Reactions with hydrogen atoms

The 0 -phenyl-0 -piperidyl-(2)-acetic acid methylester of BP 135° to 137°C under 0.6 mm pressure is obtained in theoretical yield by hydrogenation of 50 g of 0 -phenyl-0 -pyridyl-(2)-acetic acid methylester in glacial acetic acid in the presence of 1 g of platinum catalyst at room temperature, while taking up 6 hydrogen atoms. Reaction with HCI gives the hydrochloride. Resolution of stereoisomers is described in U.S. Patent 2,957,880. 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

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. 

Mezyk S, Bartels DM. (1997) Rate of hydrogen atom reaction with ethanol, ethanol-dj, 2-propanol, and 2-propanol-dy in aqueous solution. /Phys Chem A 101 1329-1333. 

Hydrogen atom, reaction with disulphides 492, 493 reaction with thiols 486, 487 thiols as source of, in solution 473-475... 

Here the phenylacetic anhydride, possessing more reactive a-hydrogen atoms, condenses with benzaldehyde to give a-phenylcinnamic acid. The preparation of the latter is an example of the Oglialoro modiflcation of the Perkin reaction. 

Alkyl sulfonyl chlorides, having an a-hydrogen atom, react with enamines derived from aldehydes and cyclic ketones in the presence of triethylamine to give cyclic sulfones. Thus the enamine (22) gave the four-membered cyclic aminosulfone (143) on reaction with methanesulfonyl chloride (95). 

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

For the cobalt-based system the molecularity of the transition state indicated by the reaction order is H3C0C4O4 and the reactants are H2 and HCoCCO). Thus, two hydrogen atoms start with values of v 3200 cm 1 and one with v 1830 cm"1. If in the transition state the strong H-H bond is not yet completely broken, then we should expect to find the H atom originally attached to cobalt bound to carbon or oxygen (v 2900-3400 cm"1) in the transition state. 

The remaining steps consist in the union of the adsorbed olefin and its derivative, the half-hydrogenated state, with an adsorbed hydrogen atom, reactions which occur with retention of configuration of the substituted carbon atom, reactions (3) and (4),... 

Although many catalytic reactions are not well understood, a large amount of work has been done on hydrogenations of double bonds. The metal surface acts as a source of electrons. The tt bonds as well as hydrogen atoms are bound to this surface. Then the hydrogen atoms react with the complexed carbons one at a time to form new C—H bonds. No reaction occurs without the metal surface. The metal in effect avoids what would otherwise have to be a free radical mechanism that would require considerably more energy. The mechanism is outlined as follows. 

Chlorine atoms react with aromatic hydrocarbons, but only at a significant rate with those having saturated side chains from which the chlorine atom can abstract a hydrogen or unsaturated side chains to which it can add. For example, the rate constant for the Cl atom reaction with benzene is 1.3 X 10"15 enr3 molecule-1 s-1 (Shi and Bernhard, 1997). On the other hand, the rate constants for the reactions with toluene and p-xylene are 0.59 X 10-10 and 1.5 X 10-l() enr3 molecule"1 s"1, respectively (Shi and Bernhard, 1997), and that for reaction with p-cymene is 2.1 X 10"10 cm3 molecule"1 s-1 (Finlayson-Pitts et al., 1999). Hence... 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

Due to the highly exothermic nature of the process, the replacement of primary, secondary and tertiary hydrogens upon reaction with electrophilic fluorine atoms is not as selective as for other radicals. For example, early work by Tedder [30,34], showed that the order of selectivity follows the usual pattern, i. e. tert > sec > prim, but the relative selectivity of fluorine atoms is less than chlorine atoms (Table 2). 

The formation of formaldehyde by reaction (17) is not consistent with the experimental data. H02 yielding peroxide is obtained besides formaldehyde. The reaction of the CH2OH radical with 02 was investigated63 under conditions comparable to those for the reaction O + CH3OH. The CH2OH radical was obtained by the reaction of hydrogen atoms with methyl alcohol. It was found that hydrogen atoms reacted with alcohol only, by abstraction of the H atom. The radical obtained yielded... 

If hydroxyl were to react by H atom abstraction in the case of ethane, and by addition with ethylene, an inverse dependence would be observed for the activation energy, namely, a decrease from ethane to ethylene, as is the case for hydrogen atom reactions. The activation energies for reactions of oxygen atoms with ethane and ethylene decrease from ethane... 

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