these heats of hydrogenation can be taken to be the resonance energy of a conjugated diene. The industrial uses illustrate the tendency of butadiene to polymerize 1,3-Butadiene ( ) is an organic compound with the formula CH2=CH-CH=CH2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two vinyl groups. It is the simplest conjugated diene. Butadiene undergoes chemical degradation in the atmosphere, but it can nevertheless be found in low leve The most stable conformer of 1,3-butadiene is the s-trans conformation, in which the molecule is planar, with the two pairs of double bonds facing opposite directions. This conformation is most stable because orbital overlap between double bonds is maximized, allowing for maximum conjugation, while steric effects are minimized. Conventionally, the s-trans conformation is considered to have a C2-C3 dihedral angle of 180°. In contrast, the s-cis conformation, in which the dihedral angle is 0°, with the pair of double bonds facing the same direction is approximately 16.5 kJ/mol (3.9 kcal/mol) higher in energy, due to steric hindrance. This geometry is a local energy maximum, so in contrast to the s-trans geometry, it is not a conformer. The gauche geometry, in which the double bonds of the s-cis geometry are twisted to give a dihedral angle of around 38°, is a second conformer that is around 12.0 kJ/mol (2.9 kcal/mol) higher in energy than the s-trans conformer. Overall, there is a barrier of 24.8 kJ/mol (5.9 kcal/mol) for isomerization between the two conformers. This increased rotational barrier and strong overall preference for a near-planar geometry is evidence for a delocalized π system and a small degree of partial double bond character in… A qualitative picture of the molecular orbitals of 1,3-butadiene is readily obtained by applying Hückel theory. (The article on Hückel theory gives a derivation for the 1,3-Butadiene ( ) is an organic compound with the formula CH2=CH-CH=CH2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two vinyl groups. It is the simplest conjugated diene. Butadiene undergoes chemical degradation in the atmosphere, but it can nevertheless be found in low levels in the air of urban and suburban areas due to emission by motor vehicles. The name butadiene can also refer to the isomer, 1,2-butadiene, which is a cumulated diene with structure H2C=C=CH−CH3. This allene has no industrial significance. The most stable conformer of 1,3-butadiene is the s-trans conformation, in which the molecule is planar, with the two pairs of double bonds facing opposite directions. This conformation is most stable because orbital overlap between double bonds is maximized, allowing for maximum conjugation, while steric effects are minimized. Conventionally, the s-trans conformation is considered to have a C2-C3 dihedral angle of 180°. In contrast, the s-cis conformation, in which the dihedral angle is 0°, with the pair of double bonds facing the same direction is approximately 16.5 kJ/mol (3.9 kcal/mol) higher in energy, due to steric hindrance. This geometry is a local energy maximum, so in contrast to the s-trans geometry, it is not a conformer. The gauche geometry, in which the double bonds of the s-cis geometry are twisted to give a dihedral angle of around 38°, is a second conformer that is around 12.0 kJ/mol (2.9 kcal/mol) higher in energy than the s-trans conformer. Overall, there is a barrier of 24.8 kJ/mol (5.9 kcal/mol) for isomerization between the two conformers. This increased rotational barrier and strong overall preference for a near-planar geometry is evidence for a delocalized π system and a small degree of partial double bond character in the C–C single bond, in accord with resonance theory. Despite the high energy of the s-cis conformation, 1,3-butadiene needs to assume this conformation (or one very similar) before it can participate as the four-electron component in concerted cycloaddition reactions like the Diels-Alder reaction. Similarly, a combined experimental and computational study has found that the double bond of s-trans-butadiene has a length of 133.8 pm, while that for ethylene has a length of 133.0 pm. This was taken as evidence of a π-bond weakened and lengthened by delocalization, as depicted by the resonance structures shown below. A qualitative picture of the molecular orbitals of 1,3-butadiene is readily obtained by applying Hückel theory. (The article on Hückel theory gives a derivation for the butadiene orbitals.) 1,3-Butadiene is also thermodynamically stabilized. While a monosubstituted double bond releases about 30.3 kcal/mol of heat upon hydrogenation, 1,3-butadiene releases slightly less (57.1 kcal/mol) than twice this energy (60.6 kcal/mol), expected for two isolated double bonds. That implies a stabilization energy of 3.5 kcal/mol. Similarly, the hydrogenation of the terminal double bond of 1,4-pentadiene releases 30.1 kcal/mol of heat, while hydrogenation of the terminal double bond of conjugated (E)-1,3-pentadiene releases only 26.5 kcal/mol, implying a very similar value of 3.6 kcal/mol for the stabilization energy. The ~3.5 kcal/mol difference in these heats of hydrogenation can be taken to be the resonance energy of a conjugated diene. The industrial uses illustrate the tendency of butadiene to polymerize. Its susceptibility to 1,4-addition reactions is illustrated by its hydrocyanation. Like many dienes, it undergoes Pd-catalyzed reactions that proceed via allyl complexes. It is a partner in Diels–Alder reactions, e.g. with maleic anhydride to give tetrahydrophthalic anhydride. Like other dienes, butadiene is a ligand for low-valent metal complexes, e.g. the derivatives Fe(butadiene)(CO)3 and Mo(butadiene)3.