Types, mechanical

We make one important note here regarding nomenclature. Early explanations of CIDNP invoked an Overhauser-type mechanism, implying a dynamic process similar to spin relaxation hence the word dynamic m the CIDNP acronym. This is now known to be incorrect, but the acronym has prevailed in its infant fomi. 

Direct interception refers to a sieve-type mechanism in which contaminants larger than the filter pore size are directly trapped by the filter. This sieve retention mechanism of particle arrest is the mechanism of choice and occurs owing to geometric or spatial restraint. This type of particle arrest is considered to be absolute, that is, it is independent of filtration conditions. 

Grafting can also occur in the amide nitrogen, either through an anionic-type mechanism which is beheved to operate when ethylene oxide [75-21 -8] and similar copolymers are grafted to polyamides, or through a polycondensation mechanism when secondary amides are formed as graft copolymers (70). 

Mechanistic studies on the formation of PPS from polymerization of copper(I) 4-bromobenzenethiolate in quinoline under inert atmosphere at 200°C have been pubUshed (91). PPS synthesized by this synthetic procedure is characterized by high molar mass at low conversions and esr signals consistent with a single-electron-transfer mechanism, the Sj l-type mechanism described earlier (22). 

Oxaziridines are generally formed by the action of a peracid on a combination of a carbonyl compound and an amine, either as a Schiff base (243) or a simple mixture. Yields are between 65 and 90%. Although oxygenation of Schiff bases is formally analogous to epoxidation of alkenes, the true mechanism is still under discussion. More favored than an epoxidation-type mechanism is formation of a condensation product (244), from which an acyloxy group is displaced with formation of an O—N bond. 

FIG. 12-82 Mannheim-type mechanical hydrochloric acid furnace. 

The most common seals are the lip type and the labyrinth type. Mechanical seals are available where seal leakage must be controlled. 

Although in principle the high-pressure polymerisation of ethylene follows the free-radical-type mechanism discussed in Chapter 2 the reaction has two particular characteristics, the high exothermic reaction and a critical dependence on the monomer concentration. 

One class of enzymes that follow a ping-pong-type mechanism are aminotransferases (previously known as transaminases). These enzymes catalyze the transfer of an amino group from an amino acid to an a-keto acid. The products are a new amino acid and the keto acid corresponding to the carbon skeleton of the amino donor ... 

In aprotic solvents (e.g. in DMF), the reduction does not take place by an ECE-type mechanism. In the case of diphenyl sulphoxide75 it can be shown that a fast disproportionation process occurs at the stage of the substrate anion radical ... 

Eq. 14) [81]. Although this transformation does not appear to be a metathesis reaction, it is thought to proceed via the formation of ruthenium carbene species and not via classical [2+2+2]-cycloaddition pathways. A rationale for the strong preference of the meta isomer 99 was provided on the basis of a metathesis-type mechanism. 

In the case of electrophiles like Br", which can form cyclic intermediates, both 1,2-and 1,4-addition products can be rationalized as stemming from an intermediate like 17. Direct nucleophilic attack by W would give the 1,2 product, while the 1,4 product could be formed by attack at the 4 position, by an Sn2 type mechanism (see p. 422). Intermediates like 18 have been postulated but ruled out for Br and Cl by the observation that chlorination or bromination of butadiene gives trans 1,4... 

Horne has studied the kinetics of exchange in aqueous perchlorate media at temperatures down to —78 °C by the isotopic method ( Fe) and dipyridyl separation. The same rate law in these ice media as in aqueous solution was observed, although the acid dependence was small. Horne concluded that the same exchange mechanism occurs in solid and liquid solvent. Evidence for a Grotthus-type mechanism has been summarised. ... 

Kemp and Waters found a primary kinetic isotope effect of 8.7 for oxidation of C-deuterated mandelic acid and noted a large difference in rate between the oxidations of mandelic acid k at 24.4 °C = 1.7 l.mole . sec ) and a-hydroxy-isobutyric acid ( 2 at 24.4 °C = 5.6 x 10 l.mole . sec ) — a difference not reproduced for the oxidation of these compounds by the one-equivalent reagent, manganic sulphate. The various data are fully in accord with a Westheimer-type mechanism, viz. 

Bode and co-workers have extended the synthetic ntility of homoenolates to the formation of enantiomerically enriched IV-protected y-butyrolactams 169 from saccharin-derived cyclic sulfonylimines 167. While racemic products have been prepared from a range of P-alkyl and P-aryl substitnted enals and substitnted imi-nes, only a single example of an asymmetric variant has been shown, affording the lactam prodnct 169 with good levels of enantioselectivity and diastereoselectivity (Scheme 12.36) [71], As noted in the racemic series (see Section 12.2.2), two mechanisms have been proposed for this type of transformation, either by addition of a homoenolate to the imine or via an ene-type mechanism. 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

From an electrochemical viewpoint, biological systems are highly branched circuits consisting of ionic conductors of aqueous electrolyte solutions and highly selective membranes. These circuits lack metallic conductors, but it has been found relatively recently that they contain sections that behave like electronic conductors (i.e., sections in which electrons can be transferred over macroscopic distances, owing to a peculiar relay-type mechanism). 

---