CONJUGATED DIENES

Butadiene

From the point of applied chemistry butadiene is the most interesting diene. The first observation is that the four p−orbitals of the two π−bonds are all perpendicular to the same sp�−plane. In chemistry this is very significant. If the center two p−orbitals overlap we would land up with one π−bond and two unpaired p−orbital at both ends, and the molecule would not be stable. The four carbons came to a compromise for the common good. The p−orbitals of carbon-1 and carbon-2 (and simultaneously carbon-3 and carbon-4) will overlap ca 36% of the time, the reminder 28% of the time the overlap will be between the p−orbitals of carbon-2 and carbon-3. By doing so the stability of the compound can be improved by ca 6%; as shown by experimental data below.

BOND LENGTH

1.34A�1.48A�1.34A�

The bond distance for a typical CC σ-bond is ca 1.54A� and for a CC double bond is ca 1.33A�. The bond distance between carbon−2 and carbon−3 for butadiene has about 28% of CC double bond.

STABILITY of MOLECULE

The enthalpy of hydrogenation (adding hydrogen to) of a π-bond is often taken as an indication of the stability of the π−bond. For a simple alkene with just one π−bond the value is ca   −126.4 kJ mole‾�. Conversely we can say we need 126.4 kJ mmole‾� to get rid of two protons to form the π−bond.

The enthalpy of hydrogenation of 1-propene to propane is   −126.4 kJ mole‾� and the enthalpy of hydrogenation for 1,5-pentadiene to pentane is   −252.7 kJ mole‾� (that is 2 x −126.4 kJ mole‾�). However the enthalpy of hydrogenation of 1,3-butadiene to butane is   −236.4 kJ mole‾�. That is it requires 6.4% less energy to remove the protons to form two π-bonds if they were conjugated.

This introduces a new outlook towards the Valence Bond Theory. The movement of the electrons forming a bond is no longer restricted to the two atoms involved. It can move along p-orbitals capable of forming π−bonds, like current along a copper wire. This became the fundamental concept for the development of electricity conducting polymers.

However for the electron to move along conjugated π-bonds the p−orbitals forming the π−bonds must all be perpendicular to the same plane. Such movement of electrons of a bond beyond two participating atomic centres is known as resonance (or the freedom of the valence electrons to roam). Resonance would increases the stability of the π−bonds.

Resonance also alters the chemistry of the Electrophilic Reaction and the Free Radical Reaction of the π−bonds. For a start the order of stability for carbonium ion or free radical are now;

ELECTROPHILIC ADDITION

Of course the π−bond would still show the usual electrophilic addition but because of the conjugation there is a certain twist to the reaction. Let us consider the addition of hydrogen bromide onto butadiene. The first step is the usual addition of the proton to carbon−1 to give a secondary carbonium ion.

For a normal electrophilic addition reaction the bromide would be attached to carbon−2 to give a 3−bromo−1−butene. But what we got were 80% of 3−bromo−1−butene and 20% 1−bromo−2−butene.

The only explanation is that after the initial attack of the proton the carbonium ion was able to migrate across the conjugation. Or shall we say the resonance resulted in the carbonium ion being moved to carbon−4;

CH₃C(+)H−CH=CH₂   ↔   CH₃CH=CH−C(+)H₂

The bromide anion then reacts with carbon−4 to give the 1−bromo−2−butene.

So for the electrophilic addition of conjugated diene we have to consider both the 1,2−addition and the 1,4−addition. The number refers more to the conjugated carbon rather than the position of the carbon in the molecule. For example we still refer to 1,2− and 1,4−addition even for molecules like 2,5−hexadiene.

For slower reaction like the addition of bromine to butadiene, the products are 54% 1,2−dibromo−butadiene and 46% 1,4−dibromo−butadiene. The increase in the 1,4−addition product is in agreement with the principle that slower reactions favour thermodynamically stable products.

FREE RADICAL REACTION

Styrene−Butadiene Rubber

Natural Rubber

A similar resonance exist for free radical reaction of a conjugated diene leading to 1,2− and 1,4−products. An important use of butadiene is the preparation of synthetic rubber, commercially known as SBR (Styrene−Butadiene Rubber), for the manufacture of tyres. This poly(styrene−butadiene) is prepared by adding a free radical initiator to a mixture of styrene and butadiene. The butadiene added mainly in a 1,4−mode.

SYN ADDITION