Propene allylic chlorination

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

Earlier, tertiary chlorine was generally considered to be less reactive than internal double bonds. This was based on experiments with low molecular weight model substances, see e.g. ref. 45. However, using copolymers between vinyl chloride and 2-chloro-propene, Berens (46) stated that the presence of 1-2 tertiary chlorine per 1000 VC per se would account for the thermal lability observed in ordinary PVC. Furthermore, our previous investigation indicated that the thermal reactivity of internal allylic chlorine is of the same order as that of tertiary chlorine (8). We, therefore, consider it justifiable to use the total content of labile chlorine atoms. As shown in Figure 10, there is a very good relation between the rate of dehydrochlorination and the amount of labile chlorine obtained in this way. For comparison, it can be mentioned that the degradation rate of commercial samples is found in the interval 1.5-3.5 10 %... 

Subjecting propene labeled with at carbon 1 to allylic chlorination (see below) leads to a 50 50 mixture of... 

Propene undergoes allylic chlorination when propene and chlorine react in the gas phase at 400°C. This method for synthesizing aUyl chloride is called the Shell process. 

Subjecting propene labeled with at carbon 1 to allylic chlorination (see below) leads to a 50 50 mixture of 1 -chloropropene labeled at C1 and at C3. Write a mechanism that explains this result. (An asterisk next to a carbon atom indicates that the carbon atom is C.)... 

Allylic chlorinations are important in industry because chlorine is relatively cheap. For example, 3-chloropropene (allyl chloride) is made commercially by the gas-phase chlorination of propene at 400°C. It is a building block for the synthesis of epoxy resin and many other useful substances. 

CH2 CH.CH2.OH mw 58.09 OB to C02 -220.35% colorl, mobile liq with a pungent odor mp —129° bp 96.9°, d 0.8520g/cc at 20/4° RI 1.4127, 1.4133, 1.4135 (Sep values). V sol in w, ethanol and ether. Prepn is by the high temp chlorination of propene, followed by hydrolysis of allyl chloride. Thus,. . Allyl chloride is hydrolyzed at 200psi pressure, 150°C and a pH range of 10—12. Injection of steam forms a water-allyl alcohol azeotrope, which is then treated with diallyl ether to remove water. Final purification by distn leads to a 98% min assay. . (Ref 2). Using procedures such as the above, ... at least two American companies (Shell Chemical Co and Dow Chemical Co) produce a total of several million lbs per year... (Ref 2)... 

With propene, CH3CH=CH2 (79), there is the possibility of either addition of chlorine to the double bond, or of attack on the CH3 group. It is found that at elevated temperatures, e.g. 450° (Cl then being provided by thermolysis of Cl2), substitution occurs to the total exclusion of addition. This is because the allyl radical (80) obtained by H-abstraction is stabilised by delocalisation, whereas the one (81) obtained on Cl addition is not, and its formation is in any case reversible at elevated temperatures, the equilibrium lying over to the left ... 

Microwave spectroscopic studies of l-fluoro-2-propene show that the cis conformation is more stable than the gauche conformation by approximately 306 cal/mol293 However, when fluorine is replaced by chlorine, bromine, or iodine, the gauche conformation becomes more stable294. These results confirm our expectations that the conformational preferences of allyl halides may depend on a balance of nonbonded attractive effects and a—it interaction effects. 

Propene Chlorine Allyl chloride Hydrogen chloride... 

Halogen compounds in which the carbon-halogen bond is adjacent to a double bond, as in C=C—C—X are known as allylic halides. The simplest example is 3-chloropropene, CH2=CHCH2C1, which is made on a large scale by the radical chlorination of propene at 400° ... 

This reaction, like the chlorination of propene, is highly selective in that the so-called allylic C—H is attacked preferentially. 

From bond energies (Table 4-6) we know that the weakest C—H bonds of propene are to the allylic hydrogens, H2C=CHCH2—H. Therefore, in the first step of radical-chain chlorination of propene, an allylic hydrogen is removed by a chlorine atom (Equation 14-1). The allylic C-H bonds are weaker than the alkenic C-H bonds because of the extra stabilization of the radical obtained on hydrogen abstraction (Equation 14-1). Two equivalent valence-bond structures (1a and 1b) can be written for the 2-propenyl radical the electron delocalization enhances the stability of the radical (see Section 6-5C) ... 

Allyl chloride is prepared by free-radical chlorination of propene (see text page 371). 

The two chlorinated propene compounds shown are colorless liquids with pungent, irritating odors. Allyl chloride is an intermediate in the manufacture of allyl alcohol and other allyl compounds, including pharmaceuticals, insecticides, and thermosetting varnish and plastic resins. Dichloropropene compounds have been used as soil fumigants, as well as solvents for oil, fat, dry cleaning, and metal degreasing. 

The chlorinated propenes are obnoxious compounds. Unlike other compounds discussed so far in this section, their pungent odors and irritating effects lead to an avoidance response in exposed subjects. They are irritants to the eyes, skin, and respiratory tract. Contact with the skin can result in rashes, blisters, and bums. Chronic exposure to allyl chloride is manifested by aching muscles and bones it damages the liver, lungs, and kidney and causes pulmonary edema. 

The first significant attempt to study the kinetics of the thermal decomposition of chlorinated and brominated hydrocarbons and related compounds was by Lessig in 1932. These studies were complicated by the catalytic activity of the pyrex or quartz glass walls of the vessels in which the gas phase reactions were carried out. It was later found that the heterogeneous reactions could be suppressed by using seasoned reaction vessels in which a fine coating of carbonised material had formed on the walls from previous decompositions of similar compounds. Substances such as allyl bromide and 3-chloro-2-methyl-propene have been found particularly effective in producing carbonised films. 

This principle may also be illustrated by some real cases. In the codimerization of propene and hexene it is important primarily to minimize the dimerization of the reactive propene. In order to favor the codimerization, a stage injection of propene according to the principle in Fig. 1 was therefore performed [2]. A similar process design with distributed additions of chlorine was applied in the chlorination of propene to allyl chloride in order to suppress different side reactions [3]. For liquid-phase processes, a distributed feed to the cascade of stirred reactors was a more natural variant. This was applied in the sulfuric acid alkylation of / obutane, where the olefin feed has to be subdivided due to selectivity reasons and the goal was to reach a desired octane number of the product [4]. 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

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