S jAr mechanism

There is no clear-cut proof that a one-step Sn2 mechanism, so important at a saturated carbon, ever actually occurs with an aromatic substrate. The hypothetical aromatic Sn2 process is sometimes called the one-stage mechanism to distinguish it from the two-stage S jAr mechanism. A clean example of a Srn2 reaction has been reported, the conversion of 10 to 11 in methanol.Both the SrnI and Srn2 reactions have been reviewed. ... 

As base. Many reactions that are promoted by a base can be effected with KF/AljO, for example, in deprotonating malononitrile for Michael reaction, in N-pyridylation of indole which is based on the S,.jAr mechanism, and in the preparation of a-heterosubstituted Weinreb amides from 7V-methoxy-A(-methyl chloroacetamide. ... 

A seemingly simple reaction is the rapid conversion by amide ion in ammonia of 2-halo-4-phenylpyrimidines to the corresponding 2-amino compound in high yield, Eq. (8), R = H. A straightforward interpretation of this result employs an S jAr mechanism in which amide ion adds to position 2, the center bearing the halogen... 

Studies of the S Ar mechanism have involved theuse of 2,4-dmitrofluoroben-zene as a substrate The S, jAr reaction of fluoroaromatics is catalyzed by both aromatic and alphatic amines [42. 43 44, 45, 46 47, 48, 49] Diamines [50] and... 

When Bunnett and Zahler [1] published their landmark review in 1951, only two mechanisms of nucleophilic aromatic substitution were known. These were the unimolecular S l process, typically observed with arenediazonium salts, as in Scheme 6.1, and the bimolecular S,.jAr pathway, which is shown in Scheme 6.2 involving substitution of a halide ion by an anionic nucleophile and involving an anionic intermediate (1). As in aliphatic substitutions, both unimolecular and bimolecular pathways are possible. 

The S,jAr reaction is circumscribed by the major mechanisms of nucleophilic aromatic substitution the most common S, Ar addition-ehmination (Chapter 6), the oxidative nucleophilic aromatic substitution of hydrogen (ONSH Chapter 11), the benzyne mechanism (Chapter 12), and the radical-nucleophilic aromatic substitution Sj j l (Chapter 10). The chiral inductor can be bonded (Section 8.2) or nonbonded to the reactants (Section 8.3) (Scheme 8.1). 

VNS hydroxylation of nitroarenes with alkylhydroperoxides proceeds according to mechanism similar to that of the reaction with a-halocarbanions— reversible nucleophiUc addition, followed by the base-induced p-ehmination of alcohols from the intermediate a"-adducts. These mechanistic features were established by detramination of effects of base on the rate of the reaction, namely, on the competition between VNS hydroxylation and S Ar of chlorine in 4-chloronitrobenzene and 2,4-dinitrochlorobenzene. For instance, the reaction of these hydropaoxides with 2,4-dinitrochlorobenzene carried out in the presence of an excess of f-BuOK proceeds exclusively as VNS to give 2,4-dinitro-5-chlorophenol, whereas in the presence of equimolar amount of f-BuOK substantial quantity of 2,4-dinitrophenol, product of S,.jAr is formed (Scheme 11.43) [66]. 

Direct benzo[b]thiophene formation via S jAr Scheme 4.2 The proposed mechanism. 

In a 4-I. wide-mouthed glass jar, fitted with a mechanical stirrer, is placed a solution of 150 g. (3 moles) of sodium cyanide (Note i) in 500 cc. of water and 318 g. (3 moles) of u.s.P. benz-aldehyde. The stirrer is started and 850 cc. of a saturated solution of sodium bisulfite (Note 2) is added to the mixture, slowly at first and then in a thm stream. The time of addition is ten to fifteen minutes. During the addition of the first half of this solution, 900 g. of cracked ice is added to the reaction mixture, a handful at a time. The layer of mandelonitrile which appears during the addition of the sulfite solution is separated from the water in a separatory funnel. The water is extracted once with about 150 cc. of benzene, the benzene is evaporated, and the residual mandelonitrile is added to the main portion. 

D. Stevanovic, A. Lowe, S. Kalyanasundaram, P.Y.B. Jar, and V. Otieno-Alego, Chemical and mechanical properties of vinyl-es-ter/ABS blends, Polymer, 43(16) 4503-4514, July 2002. 

Oxide mixtures were mechanically activated in a planetary mill with zirconium jars (jar s volume 0.15 dm ) and balls. The mill rotation frequency was 12.5 s energy load was 10 W/g of mixture. The mechanically activated mixtures were thermally treated in air in a VTP-12/15 resistance furnace with lanthanum chromite heaters. Thermal treatment mode was as follows a rate of temperature rise 300 deg/h, an isothermal exposure at 1200°C for 2 h. 

Jar P-YB, Mulone R, Davies P, Kausch HH. A study of the effect of forming temperature on the mechanical behavior of carbon-fibre/PEEK composites. Compos Sd Technol 1993 46 7-19. http //dx.doi.org/10.1016/0266-3538(93)90076-S. 

Step 2. Place the spotted thin-layer plate in a screw-capped, wide-mouth jar, or a beaker with a watch glass cover, containing a small amount of elution solvent (Fig. 5.36). It helps if one side of the jar s (beaker s) interior is covered with a piece of filter paper that wicks the solvent up to increase the surface area of the 100 solvent. The TLC plate must be positioned so that the spot of your sample is above the solvent. Cap the jar, or replace the watch glass on the beaker, to maintain an atmosphere saturated with the elution solvent. The elution solvent climbs the plate by capillary action, eluting the sample up the plate. Do not move the developing chamber after the action has started. Separation of mixtures into individual spots occurs by exactly the same mechanism as in column chromatography. Stop the elution by removing the plate from the jar or beaker when the solvent front nears the top of the TLC plate. Quickly (before the solvent evaporates) mark the position of the solvent front on the plate. 

A major advantage is polypropylene s higher temperature resistance, which makes PP particularly suitable for items such as trays, funnels, pails, bottles, carboys and instrument jars that have to be sterilized frequently for use in a clinical environment. Polypropylene is a translucent material with excellent mechanical properties. PP has a wide range of applications such as packaging, fibers, automobile industry, nondurable goods and in building construction. 

---