Fragment attack

The reaction can be rationalized as involving an E2 fragmentative attack by the teUuride anion at the halogen atom. Sodium hydrogen tellnride (method B) requires a shorter reaction time than sodinm tellnride (method A), the 2-bromoalkyl esters reacting very rapidly. Sodinm hydrogen selenide reacts similarly but more slowly. ... 

Competition between monomer and backbone for the initiator radicals. Under some conditions, the initiating fragment attacks the backbone directly, resulting in the formation of a macroradical capable of initiating graft copolymerization. 

Rubber is the most important of all elastomers. Natural rubber is obtained from the bark of the rubber tree and has been used by humans for many centuries. It is a polymer with repeating units of isoprene. In 1823, rubber was vulcanized with sulfur whilst heated and this process made it to become the valuable material it is today. In this process, sulfur chain fragments attack the rubber polymer chains, which leads to cross-linking. Most of the rubber used in the world is a synthetic variety called styrene-butadiene rubber. 

A testing method developed in Great Britain (RARDE Fragment Attack Test) (Barker et al., 1985) will be described here. 

As already seen in Schemes 11.28 (pyruvate to ethanol) and 11.30 (pyruvate to acetyl-CoA), the three-carbon unit of pyruvate is a good substrate for reaction with the thiamine diphosphate cofactor. Indeed, as shown in Scheme 11.42, the same two-carbon unit found in Scheme 11.30 is generated, but now, in place of a lipoic acid fragment, attack occurs on the carbonyl of glyceraldehyde 3-phosphate under the influence of l-deoxy-o-xylulose synthase (EC 2.2.1.7). 

Fig. 1 Propellant fragment attack results, violence as a function of temperature...

If there is an o-fluorine-substituted phenyl group in the amide fragment, attack by the rearranged anion 301 leads both to cycUzation with substitution of the nitro group in one of the aryl fragments and the formation of the pyrroloquinoxaline 302 and to substitution of the fluorine atom in other aryl group and the formation of pyrroloquinoxaline 303 (Scheme 3.92) (Rotas et al. 2004). 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The reducing-end units (see Fig. 8) are highly labile in alkaline solutions. After an initial attack by hydroxide ions at the hemiacetal function, C-1, a series of enoHzations and rearrangements leads to deoxy acids, ie, saccharinic acids, and fragmentation. Substituents on one or more hydroxyl groups influence the direction, rate, and products of reaction. 

Another subclass of proteases attacks internal peptide bonds and Hberates large peptide fragments. Bromelain, a plant protease derived from the stem of the pineapple plant, can even produce detectable semm proteolysis after oral adrninistration (180). Oral therapy with bromelain significantly reduces bmising that stems from obstetrical manipulations (181). Bromelain—pancreatin combinations have been more effective in digestive insufficiency compared to either pancreatin or placebo (182,183). Bromelain may also enhance the activity of antibiotics, especially tetracycline, when adrninistered concurrently (184). 

Ring opening by intramolecular attack by an electron source located on a /3-carbon is represented by the Wharton reaction (Scheme 62) (8UCS(P1)2363) and the Eschenmoser fragmentation (Scheme 63) (81S276). 

Figure 8.12 A fragment of a severely corroded aluminum annealing sleeve. Attack was by caustic, which caked the specimen.

The currently accepted mechanism for the Favorskii rearrangement of dihalo ketones involves a cyclopropanone intermediate formed by loss of HX. This is followed by attack of alkoxide synchronous with cyclopropanone fragmentation and departure of halide ion to form the unsaturated ester... 

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