Bonding in conjugated dienes

PROBLEM 10.7 Another way in which energies of isomers may be compared is by their heats of combustion. Match the heat of combustion with the appropriate diene. 

the order of alkadiene stability decreases in the order conjugated diene (most stable) isolated diene cumulated diene (least stable). To understand this ranking, we need to look at structure and bonding in alkadienes in more detail. 

At 146 pm the C-2—C-3 distance in 1,3-butadiene is relatively short for a carbon-carbon single bond. This is most reasonably seen as a hybridization effect. In ethane both carbons are -hybridized and are separated by a distance of 153 pm. The carbon-carbon single bond in propene unites sp - and ip -hybridized carbons and is shorter than that of ethane. Both C-2 and C-3 are. sp -hybridized in 1,3-butadiene, and a decrease in bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases. 

FIGURE 10.4 (a) Isolated double bonds are separated from each other by one or more sp -hybridized carbons and cannot overlap to give an extended tt orbital. (6) In a conjugated diene, overlap of two TT orbitals gives an extended tt system encompassing four carbon atoms. 

Additional evidence for electron delocalization in 1,3-butadiene can be obtained by considering its conformations. Overlap of the two ir electron systems is optimal when the four carbon atoms are coplanar. Two conformations allow this coplanarity they are called the i-cis and i-trans conformations. 

The letter in s-cis and -trans refers to conformations around the C—C single bond in the diene. The -trans conformation of 1,3-butadiene is 12 kJ/mol (2.8 kcal/mol) more stable than the -cis, which is destabilized by van der Waals strain between the hydrogens at C-1 and C-4. 

Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum TT electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from tt electron delocalization decreases in going to the transition state for rotation about the C-2—C-3 single bond. The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

Heats of hydrogenation are used to assess the stabilities of isolated versus conjugated double bonds. Comparing the measured heats of hydrogenation (solid lines) of the four compounds shown gives the values shown by the dashed lines for the heats of hydrogenation of the terminal double bond of 1,4-pentadiene and ( )-l,3-pentadiene. A conjugated double bond is approximately 15 kJ/ mol more stable than an isolated double bond. 

The cumulated double bonds of an allenic system are of relatively high energy. The heat of hydrogenation of aUene is more than twice that of propene. 

---

Hydrozirconation of the sterically less hindered double bond in conjugated dienes is generally observed [103]. However, the presence, as the minor regioisomer, of the allyhc product has been reported [5, 86]. Depending on the reaction conditions and the reagents used in the subsequent transformation, the allylic isomer did not form (Scheme 8-12) [2]. 

Recall from Chapters 10 and 11 that the characteristic reaction of compounds with it bonds is addition. The n bonds in conjugated dienes undergo addition reactions, too, but they differ in two ways from the addition reactions to isolated double bonds. 

The addition of C-H bonds in terminal acetylenes to C-C double bonds in conjugate dienes and a,/3-unsaturated carbonyl compounds can take place with the aid of a... 

The bonding in conjugated dienes is not so obviously exotic as it is in cumulated dienes, such as the allenes. Nevertheless, there are some subtle effects with far-reaching implications. The source of these effects can be seen from simple, schematic orbital drawings of 1,3-butadiene and 1,4-pentadiene. In a conjugated diene, the orbitals of the two double bonds overlap in an unconjugated diene, they do not (Fig. 12.14). 

---