Cyclohexane, bond dissociation energy

The bond dissociation energies in the gas phase for Me—SO2, Et—SO2 and Ph— SO2 have been evaluated to be 18, 16 and 44kcalmol respectively although Horowitz, from complex kinetic studies of the radiolysis of MeS02Cl in cyclohexane, has obtained the values of 15 and 12 kcal mol for D(Me—SOy and D(c-C6Hn—SO2), respectively these bond dissociation energies are considerably lower than the gas-phase values, and it has been suggested that the possible cause of this difference is due to the heat of vaporization of sulfur dioxide . 

Use 83 kcal mole-1 for the bond-dissociation energy of a normal C-C bond and 68 kcal mole-1 for the bond-dissociation energy of a C-Br bond. (An easy way to solve a problem of this type is first to calculate AH of each step for cyclohexane, for which there is no strain, then to make suitable corrections for the strain that is present for smaller values of n)... 

Dihydro-5,10-disilanthracene and AIBN can reduce xanthates and thiocarbonates in refluxing cyclohexane solution. Reduction of xanthates with monosilanes such as PhSiH3 and Ph2SiH2 initiated by AIBN does not work effectively because of their strong Si-H bond dissociation energies. However, the same reactions using either dibenzoyl peroxide or triethylborane as an initiator do induce the effective reduction of xanthates. 

Problem 10.3 The carbon-hydrogen bond dissociation energy for benzene (112 kcal) is considerably larger than for cyclohexane. On the basis of the orbital picture of benzene, what is one factor that ntay be responsible for this What piece of physical evidence tends to support your answer Hint Look at Fig. 10.4 and )see Sec. 5.4.)... 

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). 

In the paper describing the kinetics of the 3,3-shift of l,l-dideuterio-l,5-hexadiene, the specter of a non-concerted reaction path via a 1,4-cyclohexane diyl was raised. This species could result from exclusively bond making between Cl and C6 without concomitant cleavage of the C3,C4 bond. The estimated heat of formation of this species as a non-interacting biradical was 38 kcal/mol above 1,5-hexadiene by the usual technique of addition twice the secondary C-H bond dissociation energy minus the BDE of dihydrogen to the heat of formation of cyclohexane and comparing this the heat of formation of 1,5-hexadiene. 

Orbital theories relate an increase in the electronegativity of a carbon atom relative to that of a bonded hydrogen to an increase in the s character of its bonding hybrid orbital. Thus the decrease in bond length and pK and the increase in force constant and bond dissociation energy for C—H through the series cyclohexane, ethylene and... 

Using the well-known alcoholysis reactions of titanium ethoxide in cyclohexane, the standard heat of formation of various titanium tetra-aUcoxides in liquid as well as gaseous states were determined and the average bond dissociation energy was calcu-lated ° as given in Table 2.8. 

Accordingly,the bond dissociation energy D(Me-S02) in cyclohexane is 14.94 kcal/mole. This value is lower by 6 kcal/mole than the average of the gas phase estimations (38-40). It is worth noting that the difference of 6 kcal/mole between the gas and liquid phase bond dissociation energies is almost equal to the heat of vaporization of SO2. 

Basch et al. [44] used pseudopotential HF calculations for a determination of dissociation energies for the Pt-ligand bonds in cfv-[Pt(OCH3-0)(CH20H-C)(NH3)2]. This hypothetical compound models the antitumor-active complexes cis- [Pt(ascorbate)Cdach)] (dach = cyclohexane- 1,2-di-... 

Adiabatic frequencies of 1 are compared in Table 20 with those of some other hydro-carbons The adiabatic CC frequency is about 80 and the adiabatic CH stretching frequency about 100 cm larger than the corresponding values for cyclohexane. Compared to ethene, adiabatic CH stretching frequencies are almost identical, which is in line with the high dissociation energy of the CH bond of 1 (see Section V. E). The same observation has been made by McKean using isolated CH frequencies obtained by appropriate deuteration of 1 ... 

If this relation is true for ketones, then the dissociation energies of a-C—H bonds in ketones may be estimated. Let DC H in cyclohexane be 89 kcal mole-1, then DC H for the cyclohexanone a-CH2 group will be 89 — AD = 85 kcal mole-1, since AD = 1.8/0.45 = 4 kcal mole-1. If DC H t in isopentane is taken as 85 kcal mole-1 (as in isobutane), then Dq h for methyl i-propyl ketone will be 85 — AQ = 85 — 4 = 81 kcal mole-1. The decrease in strength of the a-C—H bond of ketones in comparison with... 

Owing to the fact that the C-F compared to a C-H bond is stronger (the dissociation energy of a C-F bond is 129 kcal moh while that for a C-H bond is 100 kcal mol" ), perfluorinated solvents are generally inert. Fluorinated solvents are therefore the best medium when one wishes to avoid hydrogen abstraction reactions, which occur to some degree in almost any solvent. A remarkable influence of perfluoro solvents on the Sj lifetime of thiones and on the Tj lifetime of benzophenone at room temperature have been noted. Xanthione in hexane has an lifetime of 25 picoseconds while in perfluorohexane, the lifetime is 162 picosecond.- The triplet lifetime of benzophenone in cyclohexane is 5 microseconds while in perfluoromethylcyclohexane it is 0.7 milliseconds." An increase of three orders is observed. The quantum... 

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