Resonance Acceptors

FIGURE 12.35 Resonance structures of the possible intermediates for the electrophilic substitution of nitrobenzene. 

FIGURE 12.36 CF3 is a mefa-directing, inductive withdrawer of electron density. 

We have two effects to consider—inductive and resonance. Halogens are inductive withdraw-ers of electron density, but they also have lone pairs that can assist in stabilizing a carbocation. Initially, you may feel uncomfortable with the idea that halogens can be resonance donors, but it should not be so surprising, as we have seen it before when we considered the addition of bromine 

Although it may seem counterintuitive, there are ground-state effects that give evidence for this phenomenon. In chlorobenzene, the contribution of resonance forms such as 12.8, in which there is a positive charge on chlorine, results in a shortened carbon-chlorine bond (1.69 A vs. 1.77 A in chloroethene and 1.72-1.75 A in haloalkanes) and a lowered dipole moment. 

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Resonance acceptor effect of R3M substituents towards Rff (d- r conjugation) and X (d-n conjugation) decreases as M changes in the order Si > Ge > Sn > Pb. The question whether the R = Pb group still has resonance acceptor properties is under debate. 

The a,it- and <7,jr -orbitals also interact with the d,a -orbital. This results in the stabilization (decrease in energy) of the a,it- and -orbitals, involving the formation of two new orbitals with lower energy, namely d,jr and d,jr. The designations d,jr and d, r represent resonance acceptor properties of Me3M substituents towards the frontier it- and jr -orbitals of MesMRjr (see also References 30 and 33). 

Sn > Pb. This trend agrees with a maximal resonance acceptor effect (d-jr conjugation) for M = Si mentioned above repeatedly, which weakens as the atomic number of M increases. 

For all benzene substituents which are resonance donors, ap > ap 1. It follows from equation 23 that the differences ap 1 — ap characterizing the strengthening of the a-n conjugation increase with the enhancement of the polarizability of all the bonds within the substituent bonded to the aromatic ring. The quantitative characteristic of the overall substituent polarizability is the sum of the refractions of its bonds, E R > (see Section I). The values of ap+ and ap are approximately equal for organic substituents which are resonance acceptors. If the R3- X M substituents had only a resonance acceptor effect (the d-7T conjugation), the correlation in equation 23 would fail for compounds R3-nX MPh. In fact, equation 24... 

A complex mechanism of the resonance acceptor effect (the d-jr conjugation in R3MRjr and the d-n conjugation in R3MX, for M = Ge, Sn, Pb). This effect is absent when M = C. The acceptor effect includes the participation of unoccupied nd-orbitals of M and of antibonding a -orbitals of the M—R bonds of the R3M fragments (R = alkyl). 

The energy separation between 1T8+ and 1 7+ acceptor levels is called the acceptor spin-orbit splitting Aa and it depends on the chemical nature of the acceptor and the host crystal. The nP1/2 and nP3/2 states associated with the T7+ VB give T5 and T states distinct from those of the Ts+ VB, and are resonant with the VB. The acceptor transitions between the 1T8+ state and the odd-parity states associated with the p3/2 VB r8+ are responsible for the so-called p3/2 spectrum. Logically, transitions between the 1T7+ state and the odd-parity resonant states associated with the p1/2 VB should also produce a distinct spectrum. No such transitions have been observed for Si, as the 1T7+ state, which lies in the band gap, is depopulated up to RT, but for diamond, due to the smaller value of Asoa in this material, weak transitions between 1T7+ and the odd-parity states associated with the T7+ VB have been observed near 80 K [27]. As no selection rule forbids them, transitions between the 1T8+ state and the p7/2 resonant acceptor states have indeed been observed in silicon, reported first in [66], and they are known as the pi/2 spectrum. 

Where a benzene ring bears a resonance acceptor group, substitution occurs at the mete-position and is slow compared with the reaction of benzene. We describe these substituents as mete-directing and deactivating. We can exemplify this by thinking about the reaction of nitrobenzene with electrophiles. The nitro group is a powerful electron withdrawer, both by resonance and induction (Figure 12.35). 

Resonance acceptors are deactivating and direct meta (-COOH, -COOR, -C(=0)R, -CHO, -NOj, -SO3H, -CN). 

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