Bond Lengths in 1,3-Butadiene

Further evidence for the special nature of conjugated dienes comes from data on bond lengths (Table 14.2). If we compare the length of the carbon-carbon single bond in 1,3-butadiene (148 pm) to that in ethane (154 pm), we find that the 1,3-butadiene single bond is shorter by 6 pm. 

TABLE 14.2 Some Carbon Carbon Bond Lengths 1 

5 Electrophilic Additions to Conjugated Dienes Allylic Carbocations 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.8). Markovnikov regiochemistry is found because the more stable carbocation is involved as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than 1-ehloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the noncon-jugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. For example, addition of HBr to 1,3-butadiene yields a mixture of two products (not counting cis-tvans isomers). 3-Bromo- 1-butene is the typical Markovnikov product of 1,2 addition, but l-bromo-2-bulene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4 addition. 

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The double-bond length in 1,3-butadiene is 0.134 nm, and the ingle-bond, 0.148 nm. Since normal carbon—carbon single bonds are 0.154 nm, this indicates the extent of double-bond character in the middle single-bond. Upon complexing with metal carbonyl moieties like Fe(CO)2, the two terminal bonds lengthen to 0.141 nm, and the middle bond shortens even more to 0.145 nm (18). 

The mechanism of the carbo-Diels-Alder reaction has been a subject of controversy with respect to synchronicity or asynchronicity. With acrolein as the dieno-phile complexed to a Lewis acid, one would not expect a synchronous reaction. The C1-C6 and C4—C5 bond lengths in the NC-transition-state structure for the BF3-catalyzed reaction of acrolein with butadiene are calculated to be 2.96 A and 1.932 A, respectively [6]. The asynchronicity of the BF3-catalyzed carbo-Diels-Alder reaction is also apparent from the pyramidalization of the reacting centers C4 and C5 of NC (the short C-C bond) is pyramidalized by 11°, while Cl and C6 (the long C-C bond) are nearly planar. The lowest energy transition-state structure (NC) has the most pronounced asynchronicity, while the highest energy transition-state structure (XT) is more synchronous. 

Butadiene, C4H6, is a planar molecule that has the following carbon-carbon bond lengths in A (10" °m) ... 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

Figure 4-6. Plane wave convergence of the carbon-carbon bond length in the ethylene and butadiene molecules, from the simulations with the CPMD program13 (Troullier-Martins pseudopotentials,1415 time step 4 a.u., fictitious mass 400 a.u., unit cell 12 A x 12 A x 12 A)...

Fig. 23-4. Some structures in which butadiene units are bonded to metal atoms. The numbers give the average lengths, in A, of the C-l/C-2 and C-3/C-4 bonds and the C-2/C-3 bond length, in each case.

We take / as the zig-zag length along the carbon chain. Let /] and I2 be the carbon carbon single- and double-bond lengths in (16.9). For sr-trarw-l,3-butadiene these values are 1.46 A and 1.34 A. 

Reip, G.J. Konietzny. S. How realistic are alternating C-C-bond lengths in 5-cE-1,3-butadiene transition metal complexes J. Chem. Soc., Dalton Trans. 2002, 862-864. 

Scheme 2. Calculated E-E single bond lengths in 1,3-butadiene, 10 and 12 (at B3LYP/6-311+G ).

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