Stability of conjugated dienes

We saw earlier (Section 5.11) that we can determine the relative stabilities of isomeric alkenes by comparing their heats of hydrogenation when the hydrogenation reactions yield the 

We can show this resonance stabilization for conjugated double bonds of butadiene in terms of Lewis structures, as shown below. The dipolar form is the minor contributor to the resonance hybrid because it takes energy to separate charges. 

Problsm 11.3 Estimate the total heats of hydrogenation of 2E, 4E-heptadiene, and of 2E,5E-heptadiene. 

Arrange the following compounds in order of increasing heats of hydrogenation. 

7-Dimethyl-l,4-cycloheptadiene rV l,3-Dimethyl-l,3-cycloheptadiene V 2,4-Dimethyl-1,4-cycloheptadiene 

A resonance argument can also be used to explain the shorter C—C o bond length of 1,3-butadiene. 1,3-Butadiene can be represented by three resonance structures  

Structures B and C have charge separation and fewer bonds than A, making them less stable resonance structures and only minor contributors to the resonance hybrid. B and C both contain a double bond between the centrsil carbon atoms, however, so the hybrid must have a partial double bond there. This makes the c entral C—C bond shorter than a C-C single bond in an alkane. 

Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms. As a result, the jc electrons are not localized between the carbon atoms of the double bonds, but rather delcx alized over four atoms. This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present. This shortens the bond. Drawing resonance structures illustrates this delcx alization. 

The overiap of adjacent p orbitals increases I the electron density in the C-C o bond. 

Problem 16.11 Using hybridization, predict how the bond length of the C-C o bond in HCsC-C CH should compare with the C—Ccs bonds in CH3CH3 and CH2=CH —CH=CH2. 

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 

Since a monosubstituted alkene such as 1-butene has AH vdn s -126 kJ/mol, we might expect that a compound with two monosubstituted. 

Problem 14.2 Use the data in Table 14.1 to calculate an expected heat of hydrogenation for allene, HaC=C=CH2. The measured value is -298 kj/mol (-71.3 kcal/mol). Rank ft conjugated diene, a nonconjugated diene, and an allene in order of stability. 

In Section 12.3 we learned that hydrogen adds to alkenes to form alkanes, and that the heat released in this reaction, the heat of hydrogenation, can be used as a measure of alkene stability. 

The relative stability of conjugated and isolated dienes can also be determined by comparing their heats of hydrogenation. 

Why is a conjugated diene more stable than an isolated diene Because a conjugated diene has overlapping p orbitals on four adjacent atoms, its n electrons are delocalized over four atoms. This delocalization, which cannot occur in an isolated diene, is illustrated by drawing resonance structures. 

No resonance structures can be drawn for 1,4-pentadiene, but three can be drawn for (3 )-1.3-pentadiene (or any other conjugated diene). The hybrid of these resonance structures illustrates that the two adjacent n bonds are delocalized in a conjugated diene, making it lower in energy than an isolated diene. 

Problem 16.16 Which diene in each pair has the larger heat of hydrogenation  

TA6L 14.1 Heats of Hydrogenation for Some Afkenes and Dienes 

Confirmation of this unexpected stability comes from data on the partial hydrogenation of 1,3-butadiene to yield 1-butene. The amount of energy released is -110 kJ/mol, some 16 kJ/mol less than that for the isolated monosubstituted double bond in 1-butene. 

Two p orbitals combine to form two It molecular orbitals. When these orbitals are occupied by two eiectrons, both eiectrons occupy the low-energy, bonding orbital, leading to a net lowering of energy and formation of a stable bond. The asterisk on indicates an antibonding orbital. 

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Stability of Conjugated Dienes Molecular Orbital Theory 4133... 

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. 

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