Conformations of 1,3-butadiene

The letter sms cis and s trans refers to conformations around the C—C single bond m the diene The s trans conformation of 1 3 butadiene is 12 kJ/mol (2 8 kcal/mol) more stable than the s cis which is destabilized by van der Waals strain between the hydrogens at C 1 and C 4... 

There are two possible planar conformations of 1,3-butadiene the s-cis and the s-trans conformations. 

The s-trans conformation of 1,3-butadiene is the predominant one at room temperature. 

The pi MO s of the three conformations of 1,3-butadiene can be derived from union of the pi MO s of two ethylene molecules in the appropriate geometry. The dissection of 1,3-butadiene into two ethylenic fragments is illustrated below for the cis conformer ... 

On the basis of the above analysis, we predict that the relative stability of the various conformations of 1,3-butadiene will vary in the order gauche > tram > cis. 

In order to exemplify how the various computed indices can be used in testing the predictions of OEMO theory within the framework of the dynamic model we shall consider once again the problems of conformational isomerism of 1,3-butadiene and geometrical isomerism of 1,2-difluoroethylene. We first consider the cis and tram conformers of 1,3 butadiene. 

In other words, the apparent order of stability of 1,3-butadiene is trans > gauche > cis. We conclude, therefore, that the preferred conformation of 1,3-butadiene is dictated by steric effects which overwhelm attractive pi and sigma nonbonded interactions. 

Table 19. Computed relative energies of the conformers of 1,3-butadiene...

The reactivity of dienes in Diels-Alder reactions is also controlled by the diene conformation. The two planar conformations of 1,3-butadiene are referred to as s-trans and s-cis (equation 16). Calculations have shown the s-trans conformation to be 2-5 kcalmol-1 more stable than the s-cis conformation. Open-chain dienes can only react in their cisoid conformation. Thus, 2-substituted dienes are generally more reactive than 1,3-butadiene due to their stronger preference for the s-cis conformation. 1 -Cis substituted 1,3-butadienes are almost exclusively in the s-trans conformation and are not reactive in Diels-Alder reactions. Highly substituted dienes may, however, be present in the s-cis conformation during a sufficient amount of time to participate in Diels-Alder reactions, even if a 1 -cis substituent is present62. 

Problem 9.26 Which conformation of 1,3-butadiene participates in the Diels-Alder reaction with, e.g., ethene <... 

The conformation that has C-1 and C-4 of the double bonds on opposite sides of the single bond between C-2 and C-3 is termed the s-trans conformation, whereas the conformation that has them on the same side of the single bond is called the s-cis conformation. The s-trans conformation of 1,3-butadiene is more stable because it has less steric strain—the larger groups are farther apart. Although the interconversion of these two conformations is fast, only the s-cis conformation can react in the Diels-Alder cycloaddition. In the s-trans conformation, C-1 and C-4 are too far apart to bond simultaneously to the dienophile. 

Use SpartanView to examine bond-rotation sequences about the C2-C3 bond in both 1-butene and 1,3-butadiene. Compare the energies of the lowest-energy and transition-state conformations, and tell in which molecule rotation is more difficult. Identify the two minimum-energy conformations of 1,3-butadiene, and tell which geometry permits Diels-Alder cycloaddition. Is this the preferred geometry ... 

It is possible to draw a canonical structure for this molecule that involves a separation of charges, but which in turn confers some double bond characteristics on the central part of the molecule. As a consequence, there are two identifiable conformers of 1,3-butadiene that result from this small energy barrier to free rotation. Suggest what are these two conformers. 

The -cis and -trans conformations of 1,3-butadiene interconvert by rotation around the C-2—C-3 bond, as illustrated in Figure 10.5. The conformation at the midpoint of this rotation, the perpendicular conformation, has its 2p orbitals in a geometry that prevents extended conjugation. It has localized double bonds. The main contributor to the energy of activation for rotation about the single bond in 1,3-butadiene is the decrease in electron delocalization that attends conversion of the 5-cis or i-trans conformation to the perpendicular conformation. 

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