Reactions of OH

The reaction of OH with Br2 has been studied under crossed-mol-ecular beam conditions [38] and was found to indicate the existence of a stable HOBrBr complex with a lifetime of several rotational periods. The HOBr product translational energy distribution was found to be well described by the RRKM—AM model and to be similar to the OX distribution from the reactions O + Br2 and I2. This is despite the fact that OH is isoelectronic with a F atom and that the most relevant study shows that Cl + Br2 is a direct stripping reaction. The fraction of the total energy appearing in product translation is 36% and there is some indication that the beam source contains a small proportion of vibrationally excited OH which may account for the measured product translational energy distribution extending beyond the maximum allowed for the reaction OH(z = 0) + Br2. 

Molecular beam measurements of the translational and vibrational energy dependence of the chemiluminescence cross section for the dioextane reaction 

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Zhen-Ming Hu and NakatsujI H 1999 Adsorption and disproportionation reaction of OH on Ag surfaces dipped adcluster model study Surf. Sci. 425 296-312... 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

Problem 11.2 What product would you expect to obtain from SN2 reaction of OH- with (k)-2-bromo-butane Show the stereochemistry of both reactant and product. 

In the presence of H + this gives the acidic form (CH3)2SOH, a species which is formed also by the reaction of OH" with dimethyl thioether. 

The pKa value of the equilibrium was found to be equal to 10.2. Meissner and coworkers36 studied also the reaction of OH radicals with DMSO, however, the product of this reaction has no optical absorption in the range 270-800 nm and they measured only the rate of this reaction by a competition method and obtained k = 4.2 x 109m-1s-1. 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

Veltwisch and colleagues45 studied the reaction of OH with several sulfoxides by pulse radiolysis using electrical conductivity for the detection of formation or disappearance of ions. Pulse radio lysis of N20-saturated aqueous solution of DMSO (10-3m) leads to a decrease in conductivity at basic pH (pH = 9.0) and an increase in conductivity at acidic pH (pH = 4.4). This is explained by the reactions... 

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

The absorption at 375 nm is assigned to the radical anion. OH radicals were found to give an optically active transient which absorbs at 360 nm and which is assigned by the authors to the radical anion formed by the reaction of OH with the sulfur atom of the thiomethyl bond of MTMSO,... 

Sumiyoshi and coworkers48 suggested that the reaction of OH radical with the sulfoxides S—C bond can be studied by looking for the sulfinic acid which is expected from the reaction of OH radical and a sulfoxide37,45 (reaction 24). 

Methanol is a very minor product and the observation that its polarization is more intense in N20-saturated solution than in He-saturated solution suggests that it is formed by reaction of OH" radical, probably by a degradation of the radical formed by the addition of OH to DMSO other than the main one given in reaction 21. 

Koulkes-Pujo and coworkers5 5 studied the formation of methane in the reaction of OH radicals and H atoms with aqueous DMSO in acidic media. In the radiolysis of deaerated acidic aqueous solution of DMSO they found that G(CH4) increases monotonously with CH4 concentration up to 0.8 m DMSO. Similar results were obtained for C2H6 but the yields of C2H6 are much lower than that of CH4. 

In the case of PCSO the addition of N20 leads to increased formation of cysteic acid, alanine and dipropyl sulfide and to a decrease in the yield of dipropyl disulfide. The addition of KBr decreases the yield of all the four products. These findings indicate that cysteic acid and alanine are formed by the reaction of OH radicals in parallel reactions as given in Figure 7. 

FIGURE 7. The reactions of OH radical with alkyl-L-cysteine sulfoxide. 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

TPEs associating both rigid and soft polyester blocks have also been described. They cannot be obtained by the melt polyesterification used for polyesterether TPEs, since interchange reactions would yield random—rather than block — copolyesters. The preferred method involves the reaction of OH-terminated aliphatic and aromatic-aliphatic polyesters with chain extenders such as diisocyanates and results in copoly(ester-ester-urethane)s. 

However, experiments in the gas phase gave different results. In reactions of OH with alkyltrimethylsilanes, it is possible for either R or Me to cleave. Since the R or Me comes off as a carbanion or incipient carbanion, the product ratio RH/MeH can be used to establish the relative stabilities of various R groups. From these experiments, a stability order of neopentyl > cyclopropyl > rcrt-butyl > n-pro-pyl > methyl > isopropyl > ethyl was found. On the other hand, in a different kind of gas-phase experiment, Graul and Squires were able to observe CHi ions, but not the ethyl, isopropyl, or (ert-butyl ions. 

Figure 10. Calculated potential energies in the gas phase (bottom) and potentials of mean force in aqueous solution (top) for the Adjj reaction of OH + H2C-O. Solid lines...

Another probable source of the relatively high yield of N2O observed in this experiment is reaction of OH with the nitroso-hydrazine ... 

Secondary product formation Is also expected to result from the reaction of OH with the nltramlne, but the mechanism and products formed In this case Is more uncertain. 

Temperature programmed reaction spectra depicting the reaction of H2O and OH groups with oxygen on Pd(lOO) are shown in fig. 3. Curve (a) was obtained for H2O adsorption on the clean surface and contains two peaks, the state at 167 K of multilayer H2O, and the <>2 state at 182 K due to H2O bound directly to the surface /7/. An additional state at 255 K, labelled y, is observed following coadsorption of 1 0 and 0 (fig. 3b). This state represents the reaction of OH groups III... 

Atkinson R, J Arey, B Zielinska, SM Aschmann (1987b) Kinetics and products of the gas-phase reactions of OH radicals and NjOj with naphthalene and biphenyl. Environ Sci Technol 21 1014-1022. 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

CH3S(0)CH2)2 is observed indicating that H atoms abstract hydrogen from DMSO to form CH3S(0)CH2 which dimerizes to (CH3S(0)CH2)2- The radical CH3S(0)CH2 is not found in the reaction of OH" with DMSO. ... 

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