Butadiene

The organic compound with the formula (CH2=CH)2 is named as 1 3 Butadiene. The prominent physical property is that it is a colorless gas and can be easily condensed to a liquid. Also, it is an industrially important compound as it is a precursor to synthetic rubber. Typically, the molecule is seen as a combination of two vinyl groups and the simplest form of it exists as a diene. Butadiene is known for breaking down quickly in the atmosphere but is constantly detected in the urban and suburban air because of vehicular emissions. It is important to remember that the common name butadiene is also used to refer to the isomer, 1,2-butadiene, which is a cumulated diene with the structure H2C=C=CH−CH3. 

The History of the Butadiene

1 3 Butadiene was isolated by the French chemist E. Caventou from the pyrolysis of amyl alcohol in 1883. After Henry Edward Armstrong isolated the pyrolysis products of petroleum, the new hydrocarbon was identified as Butadiene in 1886. Following that in the year 1910, a Russian chemist Sergei Lebedev performed the polymerisation of butadiene and obtained a material with rubber-like properties. The polymer was then found to have properties of softness such that it can be used to replace natural rubber in many of its applications especially in the cases of automobile tires. 

The industrial usage and production of butadiene started significantly in the years before World War II. Many of the nations that took part in the war realized that in the incident of war, they ran the risk of being cut off from their rubber plantations controlled in regions such as the ones under the British empire. Hence, they tried to reduce their dependence on natural rubber. In the year 1929, Eduard Tschunkar and Walter Block, who were working for IG Farben in Germany, made a copolymer out of styrene and butadiene which could be used in automobile tires. This started the phenomenon of worldwide production, with butadiene being produced from sources such as grain alcohol in the Soviet Union and the United States, and from coal-derived sources such as acetylene in Germany.

Chemical Properties of Butadiene

The formula of 1 3 Butadiene structure is shown below:

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Hence, from the given structure the butadiene chemical formula can be written as C4H6.

1 3 butadiene structure is in its most stable form only when a molecule of 1 3 butadiene exists in s-trans conformation. In this conformational structure, the molecule is planar and the two pairs of the double bonds are facing the opposite directions. This planar conformation is stable because the overlap of the orbitals in-between the double bonds is in the maximum state and which allows for the maximum conjugation as the steric hindrances and their effects are minimized. In conventional terms, the s-trans conformation is said to have a C2-C3 dihedral angle of 180° unlike in the case of the s-cis conformation, which has a dihedral angle of 0° and the pair of the double bonds are pointing towards the same direction having approximately 16.5 kJ/mol higher energy, because of the steric hindrance. The geometry is formed from the local energy maxima which are opposite to the s-trans geometry. The gauche geometry states that the double bonds of the s-cis geometry are twisted with a dihedral angle which is around 38°,  and the second conformer is around 12.0 kJ/mol which is higher in energy than the s-trans conformer. Speaking overall, between the two conformers there exists a barrier of 24.8 kJ/mol for isomerization between the two. This increase observed in the rotational barrier with an overall strong preference for a near planar geometry is conclusive of the delocalized π system and the presence of a small degree of partial double bond character in the C–C single bond, in accordance with the resonance theory.

Even though there is high energy of s-cis conformation, 1 3 butadiene structure takes this conformation or a conformation similar to the s-cis conformation before the participation as a four-electron component in reactions like the Diels-Alder reaction.

1 3 Butadiene is also a stabilized molecule as per thermodynamics. This is determined by the heat released upon hydrogenation of 1, 3 butadiene which releases approximately 57.1 kcal/mol which is less than double the amount of heat released upon hydrogenation of a monosubstituted double bond which is approximately 60.6 kcal/mole an expected value for two isolated double bonds. This, in turn, gives the stabilisation energy as 3.5 kcal/mol. The same scenario is also observed for the hydrogenation of the terminal double bond present in 1,4-pentadiene which releases about 30.1 kcal/mol of heat, and the hydrogenation of the terminal double bond of the conjugated (E)1,3 pentadiene releases only 26.5 kcal/mol, giving a very similar value of 3.6 kcal/mol in the form of stabilization energy. This difference of ~3.5 kcal/mol in the heats of hydrogenation is taken as the resonance energy of the conjugated diene.

1 3 Butadiene Reactions and Uses

When undergoing Diels-Alder reactions, as butadiene reacts with the double and triple carbon-carbon bonds it synthesizes cycloalkanes and cycloalkene is also useful in the synthesis of cycloalkanes and cycloalkenes, as it reacts with double and triple carbon-carbon bonds through Diels-Alder reactions. In such reactions, the most common processes include the dimerization and trimerization of one or two other molecules of butadiene respectively. Through the dimerization process, butadiene gets converted to vinylcyclohexene and cyclooctadiene. Of these, vinylcyclohexene is one of the most common impurities that is accumulated during the storing of butadiene. Sometimes, some of these processes also use nickel or titanium-containing catalysts. 

For the making of adiponitrile which is a precursor to some nylons, small amounts of butadiene are also used. When hydrogen cyanide is added to each of the double bonds in butadiene it converts butadiene to adiponitrile in a process known as hydrocyanation.

The maximum utility of butadiene comes from its capability to polymerize. The hydrocyanation process shows the susceptibility of butadiene to the 1,4 addition reactions and like many other dienes, it also undergoes the reaction catalysed by palladium (Pd) and which proceeds through allyl complexes. It also plays the role of a partner in the Diels-Alder reaction, which is given by the example of the reaction of maleic anhydride to produce tetrahydrophthalic anhydride. Just like the other dienes, butadiene serves as a ligand for the low-valence complexes of metals, examples of which include the derivatives of iron and molybdenum, Fe(butadiene)(CO)3 and Mo(butadiene)3 respectively. Also, most of the butadiene is used for the production of synthetic rubber especially for the manufacturing of tyres, grommets and elastic bands.

These products of butadiene which are used as replacements for rubber are the polymerised products of the basic unit of 1,3 Butadiene which results in polybutadiene. This polymeric butadiene is very soft in nature and is an almost liquid material. When combined with other monomers, butadiene gives copolymers of higher value. For examples, the polymerization of butadiene along with either styrene or acrylonitrile gives acrylonitrile butadiene styrene (ABS) and styrene-butadiene (SBR) which is the most commonly used for the manufacturing and production of tyres used by the automobile industry. Other examples also include nitrile butadiene (NBR). All these copolymers are known to have physical properties of toughness and/or elasticity which depends on the ratio of the monomers that are used in their preparation. In many cases such as in the case of chloroprene, the precursor used for the preparation of synthetic rubbers is still butadiene.