminium chloride in portions of about 6 g. at intervals of 5 minutes with constant shaking. The reaction sets in upon the addition of the aluminium chloride and the liquid boils with the evolution of hydrogen chloride. Complete the reaction by refluxing for 30 minutes on a water bath. When cold, pour the contents of the flask very cautiously on to 250 g. of crushed ice and 10 ml. of concentrated hydrochloric acid. Separate the upper benzene layer, dry it with anhydrous calcium chloride or magnesium sulphate, and remove the benzene in a 100 ml. Claisen flask (see Fig. II, 13, 4) at atmospheric pressure. Distil the remaining oil under reduced pressure use the apparatus shown in Fig. 11,19, 1, and collect the fraction b.p. 190-215°/10 mm. separately. This is crude triphenylmethane and solidifies on cooling. Recrystallise it from about four times its weight of ethyl alcohol (2) the triphenylmethane separates in needles and melts at 92°. The yield is 30 g. 

Transfer constants for mercaptans with several monomers are given in Table XV. Results for the two methods described above are in satisfactory agreement. The rate of reaction with mercaptan relative to the rate of monomer addition (i.e., the transfer constant) varies considerably for different chain radicals (see Table XIV). Temperature coefBcients of the transfer constants for mercaptans are very small, which fact indicates that the activation energy for removal of a hydrogen atom from the sulfhydryl group of a mercaptan is nearly equal to that for monomer addition. 

These findings were further confirmed by data collected under isothermal conditions. Upon hydrogen addition at 350°C on the physical mixture previously saturated with NO species at the same temperature, no reaction products were detected. This indicated that the stored nitrates could not be regenerated by H2 at constant temperature, i.e. without a prior release in the gas phase. 

The aromatic-hydroxyl radical reaction has been studied by Davis et They reported rate constants for benzene and toluene and concluded that hydroxyl additions to the aromatic ring compete favorably with the abstraction of hydrogen atom from the alkyl substituent. Doyle et al recently published hydroxyl reaction rate constants for a series of alkylbenzenes. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The magnitude of the rate constants, their observed pressure dependence, and the products of the reactions are consistent with the mechanism involving the initial addition of OH to the triple bond. For example, the OH-l-butyne reaction at 298 K is about a factor of three faster than the reaction with n-butane (see Table 6.2), despite the fact that it has fewer abstractable hydrogens and the = C — H bond is much stronger than a primary -C-H bond ( 125 vs 100 kcal mol -1). In addition, a pressure dependence is not consistent with a simple hydrogen atom abstraction (see Chapter 5.A.2). 

In the majority of cases addition of substance A to the reaction zone does not change practically the pressure and, consequently, M — M, and w — w. This is indication that there is no reason for a change in (OH)0 with addition of A, and that the expression (5) is correct. Only in the case of reaction with hydrogen the pressure change amounts to 20-25%, and this is within the method s accuracy. To calculate fcx from eq. (6) it is necessary to know not only lc "oh, but the fcoH constant as well. 

Some conclusions on the reaction mechanism may be drawn from the rate constants obtained. It was shown for hydroxyl reactions with saturated compounds (propane, for example) that the main reaction of OH was the hydrogen atom abstraction in the formation of water. This is an accepted point of view. However, another route is possible for reactions with unsaturated hydrocarbons, i.e., addition at the double bond. This is the case for the H atom with saturated compounds H reacts by abstraction, and with unsaturated ones by addition. 

Develop a reaction mechanism for iodine (I2-O2-H2 system) from the information in the NIST Chemical Kinetics Database [256], Start with the H2-O2 reaction subset hydrogen.mec. Using the database, identify the relevant reactions with I2. Add these reactions to the starting mechanism, including product channels and rate constants. List the additional I-containing species formed in reactions of I2. Extend the reaction mechanism with reactions of these species. Continue this procedure until reactions of all relevant iodine species in the I2-O2-H2 system is included in the mechanism. 

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