The Stability of Conjugated Dienes

Two examples of this extra stability of conjugated dienes can be seen in an analysis of the heats of hydrogenation given in Table 13.2. 

In itself, 1,3-butadiene cannot be compared directly with an isolated diene of the same chain length. However, a comparison can be made between the heat of hydrogenation of 1,3-butadiene and that obtained when two molar equivalents of 1-butene are hydrogenated  

Because 1-butene has a monosubstituted double bond like those in 1,3-butadiene, we might expect hydrogenation of 1,3-butadiene to liberate the same amount of heat (254 kJ moF ) as two molar equivalents of 1-butene. We find, however, that 1,3-butadiene liberates only 239 kJ mor 15 kJ mol less than expected. We conclude, therefore, that conjugation imparts some extra stability to the conjugated system (Fig. 13.6). 

An assessment of the stabilization that conjugation provides trflnx-l,3-pentadiene can be made by comparing the heat of hydrogenation of tra i-l,3-pentadiene to the sum of the heats of hydrogenation of 1-pentene and trans-2-pentem. This way we are comparing double bonds of comparable types  

We see from these calculations that conjugation affords tran -l.B-pentadiene an extra stability of 15 kJ moF a value that is equivalent, to two significant figures, to the one we obtained for 1,3-butadiene (15 kJ moF ). 

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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 a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

Problem 8.23 Account for the stability of conjugated dienes by (a) extended ir bonding, (b) resonance theory. 

The relative stability of conjugated dienes versus isolated dienes was first discussed in Section 16.9. 

In the next three sections we shall see how two different factors have been invoked to account for the relative stabilities of conjugated dienes, and of simple alkenes as well (a) delocalization of tt electrons, and (b) strengthening of a bonds through changes in hybridization of carbon. 

Unusual stability of conjugated dienes is also strongly indicated by the fact that, where possible, they are the preferred diene products of elimination reactions (Sec. 5.14). 

In a similar way, the unusual stability of conjugated dienes is attributed, not to delocalization of the n electrons, but to the fact that sp"-sp- hybridization makes the C2 -C3 bond short (1.48 A) and strong. 

Electrophilic addition to simple alkenes takes place in such a way as to form the most stable intermediate carbonium ion. Addition to a,j3-unsaturatec arbonyl compounds, too, is consistent with this principle to see that this is so, however, we must look at the conjugated system as a whole. As in the case of conjugated dienes (Sec. 8.20), addition to an end of the conjugated system is preferr jd, since this yields (step 1) a resonance-stabilized carbonium ion. Addition to the carbonyl oxygen end would yield carbonium ion 1 addition to the j3-catbon end would yield carbonium ion II. 

The low lying ji -orbitals of conjugated dienes explain the stability of many diene metal carbonyl complexes e.g. butadiene iron tricarbonyl 382>. Photochemical preparation of the latter 286> is superior to the thermal procedure 272>. Mercury can be used as a sensitizer in the photoreaction of metal carbonyls with dienes I09.i7o,i76,24i)... 

Evidence for the extra stability of conjugated dienes comes from mea surements of heats of hydrogenation (Table 14.1). We saw earlier inthedit cussion of alkene stabilities (Section 6.7) that alkenes of similar substitution pattern have remarkably similar AHgydrog values. Monosubstituted alkenes such as 1-butene have values for near -126 kJ/mol... 

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