Electron donors, structural

Notice that the MO picture gives the same qualitative picture of the substituent effects as described by resonance structures. The amino group is pictured by resonance as an electron donor which causes a buildup of electron density at the /3 carbon, whereas the formyl group is an electron acceptor which diminishes electron density at the /3 carbon. 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

The heteroaromatic compounds can be divided into two broad groups, called n-excessive and n-deficient, depending on whether the heteroatom acts as an electron donor or an electron acceptor. Furan, pyrrole, thiophene, and other heterocyclics incorporating an oxygen, nitrogen, or sulfur atom that contributes two n electrons are in the rr-exeessive group. This classification is suggested by resonance structures and confirmed by various MO methods. ... 

Figure 6.21 Relation between C2B9Hn and C sHs". In this formalism the c/o.ro-carboranes C2B10H12 are considered as a coordination complex between the pentahapto 6-electron donor C2B9H] - and the acceptor BH (which has 3 vacant orbitals). The o/oso-structure can be regained by capping the open pentagonal face with an equivalent metal acceptor that has 3 vacant orbitals.

Draw a Lewis structure (or a series of Lewis structures) for nitrobenzenium ion. Where is the positive charge Examine the electrostatic potential map for nitrobenzenium ion. Where would you expect electron-donor substituents to have the greatest stabilizing effect (consider meta and para positions only) Which is the more stable, meta or para-nitrotoluenium ion (intermediates in nitration of toluene) Compare electrostatic potential maps to that for nitrobenzenium ion. Does your result suggest that methyl acts as an electron donor ... 

Systems 2 and 3 are cross-conjugated, but 5 is not, and it might have been expected that a-substituted compounds would be more prone to exist in the aromatic form 1 than the -compounds to exist as 4. From the limited evidence available, the reverse appears to be the case. When the hetero atom is not a very strong electron donor, i.e., Z — S or O, structure 3 would be expected to be relatively more stable than 2, and this is supported by the evidence available. When Z = NR, the difference in the stabilities of 2 and 3 could be smaller. 

This has a folded structure (Figure 2.27) similar to that of rhodium carbonyl chloride (Figure 2.18) with ethene acting as a two-electron donor, but ethene is more weakly held and readily displaced by CO and certain alkenes (e.g. cycloocta-1,5-diene). 

Counting NO as a three-electron donor, [IrCl(NO)(PPh3)2]+ is, therefore, a 16-electron species isoelectronic with Vaska s compound, isolable as a red crystalline hexafluorophosphate (m.p. 211°C, i/(N-0) 1870 cm-1) or similar perchlorate and tetrafluoroborate a trans-structure is indicated by spectroscopic data, and it is presumed to have a linear Ir-N-0 grouping. 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

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