Free radical chlorination

Chlorine free radicals used for the substitutioa reactioa are obtaiaed by either thermal, photochemical, or chemical means. The thermal method requites temperatures of at least 250°C to iaitiate decomposition of the diatomic chlorine molecules iato chlorine radicals. The large reaction exotherm demands close temperature control by cooling or dilution, although adiabatic reactors with an appropriate diluent are commonly used ia iadustrial processes. Thermal chlorination is iaexpeasive and less sensitive to inhibition than the photochemical process. Mercury arc lamps are the usual source of ultraviolet light for photochemical processes furnishing wavelengths from 300—500 nm. 

Methane is the most difficult alkane to chlorinate. The reaction is initiated by chlorine free radicals obtained via the application of heat (thermal) or light (hv). Thermal chlorination (more widely used industrially) occurs at approximately 350-370°C and atmospheric pressure. A typical product distribution for a CH4/CI2 feed ratio of 1.7 is mono- (58.7%), di-(29.3%) tri- (9.7%) and tetra- (2.3%) chloromethanes. 

The highly exothermic chlorination reaction produces approximately 95 KJ/mol of HCI. The first step is the breaking of the Cl-Cl bond (bond energy = -1- 584.2 KJ), which forms two chlorine free radicals (Cl atoms) ... 

The stratosphere contains, however, only small amounts--a few tenths of a ppb-of chlorine free radicals of natural origin. They are produced by the decomposition of methyl chloride, CH3Q. The nitrogen oxides (NO and NO2) are more abundant and are produced in the stratosphere by the decomposition of nitrous oxide, N2O. Both CH3CI and N2O are of biological origin these compounds, released at the Earth s surface, are sufficiently stable to reach the stratosphere in significant amounts. 

In CfE Higher Chemistry, you came across free radicals when we considered the mechanism of the substitution reaction between methane and chlorine In the presence of ultraviolet light. You will recall that the initiation step In the mechanism Is the homolytic fission of chlorine molecules to generate chlorine free radicals. 

The initiation step is homoljdic bond cleavc e where each of the chlorine atoms receives one of the two electrons originally present in the bond and two chlorine free radicals form. The chlorine free radicals, like all free radicals, are very reactive. 

A chlorine free radical attacks an alkane molecule like methane to form hydrogen chloride and a methyl radical (see Figure 2-15). 

At low temperature, propene behaves like another alkene and undergoes a simple addition of a halogen across the double bond to form 1,2-dichlo-ropropane. These conditions minimize the possibility of forming chlorine atoms (chlorine free radicals), and the presence of oxygen traps the few that do form. However, when the conditions promote the formation of chlorine atoms, a substitution occurs to produce 3-chloropropene. 

Although not in the top 50, it is an important monomer for making epoxy adhesives as well as glycerine (HO-CH2-CHOH-CH2-OH). Propylene is first chlorinated free radically at the allyl position at 500°C to give allyl chloride, which undergoes chlorohydrin chemistry as discussed previously to give epichlorohydrin. The student should review the mechanism of allyl free radical substitution from a basic organic chemistry course and also work out the mechanism for this example of a chlorohydrin reaction. 

The signihcant feature of this reaction is that it results in the regeneration of chlorine free radicals, which are then available to react with other ozone molecules. In fact, the net result of the preceding two reactions is the dissociation of ozone molecules into diatomic oxygen molecules ... 

CFCs are nearly ideal substances for attacking ozone molecules and damaging the ozone layer. On the one hand, they tend to be very stable, even in the stratosphere. Many CFCs have half-lives of 100 years or more that means that once they have escaped into the upper atmosphere, they are likely to remain there for very long periods. On the other hand, some small number of CFC molecules do dissociate to form chlorine free radicals, with the ability to destroy ozone molecules. Although the number of CFC molecules that do dissociate is relatively small, the actual number is not important since chlorine free radicals that are generated in the process are used over and over again. That is, they are catalysts in the destruction of ozone and are not, themselves, used up in their reactions with ozone molecules. 

The chlorine free radical can then interact with ozone in several different ways analogous to the NO,. At mid-latitudes the reactions are... 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

This chlorine free radical, in turn, reacts further and the process continues until all the chlorine and the methane have been used up. This type of process is known as a chain reaction and it is very fast. The overall chemical equation for this process is ... 

Because we cannot control the chlorine free radicals produced in this reaction, we also obtain small amounts of other substituted products - CH2C12 (dichloromethane), CHC13 (trichloromethane or chloroform) and CC14 (tetrachloromethane) - by further reactions such as those shown below. 

Under certain conditions, benzene can react with halogens by addition rather than by substitution. In the presence of sunlight, a free-radicaL reaction takes place with chlorine that leads to addition products in which the aromatic character has been lost. The final product is hexa-chlorocyclohexane (benzene hexachloride), which can exist in eight possible stereoisomeric forms. The process starts with the photolytic dissociation of chlorine. Free-radical addition to the 7i-electron system of the aromatic ring follows and a chain reaction ensues (Scheme 9.1). 

Given that the bottleneck establishing the rate of chlorine-induced ozone destruction is, to first order, the concentration of the rate limiting chlorine free radical in the dominant catalytic cycle destroying odd oxygen, it is essential to establish the propensity of the stratosphere for partitioning total chlorine into the rate limiting radical form. This ratio of CIO to total chlorine as a function of altitude is a quantity of first order importance. 

The mechanism of carbon tetrachloride nephrotoxicity involves the initial homolytic cleavage of carbon tetrachloride by cytochrome P450 to form the trichloromethyl and chlorine free radicals (Figure 6). The trichloromethyl free radical can then alkylate renal macromolecules or interact with membrane unsaturated fatty acids to initiate lipid peroxidation. The trichloromethyl free radical may also combine with molecular oxygen to form a peroxy free radical that is more reactive than the trichloromethyl free... 

Reaction with atomic oxygen produces another chlorine free radical ... 

CFCs have been phased out in recent years and replaced by hydrogen-containing hydrochlorofluorocarbons (HCFCs), such as CHCI2CF3 these compounds are more likely to be destroyed by atmospheric reactions before they reach the ozone layer. Also HFCs (hydrofluorocarbons) cannot produce chlorine free radicals - for this reason CH2FCF3 is now used as a domestic refrigerant. 

CI2. The free radical mechanism was clarified by Nernst It is initiated by chlorine free radical (Cl). Each cycle of the mechanism starts with one Cl and regenerates one Cl. The chain-propagating steps for the reaction of hydrogen and oxygen include the following ... 

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