Styrene, resonance energy

Steric factors appear to be dominant in determining AHv and ASP. The resonance energy lost in converting monomer to polymer is of secondary importance for most common monomers. It is thought to account for A//, for VAc and VC being lower than for acrylic and styrenic monomers. 

Problem 10.7 Cyclooctatetraene (CgH ), unlike benzene, is not aromatic it decolorizes both dil. aq. KMnO and Brj in CCI4. Its experimentally determined heat of combustion is -4581 kJ/mol. (a) Use the Hiickel rule to account for the differences in chemical properties of CgHg from those of benzene, (b) Use thermochemical data af Problem 10.4 to calculate the resonance energy, (c) Why is this compound not antiaromatic (d) Styrene, CgH5CH==CH2, with heat of combustion —4393 kJ/mol, is an isomer of cyclooctatetraene. Is styrene aromatic ... 

This trans-selectivity ultimately results from the fact that trans-alkenes are more stable than their cis-isomers. This energy difference is especially pronounced for the alkenes in Figure 4.3 because they are styrene derivatives. Styrenes with one alkyl group in the trans-position on the alkenyl C=C double bond enjoy the approximately 3 kcal/mol styrene resonance stabilization. This is lost in cis-styrenes because in that case the phenyl ring is rotated out of the plane of the alkenic C=C double bond to avoid the cis-alkyl substituent. However, the transselectivity documented in Figure 4.3 is not a consequence of thermodynamic control. This could occur only for a reversible elimination or if the alkenes could interconvert under the reaction conditions in some other way. Under the conditions of Figure 4.3, alkenes are almost always produced irreversibly and without the possibility of a subsequent cis/frans-isomeriza-tion. Therefore, the observed trans-selectivity is the result of kinetic control. 

All /1-eliminations from the benzyl derivative in Figure 4.5 exhibit a certain stereoselectivity, in this case -stcreoselectivity. This is true regardless of whether the elimination is syn- or awft -selective or neither. The reason for the preferred formation of the /. -product is again product-development control. This comes about because there is a significant energy difference between the isomeric elimination products due to the presence (E isomers) or absence (Z isomers) of styrene resonance stabilization. 

The resonance energy of the pseudobase has been approximated by that of the nearest aromatic molecule from which it can be considered to be derived. Thus, benzene is used as the reference molecule for 1,2-dihydro-2-hydroxy-l-methylquinoline, etc. Small differences in the resonance energies of benzene and aniline (1.6 kcal mol- J140 and benzene and styrene etc. have been ignored. 

Compare the value 7 kcals. for the excess resonance energy of styrene... 

Apply the SRT method to phenanthrene and to styrene in order to verify the resonance energies listed in Table 4.5. 

Correction was made for the additional resonance energy for styrene over that of benzene as ca. 2 kcal. The final resonance energy value for the isoquino-linium cation is 48 kcal/mol, when corrections are made for this and for the conjugation energies of the fragments (shown in Scheme 14, second part). ... 

The radicals formed at each step in such a polymerization clearly have identical resonance energies. Thus in the polymerization of styrene, each radical in the growing chain is an a-phenylalkyl radical, namely... 

Spiro, J.G. et al. (2006) Experimental and theoretical investigation of the lamellar structure of a styrene-butyl methacrylate diblock copolymer by fluorescence resonance energy transfer, small-angle X-ray scattering, and self-consistent-field simulations. Macromolecules, 39 (20), 7055-7063. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

Define styrene to be monomer 1 and vinyl acetate to be monomer 2. The difference in resonance stabilization energy ep. - > 1, since... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

Styrene contains a benzene ring and will be appreciably stabilized by resonance, which makes it lower in energy than cyclooctatetraene. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

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