Allylic Chlorination of Propene

Initiation step A chlorine molecule dissociates to two atoms. 

Propagation steps In the first propagation step a chlorine atom abstracts a hydrogen atom from the allylic carbon of propene forming allyl radical. 

The allyl radical formed in the first propagation step reacts with CI2 to form allyl chloride. 

The chlorine atom generated in this propagation step then abstracts a hydrogen atom from another molecule of propene and the two propagation steps repeat over and over again. 

Assume that A/-bromosuccinimide serves as a source of Br2, and write equations for the propagation steps in the formation of 3-bromocyclohexene by allylic bromination of cyclohexene. 

Although allylic bromination and chlorination offer methods for attaching a reactive functional group to a hydrocarbon framework, we need to be aware of two important limitations. For allylic halogenation to be effective in a particular synthesis  

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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)... 

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). 

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]. 

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. 

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... 

To predict which of the various C—H bonds in propene is most likely to break when a mixture of propene and bromine or chlorine is heated, we need to look at bond dissociation enthalpies. We find that the bond dissociation enthalpy of an allylic C—H bond in propene (Table 8.7) is approximately 92 kj (22 kcal)/mol less than that of a vinylic C—H bond and 50 kJ (12 kcal)/mol less than a C—H bond of ethane. The allyl radical is even more stable than a 3° radical this unusual stability also applies to carbocations. The reason the allylic C—H bond is so weak is discussed in Section 8.6B. Note from Table 8.7 that the benzyl radical CgH5CH2- is stabilized in exactly the same way as the allyl radical and for the same reason benzylic compounds undergo many of the same reactions as allylic compounds (Section 21.5). 

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.)... 

This species acts as a propene metathesis catalyst even at 0°C, Apparently clean oxidation, hydrogenation, and chlorination processes can be achieved from this material to afford new metal sites. The same coordination centre as shown in Eq. 6 can also be formed from Mo2(allyl)4 [94]. In that case adsorption at 0 C did not cause loss of propene rather the molybdenum-molybdenum quadruple bond is considered... 

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. 

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. 

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