Methanol, bond dissociation energy

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

The different behavior of the alcohols probably arises from differences in bond dissociation energies. Experiments show that radical attack on methanol (4) and ethanol (27) leads to rupture of the C—H rather than the O—H bond. There appear to be no direct measurements of C—H bond energies in alcohols. However, D(R—OH) has been determined as 102 kcal. and does not appear to vary greatly with changes in R, provided R is a simple alkyl radical (16). Moreover, the heat of rearrangement of alkoxy radicals to hydroxyalkyl radicals has been determined from electron impact data (12). Considering, for example, 2-propanol and the following reactions... 

Figure 5.2 AMI and AMl-SRP parameters (eV) optimized to reproduce the C-H bond dissociation energy of methanol, the H-H bond dissociation energy of hydrogen, and the experimental energy for the illustrated hydrogen-atom transfer (kcal moL ). Note that in all cases but one, the magnitude of the parameter change on going from AMI to AMl-SRP is less than 10 percent...

Golden DM, Bierbaum VM, Howard CJ (1990) Comments on reevaluation of the bond dissociation energies AHDBE for H-OH, H-OOH, H-O, H-O, H-OO, and H-OO. J Phys Chem 94 5413-5415 Gray P, Herod AA (1968) Methyl radical reactions with isopropanol and methanol, their ethers and their deuterated derivatives. Trans Faraday Soc 64 2723-2734... 

Fig. 6 and 7 display typical results for the effect of solvent on the excitation spectra, fluorescence and lifetimes. We used these observations to measure bond dissociation energy of the different species by varying the excess vibrational energy of the parent jet-cooled molecule type modes. The threshold effect found for the dissociation of the 1 1 IQ (methanol) is evident and occurs at ca. 3 kcal mol , in agreement with solution-phase enthalpies. (The effect of higher degree of solvation on the dynamics will be discussed elsewhere.)... 

While the focus of our research is to utimately activate methane to methanol, as is readily done by methane monooxygenase, we also want to understand what types of biomimics will activate higher homologues as well (C2, C3, and cycloC ). In addition, the bond dissociation energies may play an important role in our ability to activate methane at ambient temperature, since methane has the highest C-H bond dissociation energy (kcal) of all alkanes, i.e., methane(104) ethane(98) propane(96) and cyclohexane (94). 

Use the thermochemical data of the various tables in this chapter to predict the bond dissociation energy for the O-H bond of ethanol. Does your value make sense, considering the comparable value for methanol given in Table 2.1 ... 

In the racemization process, the Co-C bond was cloven homolytically to produce the cyanoethyl radical and the Co(ll) complex. The Co-C bond dissociation energy for these cobaloxime complexes was estimated to be 117-122 kJ mob [28]. The racemization rate of the complex crystal by X-rays is far lower than that by visible light not only in the solid state but also in an aqueous methanol solntion [20]. It seemed adequate to consider that the secondary radiation produced by the interaction with the Co atom may be responsible for the cleavage of the bond. 

Tables 2.3 and 2.4 list a selection of typical dissociation energies. The values given in Table 2.4 are average dissociation energies for a number of different molecules. For instance, the strength quoted for a C—O single bond is the average strength of such bonds in a selection of organic molecules, such as methanol (CH3—OH), ethanol (CH,CH2—OH), and dimethyl ether (CH,—O—Cl l5). The values should therefore be regarded as typical rather than as accurate values for a particular molecule.

The dissociation energy Do of the phenol-methanol complex (phenol as hydrogen-bond donor), calculated with MP2 and B3LYP using the rather small 6-31G(d, p) basis set, is 21.78 and 22.91 klmoL, respectively . The calculated Do shows good agreement with the experimental value (25.56 0.75 kJmoP ) . 

Figure 4.8. (a) A model of H20ad on a Pt-M alloy, (b) activation energies for OH bond dissociation in H20ad [70]. (Reproduced by permission of ECS—The Electrochemical Society, from Anderson A, Grantscharova E, Seong S. Systematic theoretical study of alloys of platinum for enhanced methanol fuel cell performance.)... 

Homolytic dissociation energies of the C-O and O-H bonds in methanol are relatively high. Catalysts are often used to activate the bonds and to increase the selectivity to desired products. 

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