Resonance energy conjugated dienes

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

The average energy of the electrons is slightly lower in butadiene. This lower energy is the resonance stabilization of the conjugated diene. 

The extra stabilization provided by delocalization, compared with a localized stmcture. For dienes and polyenes, the resonance energy is the extra stability of the conjugated system compared with the energy of a compound with an equivalent number of isolated double bonds, (p. 669)... 

Hydrogenation of cyclohexa-1,3-diene is exothermic by 232 kJ/mol (55.4 kcal/mol), about 8 kJ (1.8 kcal) less than twice the value for cyclohexene. A resonance energy of 8 kJ (1.8 kcal) is typical for a conjugated diene. 

No resonance structures ean be drawn for 1,4-pentadiene, but three can be drawn for (3 )-l,3-pentadiene (or any other eonjugated diene). The hybrid of these resonance stmctures illustrates that the two adjacent n bonds are delocalized in a conjugated diene, making it lower in energy than an isolated diene. 

As you saw with allyl systems, the presence of conjugation leads to slabilizniion. The result is lower energy for conjugated dienes relative to the others. Again, both resonance and molecular-orbital explanations are applicable. 

Based on an analysis of the overall rate coefficients for the reactions of OH radicals with alkenes, Peeters has developed a structure-reactivity relationship for prediction of the relative yields of OH addition to specific sites in the parent alkenes. For mono-alkenes and non-conjugated poly-alkenes the site-specific rate coefficients depend on the stability of the ensuing hydroxyalkyl radical, which can be either a primary, secondary or tertiary radical. For conjugated dienes, the structure-reactivity analysis requires additional consideration of the resonance stabilisation energies of the adduct radicals. The quality of the predictions have been tested for a large number of alkenes and found in most cases to be in agreement with experiment within 10 %. 

The first step of the mechanism is identical for both 1,2-addition and 1,4-addition, namely, the conjugated diene is protonated to give a resonance-stabilized, aUyfic carbocation and a bromide ion. However, the second step of the mechanism can occur via either of two competing pathways (shown in blue and red). Comparing these pathways, we see that 1,4-addition leads to a more stable product (lower in energy), while 1,2-addition occurs more rapidly (lower energy of activation). The 1,4-adduct is lower In energy because It exhibits a more substituted double bond ... 

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