Butadiene forms

FIGURE 2.3 The four tc orbitals of butadiene, formed by overlap of four p orbitals. 

The ketene 37 reacjs with 2-methoxybutadiene to afford a 63% yield of the rearranged methylenedihydropyran 38 (equation 22)24. In contrast, dimethylketene and 1-methoxy-butadiene form a normal cycloadduct , the cyclobutanone 39 (equation 23)24. 

During this time, other materials that gave rubberlike materials were found. In 1901, I. Kondakov, a Russian, discovered that dimethyl butadiene when heated with potash formed a rubberlike material. In 1910, S.V. Lebedev, another Russian, reacted butadiene forming a rubberlike material (structure 9.33). 

Substituted butadienes form [4-1-2] cycloadducts with Cgg that show sufficient stability even without further stabilization. Table 4.1 shows some butadiene derivatives used as substrates. Most of these dienes form stable, isolable products in good yields. 

Dienes were treated with xenon difluoride in the presence of a boron trifluoride-diethyl ether complex using mild conditions this formed mainly the products of 1,2-addition.28 Butadiene formed 3,4-difluorobut-1-ene (87%) and the isomer l,4-difluorobut-2-ene (13%), whereas 2,3-dimethylbutadiene quantitatively produced the 1,2-addition product. 

A useful group of rubbers are the stereo specific poly(butadiene) rubbers formed by the polymerization of 1,3-butadiene. These rubbers have a ris-isomer content of more than 30%. They contain at least about 85% of poly(butadiene) formed by 1,4 addition. Further, the rubber should have a second order transition temperature of preferably not higher than -20°C (8). 

Ratios of the amount of butadiene formed in experiments where pulses of butene were passed over the catalyst at 200°C in a stream of He. 

We indicated previously that sulfur dioxide (S02) and 1,3-butadiene form a [4 + 1 ] cycloaddition product ... 

In addition to protection, a change of diene reactivity is effected by coordination to carbonyl. Butadiene forms the very stable complex 56 and its reactions are different from those of free butadiene. Electrophiles attack C(l) or C(4) of the complexed dienes, and reactions that are impossible with uncomplexed dienes now become possible. 

Some of these molecules, such as the first and the second (ethylene and propylene) can be used to build saturated chains (PE and PP). Other ones, such as the third one (butadiene) form unsaturated chains, which can react with sulphur to form vulcanized rubbers. (See Qu. 1.3,1.4 and 1.5). 

Explain why butadiene forms alternating copolymers with ethylene in copolymerisation with some Ziegler-Natta catalysts. 

Fig. 15.14. Evidence for stereoselectivity and stereospecificity with regard to the butadiene moiety in a pair of Diels-Alder reactions. The cis,trans-l,4-disubstituted 1,3-butadiene forms cyclohexene with a trans arrangement of the methyl groups. The trans,trans-1,4-disubstituted 1,3-butadiene forms cyclohexene with ris-methyl groups.

Acrylonitrile, butadiene and styrene can be polymerised individually. Acrylonitrile yields polyacrylonitrile which makes soft fibres for clothing (sold as Orion and Courtelle), while butadiene forms polybutadiene or synthetic rubber, and styrene as polystyrene is known for its excellent insulation properties. 

The gas-phase elimination of bromocyclobutane was studied at 791-1224 K using the very low-pressure pyrolysis (VLPP) technique114. Hydrogen bromide and 1,3-butadiene formed from isomerization of the product cyclobutene (equation 24) were the products obtained under these experimental conditions. 

The reactants in the presence of 2,3-dimethyl-1,3-butadiene forms the Diels-Alder adduct—4,5-dimethyl-2-pentafluoroethyl-2-trifluoromethyl-3,6-dihy-dro-277-thiopyran (82CL201). 

The phase relationships of two-phase polymer systems also have been of considerable interest in recent years. In an important series of papers, Molau and co-workers (19-24) studied systems, which were denoted POO emulsions (polymeric oil-in-oil), prepared by dissolving a given polymer in monomer and then polymerizing the monomer. During polymerizations of this type the composition of the respective phases reverses, and a phase inversion process was proposed to explain this. A similar process has been suggested as the mechanism by which poly-butadiene forms the dispersed phase in the manufacture of high-impact polystyrenes (22,25). Recently, Kruse has pointed out that this phase-inversion point may correspond to that point on a ternary phase diagram at which the reaction line bisects a tie line (26), and we have advanced a similar point of view in our earlier reports (17,18, 27). 

A symmetry-allowed reaction. The HOMO of butadiene forms a bonding overlap with the LUMO of ethylene because the orbitals have similar symmetry. This reaction is therefore symmetry-allowed. 

Two potassium atoms transfer an electron each to butadiene forming a dianion transmetallation with o-xylene then gives the potassium-bonded carbanion, which inserts butadiene. A second transmetallation with o-xylene liberates the potassium-stabilized benzylcarbanion, which is the actual catalytic species and generates o-pentenyltoluene. This can then be cyclized to 1,5-dimethyltetralin, which, after dehydrogenation to the corresponding naphthalene and isomerization to the 2,6-isomer, affords 2,6-naphthalenedicarboxylic acid by oxidation. 

The overall two-step mechanism for addition of HBr to 1,3-butadiene, forming a 1.2-addition product and 1.4-addition product, is illustrated with the energy diagram in Figure 16.7. 

The secondary reactions which follow process (5) are fascinating in their complexity. The details of these reactions were first unraveled hy Collin and Lossing in the mercury-photosensitized system. At low pressure (Pc,He = few microns Phb = 10 mm.) the hot molecule of 1,2-butadiene formed in reaction (5) invariably decomposed to two radicals. 

In coordination polymerization it is generally accepted that the monomer forms a 7r-complex with the transition metal prior to insertion into the growing chain. In general these complexes are insufficiently stable to be isolated although complexes of allene [69] and butadiene [70] have been reported. With allene the complex was formed prior to polymerization with soluble nickel catalysts, and cis coordinated butadiene forms part of the cobalt complex, CoCj 2H19, which is a dimerization cateilyst. 

Butadiene is carbonylated catalytically to form 3-pentenoate in the presence of palladium and hydrogen chloride in alcohol (16). In this reaction, butadiene forms an unsymmetrical 7r-allylic complex by the insertion of one of the double bonds into the palladium-hydrogen bond. Then the insertion of carbon monoxide takes place at the less hindered carbon of the complex to give 3-pentenoate. 

As a third system, the photoreactivity of long chain butadienes forming lipid layer-type structures was studied (12-ii.) Investigations were carried out in the crystalline state, in monomolecular layers and in LB-type multilayers. The individual compounds and their photoreactivities are listed in Table IV. 

Ti02) is effective in this reaction, giving substantial yields of maleic anhydride (Figure 10) The butadiene formed does not desorb and reacts at the surface of the catalyst according to the known mechanism for the oxidation of butene to maleic anhydride The notion of interfacial effects has therefore been applied, with some success, to finding a new catalyst ... 

Butadiene forms the products of 1,2- and of 1,4-addition in approximately equal amounts (Scheme 74). 

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