Acid-base chemistry protic

Let s begin by defining a protic solvent. We will need to jog our memories about acid-base chemistry. Recall that in Chapter 3 we talked about the acidities of protons (these are hydrogen atoms without the electrons, symbolized by H+), and we... 

Theoretical studies aimed at rationalizing the interaction between the chiral modifier and the pyruvate have been undertaken using quantum chemistry techniques, at both ab initio and semi-empirical levels, and molecular mechanics. The studies were based on the experimental observation that the quinuclidine nitrogen is the main interaction center between cinchonidine and the reactant pyruvate. This center can either act as a nucleophile or after protonation (protic solvent) as an electrophile. In a first step, NH3 and NH4 have been used as models of this reaction center, and the optimal structures and complexation energies of the pyruvate with NH3 and NHa, respectively, were calculated [40]. The pyruvate—NHa complex was found to be much more stable (by 25 kcal/mol) due to favorable electrostatic interaction, indicating that in acidic solvents the protonated cinchonidine will interact with the pyruvate. 

Another fundamental reaction of >C=0 involves its reactivity as a base. In the Brpnsted sense, >C=0 - may react with a proton donor to produce a neutral ketyl radical (>C(.)OH, Figure 2, reaction 2). This is an important process when the reduction of a carbonyl compound is carried out under acidic conditions or in a protic media (e.g. elec-trochemically, with less reactive reducing reagents such as Mg or Zn, or when >C=0"-is produced via PIET and R3N"+ has available a-protons). The follow-up chemistry of >C(.)OH is that of a neutral free radical (dimerization to form pinacols, addition to unsaturated compounds, fragmentations/ring-openings, etc.), and thus beyond the scope of this chapter. 

It is practical to divide solvents into two groups, protic and aprotic solvents. Protic solvents are those that have protons bonded to heteroatoms and include acids, neutral solvents, and some bases. A review of solvents useful for electrochemistry has appeared [289]. Electrochemical reactions in nonaqueous systems [290] and the chemistry of non-aqueous solvents [291] have been treated in monographs. 

Chemically inert, solid acid catalysts that have strong and even superacid characters are needed. The role and the specific mechanism of protic and Lewis acid site interactions must be elucidated by both theoretical modeling and experimentation. Based on an analogy with the chemistry of molecular acids, the interacting H+ Lewis-acid system offers the best chance to achieve high acid strengths. 

The reaction of 1,3-dinitrobenzenes with aryl iodides can be accomplished by using copper(I) tert-butoxide in situ prepared from CuCl and potassium rert-butoxide in dry DME. In this case, the reaction can be realized under essentially milder reaction conditions, at 70-110 C in the presence of pyridine as the base with respective yields. For instance, the biaryl 40 was produced from compounds 38 and 39 after 20 h at 67 C in DME / pyridine as the reaction solvent in 95% yield, respectively [38]. Dehalogenation products were isolated from many Ullmann reactions performed with the (relatively acidic) nitro-aryl halides, even when exclusion of water or acidic (protic) substances was provided. Furthermore, nitro-aryl halides are partially converted to triarylamines by reduction and subsequent exhaustive A -arylation [44,45]. Finnaly, it can be concluded that the Ullmann and related reactions are still valuable tools in the preparative organic chemistry providing a simple and usually efficient approach to simple symmetrical and certain classes of unsymmetrical biaryls. 

The donor number is frequently used in various fields of polymer chemistry (see Chapter 10). Another classification based on acidity/basicity of solvents allows the division of solvents into six groups containing protic-neutral protogenic protophilic aprotic-protophilic aprotic-protophobic and aprotic-inert. ... 

Several reaction conditions feature in this traditional transformation. First, the reaction is run in protic solvents and can be mediated either by acid or by base. Second, the reaction is reversible, particularly under these conditions. Finally, the enol or the enolate, vhich acts as a nucleophile, is inevitably generated in the presence of the aldehyde or ketone that functions as an electrophile. The aldol reaction performed under these conditions (Scheme 1.1) vas the state of the art until the early nineteen-seventies, vhen the chemistry of preformed enolates emerged. 

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