Benzene, bond dissociation energy

Table 17. Bond Dissociation Energies (D ) of Ethylenes and Benzenes [78]...

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

This result can be explained by the following fact. The bond dissociation energies of the C-H bond in (CH3)3C-H (isobutane) and C6H5-H (benzene) are 91 kcal/mol and 112 kcal/mol, respectively. So, the bond dissociation energy of the C-H bond in benzene is 21 kcal/mol stronger than that in isobutane. This suggests that the phenyl radical is more unstable by about 21 kcal/mol than the Lbutyl radical, and therefore should be more reactive. 

Data on the physical and chemical properties of PCDTs and PCTAs are scarce. Due to their structural similarity to PCDFs and PCDDs they are also supposed to possess some likeness in their physical and chemical properties. Sulfur and oxygen are both Group VI elements with two outer shell electrons available for covalent bonding. Structures of thiophene and furan with benzene carbon-sulfur (Cb-S) and carbon-oxygen bond (Cb-0), in PCDTs and PCDFs respectively, suggest similar chemical behavior. The bond dissociation energies (AH) show that less energy is required to break the Cb-S bond than the Cb-0 bond [17,36,37]. 

The main products of the reaction are hydrogen bromide, propene and benzene, with smaller amounts of 1-bromopropene and 2-bromopropane. The mechanism requires the overall activation energy to be equal to that for the first step. While these two energies are very close for allyl bromide (estimates are 45.5 and 47.5 kcal.mole for reaction 1, and 45.4 kcal.mole" for the overall activation energy ), the C-Br bond dissociation energy of 65 kcal.mole for ethyl bromide is considerably greater than the overall activation energy of 53 kcal. 

In so far as the rate of formation of radicals reflects their stability or reactivity the findings of Hart and Wyman are instructive. In carbon tetrachloride the rate of decomposition of benzoyl peroxide was twice as fast as that of biscyclopropanoyl peroxide. Ingold and coworkers have found that in the photodecomposition of benzoyl and biscyclopropanoyl peroxides, in carbon tetrachloride at 298 K, the phenyl radicals produced reacted faster (7.8 x 10 M s ) than the cyclopropyl radicals (1.5 X 10 M s ). These results are consistent with C-H bond dissociation energies for benzene (llOkcalmol) and cyclopropane (106kcal mol ) which implies that the cyclopropyl radical should be less reactive than the phenyl radical. In subsequent work they also showed that at ambient temperatures radical reactivities decreased along the series /c = Ph > (Me)2 C=CH > cyclopropyl > Me. Table 4 records the absolute rate constants for the reaction of these radicals with tri-n-butylgermane. 

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.)... 

Consideration of bond dissociation energies has already show n us that a benzyl free radical is an extremely stable one. We have accounted for this stability on the basis of resonance involving the benzene ring (Sec. 12.14). 

Even these three types do not cover all the carbon-carbon bonds found in nature, however. In benzene (CgHg), the experimental carbon-carbon bond length is 1.397 A, and its bond dissociation energy is 505 kj mol . This bond is intermediate between a single bond and a double bond (its bond order is l ). In fact, the bonding in compounds such as benzene differs from that in many other compounds (see Chapter 7). Although many bonds have properties that depend primarily on the two atoms that form the bond (and thus are similar from one compound to another), bonding in benzene and related molecules, and a few other classes of compounds, depends on the nature of the whole molecule. 

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

PROBLEM 23.1 Consider all the isomers of C7H7CI containing a benzene ring and write the structure of the one that has the weakest carbon-chlorine bond as measured by its bond dissociation energy. 

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