Double bond-breaking

Natural rubber is composed of polymerized isoprene units. When rubber is under tension, ozone attacks the carbon-carbon double bond, breaking the bond. The broken bond leaves adjacent C = C bonds under additional stress, eventually breaking and placing shll more stress on surrounding C = C bonds. This "domino" effect can be discerned from the structural formulas in Fig. 9-4. The number of cracks and the depth of the cracks in rubber under tension are related to ambient concentrations of ozone. 

If the carbon/oxygen double bond breaks heterolytically, placing the negative charge upon the oxygen, then the carboxylate carbon will be left with a positive charge. Draw the mechanism for this initial step. 

The electron-rich region of the % electrons cloud attacks the electropositive end of the bromine molecule. In so doing, the carbon/carbon double bond breaks, leaving one carbon with a positive charge. The bromine/bromine bond also breaks to form a bromide anion, thus conserving charge. Both of these bond cleavages are heterolytic in nature. 

To picture what polymers are, it is helpful to start with small synthetic polymers. You use such polymers every day. Plastics, synthetic fabrics, and nonstick surfaces on cookware are polymers. The unsaturated hydrocarbon ethylene, C2H4, is the monomer of a common polymer used often in plastic bags. The monomers are bonded together in a chemical reaction called polymerization (puh lih muh ruh ZAY shun). As you can see in Figure 15, the double bond breaks in each ethylene molecule. The two carbon atoms then form new bonds with carbon atoms in other ethylene molecules. This process is repeated many times and results in a much larger molecule called polyethylene. A polyethylene molecule can contain 10,000 ethylene units. 

In the addition of HCI to propene, the hydrogen-chlorine bond breaks first. As the new carbon-hydrogen bond forms, the carbon-carbon double bond breaks and becomes a single bond. Finally, a new carbon-chlorine bond is formed. 

We have seen that alkenes can be oxidized to 1,2-diols and that 1,2-diols can be further oxidized to aldehydes and ketones (Sections 20.6 and 20.7, respectively). Alternatively, alkenes can be directly oxidized to aldehydes and ketones by ozone (O3). When an alkene is treated with ozone at low temperatures, the double bond breaks and the carbons that were doubly bonded to each other find themselves doubly bonded to oxygens instead. This oxidation reaction is known as ozonolysis. 

Under the right conditions, alkenes will undergo addition reactions with each other. The reaction is called polymerization. For example, many ethene molecules will join together to form polythene - the double bonds break and the molecules link up ... 

Cycloadditions. In these reactions a conjugated diene adds to a double bond to form a ring. This involves the formation of two new sigma bonds. The DIELS-ALDER REACTION is a cycloaddition. The reverse reaction, in which a ring containing a double bond breaks to a diene and a compound containing a double bond, is also a pericyclic reaction. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

The most characteristic reaction of alkenes is an addition reaction to the carbon-carbon double bond, breaking the tt bond and producing two new tr bonds to two new atoms or groups of atoms. 

This attack would result in the carbon atom in question having more than eight electrons around it. To keep the carbon atom ftom having too many electrons around it the double bond breaks. 

Rotation about the carbon-carbon double bond breaks the tt bond. 

During electrolysis of solutions of unsaturated compounds the carbon —metal bond forms at the point where the double bond breaks, and one carbon atom is protonated ... 

Whether deleting these terms that prevent correct dissociation in the model double bond-breaking also helps to avoid the nonvariational catastrophes that occur in CCD and IP is unclear it must be answered by numerical tests. However the answer appears to be affirmative. To illustrate the performance of... 

Electrocyclic reactions. In these reactions a ring is formed across the ends of a conjugated system of double bonds. The reverse reaction, in which a ring containing two double bonds breaks by movement of electrons in a cycle, is also an electrocyclic reaction. 

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