Barrier Butadiene

A con jugated sp - -sp --" single bond (for example, the bond joining the tw o phenyl rings of biphenyl, the central bond of butadiene, with delocali/ed aromatic bonds, or phenyl amine, where N-G bond is labeled aromatic and nitrogen is sp2 b h ybridi/ed) IS described by a two-fold barrier, V2=l() kcal/mol. 

The faet that the lowest two orbitals of the reaetants, whieh are those oeeupied by the four 71 eleetrons of the reaetant, do not eorrelate to the lowest two orbitals of the produets, whieh are the orbitals oeeupied by the two a and two n eleetrons of the produets, will be shown later in Chapter 12 to be the origin of the aetivation barrier for the thermal disrotatory rearrangement (in whieh the four aetive eleetrons oeeupy these lowest two orbitals) of 1,3-butadiene to produee eyelobutene. 

The principal monomer of nitrile resins is acrylonitrile (see Polyacrylonitrile ), which constitutes about 70% by weight of the polymer and provides the polymer with good gas barrier and chemical resistance properties. The remainder of the polymer is 20 to 30% methylacrylate (or styrene), with 0 to 10% butadiene to serve as an impact-modifying termonomer. 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

Rotational barriers for bonds which have partly double bond character are significantly too low. This is especially a problem for the rotation around the C-N bond in amides, where values of 5-10 kcal/mol are obtained. A purely ad hoc fix has been made for amides by adding a force field rotational term to the C-N bond which raises the value to 20-25 kcal/mol, and brings it in line with experimental data. Similarly, the barrier for rotation around the central bond in butadiene is calculated to be only 0.5-2.0 kcal/mol, in contrast to the experimental value of 5.9 kcal/mol. 

Platinum-cobalt alloy, enthalpy of formation, 144 Polarizability, of carbon, 75 of hydrogen molecule, 65, 75 and ionization potential data, 70 Polyamide, 181 Poly butadiene, 170, 181 Polydispersed systems, 183 Polyfunctional polymer, 178 Polymerization, of butadiene, 163 of solid acetaldehyde, 163 of vinyl monomers, 154 Polymers, star-shaped, 183 Polymethyl methacrylate, 180 Polystyrene, 172 Polystyril carbanions, 154 Potential barriers of internal rotation, 368, 374... 

The spectroscopic conclusion of Bartholome and Karweil11 that in butadiene there is essentially free rotation about the C-C bond with a barrier not greater than 200 cal./mole is almost certainly incorrect. 

Figure 13.10 A schematic free-energy versus reaction coordinate diagram for the 1,2 and 1,4 addition of hbr to 1,3-butadiene. An allylic carbocation is common to both pathways. The energy barrier for attack of bromide on the allylic cation to form the 1,2-addition product is less than that to form the 1,4-addition product. The 1,2-addition product is kinetically favored. The 1,4-addition product is more stable, and so it is the thermodynamically favored product.

The primary use of acrylonitrile is as the raw material for the manufacture of acrylic and modacrylic fibers. Other Major uses include the production of plastics (acrylonitrile-butadiene- styrene (ABS) and styrene-acrylonitrile (SAN), nitrile rubbers, nitrile barrier resins, adiponitrile and acrylamide (EPA 1984). 

The [Ni°(CDT)] product complex 8b is formed via reductive elimination under ring closure starting from the dodecatrienediyl-Ni11 complex. The formation of the several isomers of CDT occurs via competing paths for reductive elimination that involves different stereoisomers. Displacement of the cyclotrimer product in subsequent consecutive substitution steps with butadiene, which is supposed to take place without a significant barrier, regenerates the [Ni°( butadiene) J active catalyst thus completing the catalytic cycle. 

A Positive/negative sign indicates increased decreased relative barriers and reaction energies, respectively, relative to the generic [Ni0(r 2-butadiene)2PII3] catalyst. For computational details see Reference lib. 

Scheme 4. Condensed free-energy profile (kcalmol-1) of the complete catalytic cycle of the C8-reaction channel of the nickel-catalyzed cyclo-oligomerization of 1,3-butadiene for catalyst IV with L = P(OPh)3. The favorable [Ni°(p2-tr

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