Direct Acid-Base Reaction

Evidence in favour of direct initiation involving the attack of an undissociated Lewis acid ito an olefin is essentially confined to systems containing titanium tetrachloride. This section is devoted to an analysis and discussion of that evidence and to the proposal of an overall mechanism compatible with it. 

The occurrence of condensation polymerisation is not unique to isobutene. Sauvet observed it with the system titanium tetrachloride-1,1, -diphenylethylene and Vairon and Sigwalt with cyclopentadiene-butoxytitanium trichloride. A general rationalisation of this phenomenology must therefore be found. This is pven at the end of this section after a airvey of other systems involving titanium tetrachloride. 

A few years after the discovery of ccxidensation polymerisation, the behaviour of prepolymerisation was reinvestigated in the same laboratory using highly purified isobutene, TiCl4 and dichloromethane in the temperature range —60 to —30 °C. It was concluded that direct initiation was responsible for most of the polymerisation observed, and the limited yields wre attributed to the consumption of the catalyst in the formation of an inactive complex with the monomer. No indications exist however to support 

No other aliphatic olefin has been investigated in detail with titanium tetrachloride. 2-methyl-2-butene is known to oligomerise in methylene chloride at —78 but 

The interaction between titanium tetrachloride and model aromatic olefins has been extensively studied and has afforded a great deal of fundamental information. 1,1-diphenylethylene was the first one to be investigated. Sauvet et riiowed that initiation took place both with and without a cocatalyst. Later work by the same group, focussed on the behaviour of direct initiation, indicated that the concentration of active species decreased iriien the monomer concentration was increased  

---

This has been mentioned at various points in this paper and may involve either a direct acid-base reaction of nitrene and nucleophile or, in some instances, reaction of the nitrene precursor with the nucleophile (or 1,3-dipolarophile) followed by loss of nitrogen. For example, the reaction of benzenesulphonyl azide with pyridine to give 31 (Ar=Ph) 69> could either involve a free nitrene or a concerted process in which the lone pair on the pyridine nitrogen atom assists the elimination of molecular nitrogen. That some free nitrene can be involved in these reactions is clear from the isolation of some 3-benzenesulphonamido-2,6-lutidine... 

An alternative, widely used, approach to charge assisted networks is based on the exploitation of direct acid-base reactions. There are, broadly speaking, essentially two different means to obtain charge-assisted interactions, which depend on whether the network is constructed of ions of the same charge (homoionic hydrogen bonded networks) or of ions of opposite charge (hetero-ionic networks). These two limiting situations are shown in Fig. lc,d. The utilization of... 

This review has clearly matured on the basis of long and arduous experience. Years ago, Cheradame and Sigwalt demonstrated the occurrence of initiation by direct acid-base reaction between a Lewis acid and a vinyl monomer, wiiile Gandini and Flesch first put pseudocationic polymerisation involving propagation by undissociated ester intermediates on the map. Each of the two authors of this review was, accordingly, trained by one of the pioneers of cationic polymerisation. From them they learned the rigorous vacuum techniques which no other methods have yet been able to displace. 

Even faster setting and strength gain can be achieved by direct acid-base reactions. For example, magnesium chloride and magnesium oxide (Sorel cement) react very rapidly in water to form... 

Amides are sometimes prepared directly from carboxylic acids and amines by a two step process The first step is an acid-base reaction m which the acid and the amine combine to form an ammonium carboxylate salt On heating the ammonium carboxy late salt loses water to form an amide... 

The electric field-jump method is applicable to reactions of ions and dipoles. Application of a powerful electric field to a solution will favor the production of ions from a neutral species, and it will orient dipoles with the direction of the applied field. The method has been used to study metal ion complex formation, the binding of ions to macromolecules, and acid-base reactions. 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

The favorability of acid-base reactions is affected, in pa by electrostatic interactions between charged atoms a dipoles within the same molecule. The equilibrium w shift in the direction of an ion that is stabilized 1 intramolecular ion-dipole interactions. 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

First, the simple thermodynamic description of pe (or Eh) and pH are both most directly applicable to the liquid aqueous phase. Redox reactions can and do occur in the gas phase, but the rates of such processes are described by chemical kinetics and not by equilibrium concepts of thermodynamics. For example, the acid-base reaction... 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

The reversibility of reactions is another important characteristic in assessing the fate of deep-well-injected wastes. Depending on environmental conditions, reversible reactions readily proceed in either or both directions. Most acid-base reactions exemplify reversible processes. In aqueous solutions, relatively minor changes in such factors as pH or concentration can change the direction of these reactions. Irreversible reactions, typified by hydrolysis, have a strong tendency to go in one direction only. 

The limitation of using such a model is the assumption that the diffusional boundary layer, as defined by the effective diffusivity, is the same for both the solute and the micelle [45], This is a good approximation when the diffusivities of all species are similar. However, if the micelle is much larger than the free solute, then the difference between the diffusional boundary layer of the two species, as defined by Eq. (24), is significant since 8 is directly proportional to the diffusion coefficient. If known, the thickness of the diffusional boundary layer for each species can be included directly in the definition of the effective diffusivity. This approach is similar to the reaction plane model which has been used to describe acid-base reactions. 

As we have seen earlier in the case of proton transfer reactions such as occurs between HCl(g) and NH3(g), water is not necessary for the acid-base reaction to take place. This is also true of the reactions between the acidic oxides of nonmetals and the basic oxides of metals. In many cases, they react directly as illustrated in the following equations ... 

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. 

Proton acid-base reactions are not particularly sensitive to stoic compression, and hence provide a good measure of inductive effects. For acid-base character, three sets of reference reactions can be used. The easiest of these to perform experimentally requires an analogy be drawn between the relative pK values of a series of protonated annelated pyridines and the pK values of the analogous isoelectronic benzene. The second is a direct measure of the kinetic acidity of the a- and P-sites on a soies of annelated benzenes. The third is a related direct assessment of kinetic acidity by protodetritiation. 

Direct calorimetric method or temperatnre dependence of eqnilibrium constants can be nsed to measnre enthalpies and entropies of acid-base reactions. Calorimetric techniqnes allow obtaining an interesting quantification and evaluation of the gas-solid interactions and more details on use of data from these measurements will be given in the following section. 

The second method, illustrated in Section B for the ammonium salt, relies on the direct stoichiometric acid-base reaction between V2Os and the oxide, hydroxide, carbonate, or hydrogen carbonate (bicarbonate) of the desired positive ion. Surprisingly, this method works well for quite a variety of ions, even in cases where both reactants are only slightly soluble in water (e.g., CaO + V2Os). 

In the Bronsted definition, an acid donates a proton and a base accepts a proton. The strengths of acids and bases are measured by the extent to which they lose or gain protons, respectively. In these reactions acids are converted to their copjugate bases and bases to their conjugate acids. Acid-base reactions go in the direction of forming the weaker acid and the weaker base. 

---