Carbon compounds stabilization mechanisms

With regard to the susceptibility of many organometallics to aerial oxidation, it must be remembered that most compounds containing C-H bonds are thermodynamically unstable with respect to oxidation by molecular oxygen to produce water and carbon dioxide. The mechanisms whereby (for example) many alkyls are spontaneously flammable in air are necessarily difficult to determine the mechanisms involved in the combustion of hydrocarbons are still incompletely understood. Suffice it to say that the kinetic stability of ER to air seems to be low where E is of low electronegativity. 

Most SN reactions of hydride donors, organometallic compounds, and heteroatom-stabilized carbanions at the carboxyl carbon follow the mechanism shown in Figure 6.2. Thus the substitution products, i.e., the aldehydes and ketones C, form in the presence of the nucleophiles. Thus, when the nucleophile and the acylating agent are used in a 2 1 ratio, alcohols F are always produced. 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Figure 22.5 Mechanism of enolate ion formation by abstraction of an a proton from a carbonyl compound. The enolate ion is stabilized by resonance, and the negative charge (red) is shared by the oxygen and the a carbon atom, as indicated by the electrostatic potential map.

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

---