Acid-base reactions electron density

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

In order to understand why the activation energies differ between the two pathways, Mui et al. examined the transition state geometries [279]. They found that as electron density is donated from the amine lone pair to the down silicon atom upon adsorption into the precursor state, the up Si atom in the dimer becomes electron rich. At this stage, the dative bonded precursor can be described as a quaternary ammonium ion. The N—H dissociation pathway can thus be interpreted as the transfer of a proton from the ammonium ion to the electron-rich up Si atom through a Lewis acid-base reaction. In the transition state for this proton transfer, the N—H and Si—H... 

The Laplacian of the electron density plays a dominant role throughout the theory.191 In addition, Bader has shown that the topology of the Laplacian recovers the Lewis model of the electron pair, a model that is not evident in the topology of the electron density itself. The Laplacian of the density thus provides a physical valence-shell electron pair repulsion (VSEPR) basis for the model of molecular geometry and for the prediction of the reaction sites and their relative alignment in acid-base reactions. This work is closely tied to earlier studies by Bader of the electron pair density, demonstrating that the spatial localization of electrons is a result of a corresponding localization of the Fermi correlation hole. 

The movement of electrons in Lewis acid-base reactions can be seen clearly with electrostatic potential maps. In the reaction of boron trifluoride with dimethyl ether, for instance, the ether oxygen atom becomes more positive and the boron becomes more negative as electron density is transferred and the B-0 bond forms (Figure 2.6). 

It was noted that nucleophiles are less nucleophilic if they that can delocalize electrons by resonance. An example is the phenoxide anion, where electron density is delocalized away from oxygen by the adjacent phenyl ring, meaning that oxygen cannot donate electrons to carbon as effectively and is less nucleophilic. It should also be noted that nucleophilic reactions are usually under kinetic control, whereas acid-base reactions are under thermodynamic control. Under kinetic control, the most nucleophilic species will react faster and dominate the substitution reaction. 

Relative rates of O2 uptake by tra/is-[Ir(X)(CO)(PPh2R)2] ( = Cl, Br, I R = Ph, Et, Me) in CHjCh follow the order R = Me > Et > Ph and X = I > Br > Cl > F. If dioxygen addition is considered as an acid-base reaction (metal complex = Lewis base), then factors which increase the electron density at the metal centre should (in the absence of unfevourable steric effects) enhance the addition of O2 (reaction 90). The results suggest that as the basicity of the phosphine ligand increases and the electronegatively of X decreases, the rate of O2 addition also increases. The X-ray crystal structure of [Ir(Cl)(C0)(02)(PPh2Et)2] reveals a side-on coordination with the O2 centre, CO and Cl moieties defining the equatorial plane, or a distorted octahedral iridium(III) system with... 

An explanation for the correlation of acidity with 7 3c h is that both are related to the hybridization of the carbon orbital used for C-H bonding. Because s orbitals are lower in energy than p orbitals, a hybrid orbital with more s character will be lower in energy and thus have electron density closer to the nucleus than will a hybrid with less s character. This lower energy orbital will be more electronegative and will be better able to stabilize a negative charge when a proton is removed in an acid-base reaction. 

Although benzene has six 7i-electrons, it cannot donate electrons as well as a simple alkene with a single n-bond. Benzene is less reactive than a simple alkene. The reaction of an alkene with HCl is an acid-base reaction, where the 7i-bond of the alkene is the Brpnsted-Lowry base. If benzene does not react with HCl, benzene must be a weaker base than the alkene. Benzene is a weaker base (unable to donate electrons as efficiently) because the six 7t-electrons of benzene are delocalized on six carbons, whereas the two 7t-electrons of an alkene are only distributed between two carbons. In other words, benzene is resonance stabilized, making it less reactive. The more delocalized the electrons are, the lower is the net electron density for any point between the carbon atoms. If a reaction has to occur at one carbon in any reaction, then the net electron density of one carbon in benzene is less than that of one carbon in an alkene. Benzene is less reactive than alkenes because more energy is required to disrupt the 7t-system (an endothermic process see Chapter 7, Section 7.5) and the electron delocalization makes less electron density available for donation. 

Pyridine is a tertiary amine and a good base, as noted in Section 26.1.2. Because of this property, many of the electrophilic reagents used for aromatic substitution coordinate with the electron pair on nitrogen (an acid-base reaction). Specifically, the Lewis acids used in Chapter 21 for electrophilic aromatic substitution will coordinate with the electron pair on nitrogen, so they cannot be used. If electrophilic aromatic substitution does occur, the reaction is slow, and such reactions are difficult. Carbons 3 and 5, relative to nitrogen, have the greatest x-electron density (see IOC) and they are the major sites for reaction. The intermediates generated from pyridine in electrophilic... 

Sections 4.1.2 and 4.1.3 compared complex oxides and halides with the binary (parent) compounds. In this section we attempt to show on a semiquantitative basis how complex oxides and halides favor high oxidation states, primarily because they represent acid-base reactions in which the presence of a basic oxides (e.g., BaO) or halide (e.g., CsCl) donates electron pairs (Lewis basicity) to the acidic (high charge density) f-element ion. 

All reactions are accomplished via a flow of electron density (the motion of electrons). Acid-base reactions are no exception. The flow of electron density is illustrated with curved arrows ... 

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