Stability of Conjugated Dienes Molecular Orbital Theory

Conjugated dienes can be prepared by some of the methods previously discussed for preparing alkenes (Sections 11.7-11.10). The base-induced elimination of HX from an allylic halide is one such reaction. 

Simple conjugated dienes used in polymer synthesis include 1,3-butadiene, chloroprene (2-chloro-l,3-butadiene), and isoprene (2-methyl-l,3-butadiene). Isoprene has been prepared industrially by several methods, including the acid-catalyzed double dehydration of 3-methyl-l,3-butanediol. 

One of the properties that distinguishes conjugated from nonconjugated dienes is that the central single bond in shorter than might be expected. The 

C2-C3 single bond in 1,3-butadiene, for instance, has a length of 147 pm, some 6 pm shorter than the C2-C3 bond in butane (153 pm). 

Because a monosubstituted alkene has a AfT hydrog of approximately 126 kJ/mol, we might expect that a compound with two monosubstituted double bonds would have a AfT hydrog approximately twice that value, or 

1 Stability of Conjugated Dienes Molecular Orbital Theory 

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C-C bonds like those in alkanes result from tr overlap of sp orbitals on both carbons. In a conjugated diene, however, the central C-C bond results from a overlap of sp orbitals on both carbons. Since sp orbitals have more 5 character (33% s) than s/P orbitals (25% 5), the electrons in sp orbitals are closer to the nucleus and the bonds they form are somewhat shorter and stronger. Thus, the extra stability of a conjugated diene results In part from the greater amount of 5 character in the orbitals forming the C-C bond. 

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