13-Pentadiene

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

For Al/Ti = 4, good agreement between the experimental data and theoretical curves for depletion of cis and formation of trans monomer 

Al/Ti 6, less trans isomer was formed than would be anticipated and it was suggested that polymerization without prior isomerization may have occurred. The expression of the polymerization rate in terms of a first order rate coefficient, necessary for the derivation of a simple kinetic scheme, makes it difficult to compare with other systems. However, if the coefficient fep is identified with k p [Ti], where [Ti] is the concentration of transition metal compound (23 x 10 mole 1 M) a value for k p of 

7 X 1 mole sec is obtained. This is not greatly different from the value estimated for butadiene polymerization using the same catalyst (Table 15). An interesting feature of the reaction is that chloroform inhibits the polymerization with relatively little effect on isomerization. 

The intrinsic viscosity of the polymer fell with increase in Al/Ti ratio — more so with the cis monomer at Al/Ti ratios above 4. 

(645 ml., 3.5 moles) of 1,5-pentanediol diacetate (Note 4) is added to the tube over a period of 3.5 hours. 

The contents of the three receivers are combined and distilled at atmospheric pressure through a 15-cm. indented Claisen-type still head the condenser is cooled with ice water, and the receiver is immersed in an ice bath. The fraction boiling at 25-55°, wt. 170-190 g., is redistilled through a 60-cm. column packed with glass helices or a column of similar efficiency to give 150-170 g. (63-71%) of 1,4-pentadiene, b.p. 26-27.5°/760 mm., 1.3861-1.3871 (Note 5). 

A Hershberg dropping funnel4 modified by addition of a pressure-equalizing arm (Fig. 3) makes it easy to add the diacetate at a constant rate. 

oxygen-free grade of commercial nitrogen is used. Nitrogen can be omitted without diminishing the yield by more than a few per cent. 

5-Pentanediol diacetate,5 6b.p. 85-90°/0.9 mm., 1.4253, is obtained in 92-94% yield by adding a 10% excess of acetic anhydride to 1,5-pentanediol7 at 120-140°, refluxing the mixture for 2 hours, and distilling it at reduced pressure. The practical grade of 1,5-pen tanediol sold by the Eastman Kodak Company may be used. 

---

A hydrocarbon that contains two double bonds is called an alkadiene, and the rela tionship between the double bonds may be described as isolated conjugated or cumu lated Isolated diene units are those m which two carbon-carbon double bond units are separated from each other by one or more sp hybridized carbon atoms 1 4 Pentadiene and 1 5 cyclooctadiene have isolated double bonds... 

Example 17 Estimate Liquid Heat Capacity of 1,4 Pentadiene. ... 

Example 17 Estimate liquid heat capacity at 293.15 K of 1,4-pentadiene, CH2 =CH-----CH2---CH =CH2- The atomic groups are ... 

C4H7N BUTYRONITRILE 32.613 2.4673E-01 2.5125E-05 108.66 193 C5H8 1.4-PENTADIENE 104.148 2.1318E-01 2.5915E-05 170.25... 

Pentadiene has been prepared by the interaction of allyl bromide and vinyl bromide in the presence of magnesium and by the pyrolysis of 1,5-pentanediol diacetate or 4-penten-l-ol acetate. The present procedure is essentially that of Shoemaker and Boord with some modifications. ... 

Photolysis of 1,5-diphenyl-1,4-pentadiene is another example of a reaction that takes this... 

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable ... 

The unsaturated compounds we looked at in Chapters 6 and 7 had only one double bond, but many compounds have numerous sites of unsaturation. If the different unsaturations are well separated in a molecule, they react independently, but if they re close together, they may interact with one another. In particular, compounds that have alternating single and double bonds—so-called conjugated compounds—have some distinctive characteristics. The conjugated diene 1,3-butadiene, for instance, has some properties quite different from those of the nonconjugated 1,4-pentadiene. 

Because a monosubstituted alkene has a AT/Ohyc rog of approximately -126 kj/mol, we might expect that a compound with two monosubstituted double bonds would have a Af/0hyjrog approximately twice that value, or -252 kj/mol. Nonconjugated dienes, such as 1,4-pentadiene (AH°hydrog = —253 kj/mol), meet this expectation, but the conjugated diene 1,3-butadiene (AT/°hydr0g = -236 kj/mol) does not. 1,3-Butadiene is approximately 16 kj/mol (3.8 kcal/mol) more stable than expected. 

---