Organic molecules double bonds attacked

The central Co,=Cp double bond of an allenylidene backbone can also react with a variety of dipolar organic substrates to yield cyclic adducts. Most of the cychza-tion processes reported occur in a stepwise manner via an initial nucleophilic attack at the Coi atom and further rearrangement of the molecule involving a coupling with the Cp carbon. Representative examples are the reactions of the electron-poor ruthenium-allenylidene complex 46 with ethyl diazoacetate and 1,1-diethylpropar-gylamine to yield the five- and six-membered heterocyclic compounds 82 and 83, respectively (Scheme 29) [260, 284]. 

The bio-oxidants that are formed are highly suited to attacking organic molecules by attracting the most readily available electrons in the substrates structures (Table 17.4). Often these are -electrons of aromatic rings and carbon-carbon double bonds ... 

This chapter deals with those cationic polymerisations in which the initiators contain an electrophilic organic moiety capable of attacking the vinylic double bond of the monomer to produce an active species. Included in this family are such compounds as stable carbenium and acylium salts, less stable ones which are usually prepared in the presence of monomer to avoid extensive decomposition before initiation, and ester molecules i s-sessing a high degree of polarisation. Cation-radical salts will also be considered, but ox-onium salts will not, because they are too stable to initiate the polymerisation of vinyl monomers. 

In the modern urban atmosphere, 03 may be the pollutant of particular concern for health. However, it is a reactive gas that will also attack the double bonds of organic molecules (see Section 2.7) very readily. Rubber is a polymeric material with many double bonds, so it is degraded and cracked by 03. Tyres and windscreen wiper blades are especially vulnerable to oxidants, although newer synthetic rubbers have double bonds protected by other chemical groups, which can make them more resistant to damage by 03. 

A great variety of organic compounds are emitted by vegetation. These biogenic compounds are highly reactive in the atmosphere. They are basically alkenes or cycloalkenes, and their atmospheric chemistry is generally analogous to that of alkenes. Because of the presence of C=C double bonds these molecules are susceptible to attack by 03 and N03, in addition to the customary reaction with OH radicals. 

Organic compounds with carbon-carbon unsaturated double bonds, such as carbon disulfide, are powerful swelling solvents and show greater swelling action than their saturated counterparts. Smaller solvent molecules can penetrate a polymer matrix more effectively. The degree of similarity between solvent and resin is important. Slightly polar resins, such as polyesters and the vinyl esters, are attacked by mildly polar solvents. 

Styrene, of course, can be polymerized alone to give the brittle, transparent, glassy polystyrene, much used for injection-moulded items. To do this it is usually treated with a small amount of an organic peroxide, e.g. benzoyl peroxide. This has the effect of attacking the double bond, and opening it, to allow styrene molecules to add to one another in the process known as free radical addition polymerization. The result is a long-chain polymer molecule. 

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