Hybridization and aromaticity

The out-of-plane terms (Eq. 3.33) are used for sp2 hybridized and aromatic systems such as carboxylates and phenyl groups. In metal compounds, out-of-plane terms are sometimes used for the modeling of square planar complexes (see Fig. 3.15, Section 3.6 below). 

Compare the basicity of amines with other common bases, and explain how their basicity varies with hybridization and aromaticity. 

Explain how the basicity of amines varies with hybridization and aromaticity. 

Assume that the van der Waals radius of the lanthanum atom is about 3.5 A. Use the carbon-carbon bond lengths in Table 5.1 to estimate the minimum number of interconnected, sp hybridized, and aromatic carbon atoms that would be required to completely encase one lanthanum atom in a roughly spherical shell. 

In pyrrole on the other hand the unshared pair belonging to nitrogen must be added to the four tt electrons of the two double bonds m order to meet the six tt elec tron requirement As shown m Figure 11 166 the nitrogen of pyrrole is sp hybridized and the pair of electrons occupies a p orbital where both electrons can participate m the aromatic tt system... 

CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

The database of Garbon Atom Gontribuhons contains increments for different types of carbons that are not involved in any funchonal group. They differ by their state of hybridization (sp, sp sp ), number of attached hydrogens or branching (primary, secondary, tertiary and quaternary), cyclization (cyclic and noncyclic) and aromaticity (nonaromahc, aromatic and fused aromatic). 

The structure of a-C H films may be thus pictured as sp--carbon atoms in condensed aromatic clusters, dispersed in an sp- -rich matrix, which confers to the network its characteristic rigidity. This situation can also be regarded as a random covalent network in which the sp" clusters of a defined size take part in the structure as an individual composed atom with its corresponding coordination number [17]. Such kinds of models have been successfully used to describe the dependence of a-C H film mechanical properties on composition, hybridization, and sp" clustering [23]. 

Alkorta, I., Elguero, J., 1998, Ab Initio Hybrid DFT-GIAO Calculations of the Shielding Produced by Carbon-Carbon Bonds and Aromatic Rings in H NMR Spectroscopy , New J. Chem., 381. 

The synthesis of block polymers of diacetylene-silarylene and diacetylene-carboranylenesiloxane polymers (99a-e) (Fig. 61) by the polycondensation reaction of 1,4-dilithiobutadiyne with l,4-bis(dimethylchlorosilyl) benzene and/or l,7-bis(tetramethylchlorodisiloxane)-m-carborane have been reported by Sundar and Keller.129 These polymers are a hybrid between the carboranylenesiloxane and silarylene-siloxane polymers and have high char yields (up to 85%) at 1000°C in N2 and in air, reflecting the thermal stabilizing effects of the carborane and aromatic units in the polymeric backbone. 

To assess, at least qualitatively, how much of the observed shift in the triphenylcarbonium ion is due to the change of hybridization from to sp and how much to the effect of the positive charge, a comparison of the chemical shifts of the triphenyl-C -carbonium and trimethyl-C -carbonium ions with their parent sp -hybridized covalent precursors and with some C -compounds having p -hybridization is useful. Data of Table 9, indicate that the C -shifts of ap -hybridized compounds (olefins and aromatic hydrocarbons), at least in the molecules studied (Lauterbur, 1957,1962), are very similar and fairly independent of the nature of the molecules. 

In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two electrons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp hybridized, and one sp orbital on each is bonded to hydrogen. To complete the six 7i-electron system the non-hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of electrons, and the compound cannot function as a base without losing its aromatic character. 

Pyridine has tt electron structure similar to that of benzene. Each of the five 5 p -hybridized carbons has a p orbital perpendicular to the plane of the ring. Each p orbital has one tt electron. The nitrogen atom is also sp -hybridized and has one electron in the p orbital. So, there are six tt electrons in the ring. The nitrogen lone pair electrons are in an sp orbital in the plane of the ring and are not a part of the aromatic tt system. 

The situation in pyrrole is slightly different. Pyrrole has a tt electron system similar to that of the cyclopentadienyl anion. It has four 5p -hybridized carbons, each of which has a p orbital perpendicular to the ring and contributes one tt electron. The nitrogen atom is also 5p -hybridized and its lone pair electrons occupies a p orbital. Therefore, there is a total of six tt electrons, which makes pyrrole an aromatic compound. 

The question of aromaticity arises. Neither thiophenium salts nor thiophene sulfoxides are especially stable, making the classical reactivity test of electrophilic aromatic substitution difficult. The former dealkylate readily and the latter, at least for the case of thiophene sulfoxide, readily undergo self-dimerization (65CCC1158) (the bulky substituents of (57) impede this reaction). Aromaticity requires that the lone pair on sulfur participate in the aromatic sextet. If the lone pair, because of sp3 hybridization and improper symmetry, is not delocalized into the butadiene segment, the system will be antiaromatic. 

We restrict our attention in this chapter to the simple but widely used Hiickel MO (HMO) method for calculating orbitals for rc-electron and aromatic molecules [2], The HMO scheme assumes that a conjugated n-electron molecule consists of a network of sp2-hybridized carbon atoms lying in a plane and each participating atom i has a 2p electron in an atomic orbital, < ) , perpendicular to this plane. Linear combinations of these atomic orbitals (LCAO) result in the molecular n wavefunctions, q/j, each of which has a discrete energy, Ej. In terms of the parameters used in HMO computations,... 

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