3.3- Dichloropentane

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

We have chosen the PVC diad and triad compounds 2,4-dichloropentane (DCP) and 2,4,6-trichloroheptane(TCH) as subjects for our attempt to obtain quantitative kinetic data characterizing their (n-Bu)3SnH reduction in the hope that they will serve as useful models tor the reduction of PVC to E-V copolymers. Unlike the polymers (PVC and E-V), DCP and TCH are low molecular weight liquids whose high resolution 13C NMR spectra can be recorded from their concentrated solutions in a matter of minutes. Thus, it is possible to monitor their (n-Bu)3SnH reduction directly in the NMR tube and follow the kinetics of their dechlorination. 

A small quantity of 2,4-dichloropentan-3-one (5%) is obtained with the a-monochloropentanone 2 during the distillation. The NMR 8uialysis for 2-chloropentan-3-one 2 is described by Wyman and Kaufman. ... 

Lim and Kolinsky (203) estimated the chain-transfer coefficients of 2,4-dichloropentane and 2,4,6-trichloroheptane (dimer and trimer of vinyl chloride, plus H and CH3 end-groups) as 5 x 10 4 for both compounds at 50° C, though a higher value would be expected for the latter, which has one more —CH2 CHCl-group, than for the former they suggest that this value is appropriate for Ctp for vinyl chloride polymerization. However, the poly-... 

Inductive effects on dipole moments and the effects of intervening atoms on electrostatic interaction energies are represented by polarizability centers In conjunction with bond centered dipoles. Solvation energies are estimated by means of a continuum dlpole-quadrupole electrostatic model. Calculated energies of a number of conformations of meso and racemic 2,4-dichloropentane and the iso, syndio, and hetero forms of 2,4,6-triehloroheptane give satisfactory representations of isomer and conformer populations. Electrostatic effects are found to be quite important. 

As the range of chemical shifts is an order of magnitude larger for 1 C than for H, CMR is better suited for investigating stereoisomers. This has been demonstrated by 1- C NMR measurements on 2,4-dichloropentane and 2,3-dichlorobutane, which are models... 

Table 4.19. 13C Chemical Shifts of 2,4-Dichloropentanes, 2,3-Dichlorobutanes and Polyvinyl Chloride (<5C in ppm) [262],...

Comparison of the 13C NMR spectrum of commercial polyvinyl chloride with the CMR spectra of racem- and meso-2,4-dichloropentane permits signal identification of the syndiotactic, heterotactic and isotactic methine carbons of the polymer [262]. 

Stereosequences of polyvinyl chloride (PVC) are assignable by comparing its carbon shifts with those of meso and racem 2,4-dichloropentane [262] as outlined in Section 4.4.1. 2,4,6-Trichloroheptane can also be used as a model in which the three triads (mm, rr, mr) building up the stereoisomers of PVC may be observed [521]. 

Assume tliat you havecarried out a radical chlorination reaction on (P)-2-chloio-pentane and have isolated (in low yield) 2,4-dichloropentane. How many stereoisomers ot the product are toimed and in what latio Are any of the isomers optically active (See Problem 10.24.)... 

Dichloride structures were studied by Erbe et al. [190] and are thought to be more stable than 2,4-dichloropentane which represents the normal structure of the polymer. 

In 2,4-dichloropentane, for example, there are two asymmetric carbon atoms with four ligands having the seniorities Cl > CH2—CHCl—CH3 > CH3 > H (Figure 3-5). Molecule I has, according to the chirality rules, the configuration RS. The two asymmetric central atoms have opposing configurations so, the molecule is a meso compound. Since molecules I and III can be converted into each other by a rotation about 180°, these two molecules must be identical. Molecules II and IV cannot be transformed into each other by a symmetry operation, and so, they must be enantiomers a 1 1 mixture of these two is therefore racemic. 

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