Neutral organic reaction intermediates

Intermediate processes of catalyzed organic reactions may involve neutral free radicals R , positive ions R+, or negative ions R as short-lived reactants. A classification of catalysts and processes from the point of view of elementary reactions between reagents and catalysts Is logically desirable but has not yet been worked out. However, there is a wealth of practice more or less completely documented, some proprietary but available at a price. The ensuing discussions are classified into kinds of catalysts and into kinds of processes. 

The formation of the cation is the rate-determining step. You can look at this in two ways. Either you could argue that a cation is an unstable species and so it will be formed slowly from a stable neutral organic molecule, or you could argue that the cation is a very reactive species and so all its reactions will be fast, regardless of the nucleophile. Both arguments are correct. In a reaction with an unstable intermediate, the formation of that intermediate is usually the rate-determining step. 

Other than p-hydride elimination, another important pathway by which palladium(II) intermedi ates can lead to neutral organic fragments is reductive elimination. This forms the basis of the mech anism for cross-coupling reactions between an organometallic reagent and an organic halide o triflate. 

It can be assumed, unless otherwise stated, that when an organic reaction is written, the products shown have undergone any required aqueous workup, which may involve acid or base, to give a neutral organic molecule (unless salts are shown as the product). In other words, when an equation for a reaction is written in the literature or on an exam, an aqueous workup usually is assumed and intermediates, salts, etc. are not shown. 

Carbenes. Although most organic compounds possess uncharged carbon atoms that have a Lewis octet of electrons in their valence shells, i.e., they form four bonds, neutral carbon fragments can nevertheless be generated as reaction intermediates. These include carbenes. The generation of transient carbene ICs is explored in detail in this chapter. 

Radical Anions. Radical anions are common intermediates in organic reactions they are easily prepared from compounds with low-enough LUMOs by the addition of an electron (from a dissolving metal or from a cathode, or the solvated electron itself). Those derived from carbonyl groups (421) dimerize at carbon 339 those derived from a,/ -unsaturated carbonyl compounds (422) dimerize at the /1-position,340 and pyridines dimerize predominantly at the 4-position.341 In each case, the odd electron has been fed into the orbital which was the LUMO of the starting material the site of coupling therefore should, and does, correlate with the site at which nucleophiles attack the neutral compounds. 

It is significant to note that almost all carbocations have known isoelectronic and isostructural neutral boron analogs. Boron compounds also provide useful models for many types of intermediates (transition states) of electrophilic organic reactions. 

KilUlea et al. have reported on nitrogen chemistry in SCWO Both oxidized (e.g., nitro) and reduced (e.g., amino) forms of nitrogen are converted to N2 and a minor fraction of N2O in SCWO. Ammonia is a relatively refractory compoimd, requiring on the order of 700°C for complete destruction when not accompanied hy organic material. In the presence of organic material, ammonia can he completely oxidized at temperatures below 650 C. an effect believed to be due to common reaction intermediate species. Signihcant residual ammonia in pH-neutral SCWO effluent will appear as a white precipitate of ammonium bicarbonate. Nitric acid or nitro moieties in the SCWO feed will act as oxidants in SCWO systems, provided a reduced material such as organic carbon is present. 

Many important organic reactions involve nucleophilic carbon species (car-banions). The properties of carbanions will be discussed in detail in Chapter 7 and in Part B, Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve car-banion intermediates are therefore usually carried out by reaction of the neutral organic molecule and the electrophile in the presence of a base that can generate the more reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophilic and does not react with benzaldehyde. The enolate (carbanion) formed by deprotonation is much more nucleophilic. 

---