Allylic chlorinations

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

The selectivity greatly depends on the reaction temperature. Propylene,153 for example, gives allyl chloride in 25% yield at 210°C, whereas the yield is 96% at 400°C. This is explained by the reversibility of radical addition of chlorine to the double bond at high temperature, in contrast to radical substitution, which is not an equilibrium process.156 

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Hjertberg and coworkers [38-41] were able to correlate the amount of labile chlorine, tertiary and internal allylic chlorine, to the dehydrochlorination rate. They studied PVC samples with increased contents of labile chlorine, which were obtained by polymerization at reduced monomer concentration. According to their results, tertiary chlorine was the most important defect in PVC. In agreement with other reports [42,43], the results also indicated that secondary chlorine was unstable at the temperatures in question, i.e., random initiation would also occur. 

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

Caraculacu et al. [48] also quantitatively determined allylic chlorines in PVC by isotopic exchange with SO Cl2. The selective exchange of chlorine in the polymer was verified by experiments with model compounds. The number of allylic chlorines in PVC was found to be between 0.12 and 0.16 for 100 monomer units. 

Using thiophenol instead of phenol, Michel et al. [49] found a new selective reaction that takes place exclusively with allylic chlorines and not with tertiary chlorines. A single product of thioether structure is formed [Eq. (11)]. 

In usual PVC 1.53-2.54 allylic chlorines per 1,000 monomer units and in a benzene-soluble low-molecular weight PVC fraction, a higher value of 3.07 allylic chlorines per 1,000 monomer units was obtained. This method has come under certain criticism because it was found to give significantly higher values for labile (allylic) chlorines than do other methods, e.g., phenolysis [50]. 

Recent research has conclusively demonstrated the presence of extremely low levels of tertiary and allylic chlorines in PVC as structural defects [118]. Such chlorines, which are extremely labile, when present in areas of like-charge concentration in the polymer matrix would be more susceptible to breaking from the chain than the ordinary secondary chlorines. 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

Evidence of chemical interaction between the mbbers and compatibUizers was demonstrated by extracting the blends with chloroform at room temperamre and examining both soluble and insoluble fractions with Fourier transform infrared (ETIR) spectrometry. The weight of the insoluble fraction of the compatibilized melt blend was more than that in the uncompatibilized blend indicating the formation of (EP-g-MA)-g-CR due to reaction between MA and allylic chlorine of CR. The compounds containing epoxidized EPDM additive were examined by both optical and... 

Another synthesis, shown in Scheme 13.9, that starts with the same aldehyde (perillaldehyde) was completed more recently. The C(8)—C(9) bond was established by an allylic chlorination and addition of the corresponding zinc reagent to isobu-tyraldehyde. In this synthesis, the C(7) stereochemistry was established by a homogeneous hydrogenation of a methylene group, but this reaction also produces both stereoisomers. 

Cyclohexene undergoes analogous allylic chlorination for the same reasons. 

Propene undergoes allylic chlorination when propene and chlorine react in the gas... 

Refluxing with zinc in ethanol reduced a-bromocinnamic acid to cinnamic acid in 80% yield [997]. Allylic chlorines in y,y,y-trichlorocrotonic acid were partly or completely hydrogenolyzed by zinc and sodium amalgam [5/9]. Hydrogenolysis of allylic bromine in oc, -unsaturated esters with zinc in acetic acid gave predominantly, y-unsaturated esters in 65-97% yields [998]. 

The 6,7-dihydro-5/f -1,4-dioxepin (266) has been prepared (54CR(38)982). and more recently it has been shown that the 2,3-dihydro-5jF/-l,4-dioxepins (263) and (265) can be produced from 1,4-dioxine-halocarbene adducts (264), either by heating under reflux in xylene or by treatment with bases. The allylic chlorine atom in (263) is readily substituted by alkoxide or cyanide ions (77ZC331, 76UKZ968). Saturated rings of type (267) have been prepared by the treatment of cyclic acetals of ethane-1,2-diol with vinyl ethers in the presence of boron trifluoride, and l,4-dioxepan-5-one (268) has been prepared by the reaction of bromoform and silver nitrate with aqueous dioxane (60AG415). 

Another synthetic example of photoinitiated reactions of /-butyl hypochlorite is allylic chlorination. Thus, a variety of olefins, i.e., traits-2-... 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

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