Energy, bond benzene

Thermochemical stabilization (in kcal/mol) based on difference between A/ and summation of standard bond energies (benzene RE = 45.8 kcal/mol)."... 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of... 

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

Because of the high C - F bond energy, glycosyl fluorides are stable in comparison to the other glycosyl halides, and this character has attracted much attention. They have been prepared in many different ways. One of them, rather classical, is through addition of the elements of HF (for example, HF in benzene ), BrF, or IF to per-O-acylated glycals. ... 

However, direct calculations of accurate bond energies represent a major challenge. Examples are given [13,14] where the ratios of carbon-carbon bond energies, relative to that of ethane, were successfully calculated for ethylene, acetylene, benzene, and... 

No such SDCI results are presently available for benzene, but taking advantage of the fact that the carbon atoms are evidently electroneutral in graphite and not so in benzenoid hydrocarbons, the results obtained for graphite support the approximate validity of bond energies deduced for polynuclear benzenoid hydrocarbons and of the net charge, 13.2 me (probably + 1 me), deduced for the carbon atom of benzene [44]. 

The CC and CH bonds of ethane (Example 10.1), and the final selection See = 69.633 and 8ch = 106.806 kcal/mol, are used to get the CC and CH bonds found in unsaturated hydrocarbons by retaining both the contribution of Fkh Eq. (11.12), and the effect of charge variations described by Eq. (10.37). The reference CC double bond of ethylene and the reference CC bonds of benzene, however, roughly estimated along the lines described in Example 10.1, are deduced from the appropriate CH bond energies and the energy of atomization of the corresponding molecule, AE, obtained from experimental data. 

The following bonds occur in benzenoid hydrocarbons. Consider first the endocyclic carbon-carbon bonds, namely, those found in benzene, with = 115.39 (or Sqq = 124.84) kcal/mol, which—in a sketchy way—are some kind of averages between a single and a double sp -sp bond. (Their number is double that of the number of double bonds that can be written in classical Kekule structures, e.g., 2 X 5 in naphthalene, 2 x 7 in anthracene.) But in polynuclear benzenoid structures there are not twice as many averages as there are Kekule double bonds. Hence, consider the extra single C sp )—C sp ) bonds like the one found in naphthalene, or the two extra single bonds found in anthracene. The appropriate bond energy formulas are... 

The increased stability of 4n + 2 cyclic planar polyenes, relative to their imaginary classical counterparts, comes about because all the bonding energy levels within the ji-system are completely filled. For benzene and pyridine there are three such levels, each containing two spin-paired electrons. There is then an analogy between the electronic constitutions of these molecules and atoms that achieve noble gas structure. 

The Mg+—CeHe dissociation energy at 0 K was determined to be 134 4 kJmol (1.39 0.10 eV) using collision induced dissociation and 112 kJmoH by laser photodissociation . Using the radiative association kinetics approach to ion cyclotron resonance spectrometry, the value was shown to be the comparable 1.61 eV (155 kJmoU ). It was also shown that the binding of the second benzene to Mg , i.e. the Mg+ (CeHe)—CgHe bond energy, is less than 1.4 eV (135 kJmoU ). 

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