Acid—base reactions Lewis

Critical points in the VSCCs of the respective base and acid atoms. This information enables one to predict positions of attack within a molecule and, hence, the geometries of approach of the reactan For example, a keto oxygen in the formamide molecule has two large non-bonded charge concentrations in the plane of the nuclei (VV = 6.25 and — 6.30 au), while the 

Displays of — for the formamide molecule in the plane containing the nuclei and in a perpendicular plane containing the C-N axis. The values refer to the non-bonded maxima in the VSCCs of the oxygen and nitrogen atoms. The VSCC of carbon exhibits three bonded maxima in the upper figure. The Laplacian is positive over much of the VSCC of carbon in the plane illustrated in the lower diagram. The two points in this plane where is most positive are the holes in the VSCC of carbon, the points of nucleophilic attack (as indicated by 

10) and the corresponding critical points for a number of ketones are positioned to form angles of 110 + 1° with respect to the C=0 bond axis (see Fig, 7.20, as well). This is the angle of attack predicted for the approach of a nucleophile to a carbonyl carbon (Burgi and Dunitz 1983). 

Electrostatic potential maps have been used to make predictions similar to these (Scrocco and Tomasi 1978). Such maps, however, do not in general reveal the location of the sites of nucleophilic attack (Politzer et al. 1982), as the maps are determined by only the classical part of the potential. The local virial theorem, eqn (7.4), determines the sign of the Laplacian of the charge density. The potential energy density -f (r) (eqn (6.30)) appearing in eqn (7.4) involves the full quantum potential. It contains the virial of the Ehrenfest force (eqn (6.29)), the force exerted on the electronic charge at a point in space (eqns (6.16) and (6.17)). The classical electrostatic force is one component of this total force. 

Relief plots of the negative of the l-apladan distributions for N O (top), SCO. and OCO (bottom). An oxygen atom is on the right-hand side in each case. 

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Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

When a complex ion is formed from a simple cation, the electron pairs required for bond formation come solely from the ligands. Reactions such as these, in which one species donates an electron pair to another, are referred to as Lewis acid-base reactions. In particular—... 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

Dinitrogen monoxide, N,0, reacts with water to form hyponitrous acid, H,N202(aq), in a Lewis acid-base reaction. 

Self-Test 14.7B Is the reaction between CaO and SiO, a redox reaction or a Lewis acid-base reaction If redox, identify the oxidizing and reducing agents. If Lewis acid-base, identify the acid and the base. 

FIGURE 15.4 When aqueous ammonia is added to a copper(ll) sulfate solution, first a light-blue precipitate ot Cu(OH)2 forms (the cloudy region at the top, which appears dark because it is backlit). The precipitate disappears when more ammonia is added to form the dark blue complex ru(NH))4 T by a Lewis acid-base reaction. 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

B Lewis acid-base reaction CO is the Lewis acid and OH- is the Lewis base. 

Lewis Acid-base Reactions, the carbocation may combine with a species possessing an electron pair (a Lewis acid-base reaction, see Chapter 8) ... 

The most common reaction of carbanions is combination with a positive species, usually a proton, or with another species that has an empty orbital in its outer shell (a Lewis acid-base reaction) ... 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

C21-0022. Draw molecular pictures showing a t q)ical Lewis acid-base reaction and a typical Br< )nsted acid-base reaction. Describe in words the differences and similarities of these two reactions. 

C21-0093. Some pure liquid interhalogen compounds are good electrical conductors, indicating that they contain cations and anions. Show a Lewis acid-base reaction between two bromine trifluoride molecules that would generate ionic species. 

A base has the ability to donate a pair of electrons and an acid the ability to accept a pair of electrons to form a covalent bond. The product of a Lewis acid-base reaction may be called an adduct, a coordination compound or a coordination complex (Vander Werf, 1961). Neither salt nor conjugate acid-base formation is a requirement. 

The literature3 contains some limited work on decaborane-based polymer systems. Typically, most utilize the Lewis acid/base reaction between decaborane (Lewis acid, B10H12) and amines and phosphines (Lewis bases, L) resulting in the formation of complexes (see scheme 2) having the general formulas B10H12L2. For example, the... 

Step 2 is the reverse of a Lewis acid-base reaction. (The presence of a formal... 

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

The following is an example of a Lewis acid-base reaction. 

When a salt is dissolved in water, the metal ions, especially transition metal ions, form a complex ion with water molecules and/or other species. A complex ion is composed of a metal ion bonded to two or more molecules or ions called ligands. These are Lewis acid-base reactions. For example, suppose Cr(N03)3 is dissolved in water. The Cr3+ cation attracts water molecules to form the complex ion Cr(H20)63+. In this complex ion, water acts as the ligand. If ammonia is added to this solution, the ammonia can displace the water molecules from the complex ... 

The Lewis acid-base reaction leading to complex formation910 has been recently11 considered in relation to the role of solvation effects. Many scales of thermodynamic parameters have been suggested. The concept of donor number (DN) was proposed by Gutmann12, and defined as the AH (kcalmol-1) for the interaction of a basic solvent with SbCL in 1,2-dichloromethane at room temperature ... 

Proposed mechanisms for polycondensations are essentially the same as those proposed in the organic chemistry of smaller molecules. Here, we will briefly consider several examples to illustrate this similarity between reaction mechanisms for small molecules and those forming polymers. For instance, the synthesis of polyamides (nylons) is envisioned as a simple Sn2 type Lewis acid-base reaction, with the Lewis base nucleophilic amine attacking the electron-poor, electrophilic carbonyl site followed by loss of a proton. 

The majority of the condensation polymerizations can be considered extensions of typical Lewis acid-base reactions. 

Problem 3.34 Given the following Lewis acid-base reactions ... 

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 participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

Hossain, M. Z., Machida, S.-I., Nagao, M., Yamashita, Y., Mukai, K. and Yoshinobu, J. Highly selective surface Lewis acid-base reaction Trimethylamine on Si(100)c(4 x 2). Journal of Physical Chemistry 108, 4737 (2004). 

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