1- Chloro-2-pentene

Investigation of the kinetics of the reaction of 4-chloro-2-pentene, an allylic chloride model for the unstable moiety of polyfvinyl chloride), with several thermal stabilizers for the polymer has led to a better understanding of the stabilization mechanism. One general feature of the mechanism is complexing of the labile chlorine atom by the metal atom of the stabilizer. A second general feature is substitution of the complexed chlorine atom by a ligand (either carboxylate or mercaptide) bound to the metal. Stabilization requires that the new allylic substituent (ester or sulfide) be more thermally stable than the allylic chlorine. The isolation of products from stabilizer-model compound reactions supports the substitution hypothesis of poly(vinyl chloride) stabilization. 

Dodecyl 4-pent-2-enyl Sulfide. Dibutyltin oxide (21.37 grams, 0.08589 mole) reacted with 34.7 grams (0.1715 mole) of dodecyl mercaptan in 150 ml. of toluene at reflux for 39 minutes when the evolution of water had ceased. The reaction product was charcoaled, filtered, and concentrated to constant weight at 55 °C. and < 1 mm. The yield of colorless dibutyltin bis(dodecylmercaptide) was 54.7 grams (theory 54.53 grams). This mercaptide, 1.5023 grams (0.00494 mole) of dibutyltin dichloride, and 17.96 grams (0.1718 mole) of 4-chloro-2-pentene reacted in 250 ml. chlorobenzene solution for 7.2 hours when analysis of a sample of the reaction product for chloride showed the reaction to be virtually completed. 

Kinetic Measurements. All experiments were conducted in a constant temperature oil bath controlled within 0.1°C. The stabilizer was dissolved in chlorobenzene in a volumetric flask of suitable volume, usually 100 ml., and placed in the bath. After about 7 minutes the volume was adjusted to about 95 ml., and 4-chloro-2-pentene was added from a pipet with swirling for 100 ml. of solution generally 2.00 ml. (average wt. 1.7958 grams) of chloride was added to give a 0.172N solution in chloride. The volume was adjusted quickly to nearly 100 ml. with chlorobenzene, and the contents were mixed thoroughly. Final adjustment to 100 ml. was made when thermal equilibrium was reached. 

At suitable intervals 5.00-ml. samples of the reaction product were removed using a pipet which had been preheated to the bath temperature. The reaction was quenched by cooling the samples in an ice bath. The sample was concentrated under vacuum, finally at 50 °C. at aspirator vacuum (12 mm.), to remove unreacted 4-chloro-2-pentene which reacts modeartely readily with silver nitrate. 

The reaction half-time for 4-chloro-2-pentene, the allylic chloride model, with dibutyltin -mercaptopropionate is about 1/20 that for 2-chloro-2-methylbutane, a tert-chloride model, with the same stabilizer. This result supports the choice of an allylic chloride as the most important unstable functionality of poly (vinyl chloride). 

Allylic Chloride vs. tert-Chloride Reactivity. There is some question in the literature as to whether the allylic chloride moiety or ferf-chloride group is more responsible for the thermal instability of poly (vinyl chloride) (I, 2). To shed some light on this problem we compared the relative reactivities at 100 °C. in chlorobenzene of 4-chloro-2-pentene and 2-chloro-2-methylbutane with dibutyltin -mercaptopropionate. Data are summarized in Table I. The half-time for the reaction of the allylic chloride with the stabilizer mercaptide group was less than 15 minutes, whereas the half-time for the tert-chloride was nearly 20 times longer. The greater reactivity of the allyl chloride suggests that it is the more important functionality in polymer degradation. However, these results on rates of chlorine substitution are not necessarily an exact measure of thermal instability. 

Kinetic Results. The reactions were followed by measuring the rate of liberation of chloride ion from the reaction of 4-chloro-2-pentene with stabilizer in chlorobenzene. 

The reaction of 4-chloro-2-pentene with dibutyltin dilaurate, one of the first to be studied, provided some interesting observations. A plot of liberated chlorine vs. time (Figure 1) shows the reaction to be auto-catalytic. Investigation led to the discovery that dibutyltin dichloride, a product of the stabilization reaction, is a catalyst for the reaction (Figure 1). Further investigation revealed that at the outset all the reaction takes place by the unimolecular decomposition of the allylic chloride and that not until species with tin—chlorine bonds are present does this stabilizer undergo reaction with the allylic chloride. The reactive stabilizer in this instance is dibutyltin laurate chloride. Dibutyltin dilaurate does not react with the allylic chloride, and dibutyltin dichloride, which is very reactive,... 

Figure I. Reaction of dibutyltin dilaurate with 4-chloro-2-pentene at 100° C.

The behavior of dibutyltin bis(dodecylmercaptide) on reaction with 4-chloro-2-pentene proved interesting (Figure 3). With only the two reactants in chlorobenzene, virtually no reaction took place up to 5 hours. However, the addition of dibutyltin dichloride resulted in a rapid reaction. Furthermore, the addition of a few milligrams of azobisisobutyro-nitrile eliminated any induction period. This latter consequence is not interpreted to result from a free radical stabilization mechanism, but it is presumed to be caused by free radical-catalyzed hydrogen chloride elimination, resulting (by neutralization with the stabilizer) in the formation... 

Table IV. Reaction of Dibutyltin Dilaurate With 4-Chloro-2-pentene at 80°C.

Figure 4. RCl vs. time for the reaction of 4-chloro-2-pentene with dibutyltin bis(monobutylmaleate) at 80° C. Letters L, /, N, M designate different...

CH3CH = CHCHCH3 Cl 4-Chloro-2-pentene 1,2 addition 1,4 addition... 

A and D, which are resonance-stabilized, are formed in preference to B and C, which are not. The positive charge of allylic carbocation A is delocalized over two secondary carbons, while the positive charge of carbocation D is delocalized over one secondary and one primary carbon. We therefore predict that carbocation A is the major intermediate formed, and that 4-chloro-2-pentene predominates. Note that this product results from both 1,2 and 1,4 addition. 

We are not aware of any systematic investigations on the diastereoselectivities of the CC-bond-forming step in intermolecular reactions of carbocations with alkenes. Generally, we observed only low stereoselectivities in such cases, as illustrated for the Lewis acid catalyzed addition of 4-chloro-2-pentene to (Z)-2-butene (Scheme 19). The si,re transition state 6 is slightly favored (75 25) over the si,si (and re,re) transition state 7, and for the corresponding addition to ( )-2-butene, the advantage of si,re over si,si (or re,re) sinks to 57 43 [95]. 

Chloro-2-pentene predominates in both. l,2-.Addition 6-bromo- 1,6-dimethylcyciohexene... 

C5H9CI 4-chloro-2-pentene 1458-99-7 159.69 10.598 2 5289 C5H10 trans-2-pentene 646-04-8 132.89 8.350 1... 

Allene is less stable than either a conjugated or a nonconjugated diene. l-ChIoro-2-pentene, 3-chIoro- 1-pentene, 4-chloro-2-pentene... 

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