Chlorination free-radical substitution

As has been noted earlier, the solvent usually has little effect on free-radical substitutions in contrast to ionic ones indeed, reactions in solution are often quite similar in character to those in the gas phase, where there is no solvent at all. However, in certain cases the solvent can make an appreciable difference. Chlorination of 2,3-dimethylbutane in aliphatic solvents gave 60% (CH3)2CHCH-(CH3)CH2C1 and 40% (CH3)2CHCC1(CH3)2, while in aromatic solvents the ratio became 10 90. This result is attributed to complex formation between the... 

This is a free radical substitution reaction. Because chlorine atoms have 7 outer shell electrons, each will possess an unpaired electron. So 2 chlorine radicals are produced. A radical is a species that has a single unpaired electron. 

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. 

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. 

This order of reactivity was observ for add dedeuteration, but for acetylation, formylation, and chlorination it was slightly different thieno[3,2-h]thiophene (2) > thieno[2,3-h]thiophene (1) > thiophene thieno[3,4-6]thiophene (3) was not studied. A substantially greater discrepancy between theoretical and experimental data was observed for nucleophilic substitution from the data on base dedeuteration and competitive metalation reactions/ the order of decreasing reactivity was as follows thieno[2,3-h]thiophene (1) > thieno[3,2-h]thiophene (2) > thiophene. To a certain extent this may be explained by differences in the mechanism of metalation and deuterium exchange with a base. A discrepancy between calculation and experiment was also found for free-radical substitution. ... 

As noted previously, free radical substitution of hydrogen by chlorine received early study, and the basic mechanisms of this process were delin-... 

Chlorination of Alkanes. The most direct and economical method for the manufacture of chloromethanes is the thermal free-radical chlorination of methane.176 177 Whereas in the 1940s and 1950s photochlorination was practiced in some plants, thermal chlorination is the principal industrial process today. The product chloromethanes are important solvents and intermediates. Commercial operations perform thermal chlorination at about 400-450°C. Vapor-phase photochemical chlorination of methane may be accomplished at 50-60°C. Fast and effective removal of heat associated with thermally induced free-radical substitution is a crucial point. Inadequate heat control may lead to explosion attributed to the uncontrollable pyrolysis liberating free carbon and much heat ... 

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. 

Such normal free-radical substitution processes might be geometrically directed to otherwise unremarkable positions in substrates, as the benzophenone reactions had been. Consequently, in our first example, we found that intramolecular chlorination could be directed by attachment of a PhICl2 group to the steroid [156], i.e., compound 82 directed chlorination exclusively to C-14 (83), while 84 directed halogenation exclusively to C-9 (85). As shown in Scheme 1.1, an intramolecular hydrogen abstraction by... 

At 400°C and without a catalyst, chlorine reacts in its atomic form and causes a free-radical substitution. Of all the bonds to carbons of the benzene ring, the bond between carbon and nitrogen is the weakest (292 kJ, 70 kcal/mol) compared to carbon-hydrogen bond (415 kJ, 99 kcal/mol) and carbon-fluorine bond (443 kJ, 106 kcal/mol). Therefore, atomic chlorine cleaves the bond between carbon and nitrogen and replaces the nitro group. The product is l-chloro-3,4-difluorobenzene (compound Q) [36. ... 

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). 

Electrophilic addition of chlorine to double bonds was accompanied by free-radical substitution. This led to over-chlorinated by-products. However, several of... 

Alkenes can undergo free radical substitution reactions with halogens. Which of the following best represents a chain propagation step during the free radical chlorination of methane ... 

Chlorinated Polyethylene (CPE). The first patent on the chlorination of PE was awarded to ICl in 1938. CPE is polymerized by substituting select hydrogen atoms on the backbone of either HDPE or LDPE with chlorine. Chlorination can occur in the gaseous phase, in solution, or as an emulsion. In the solution phase, chlorination is random, while the emulsion process can result in uneven chlorination due to the crystalline regions. The chlorination process generally occurs by a free-radical mechanism, shown in Fig. 2.25, where the chlorine free radical is catalyzed by ultraviolet light or initiators. 

The procedures described in Sections 9.2 and 9.3 illustrate methods for transforming alkanes to alkyl chlorides and bromides by free-radical substitution reactions. These experiments give you an opportunity to analyze the mixtures of products that are obtained and to gain insights about the relative reactivities of different types of hydrogen atoms toward chlorine and bromine radicals. 

Substitution involves the replacement of one Ugand (or substituent) on carbon by another. The replacement of hydrogen in methane (CH4) by chlorine in a process (descriptively) called a free radical substitution is shown in Equation II-N. 

A mixture of propylene and chlorine (4 1 molar) is heated at about 500°C and 2 atmospheres. Under these conditions a free radical substitution reaction occurs rather than addition at the double bond and allyl chloride is the main product. The allyl chloride may be converted to glycerol in one of two ways, namely ... 

Figure 10.68 The reaction mechanism for the free-radical substitution between methane and chlorine in ultraviolet light... 

Carbon tetrachloride may also be prepared by the free radical substitution of the hydrogen atoms of methane by chlorine. 

Substitution reactions involve the replacement of one atom, or a group of atoms, by another. For example, the free-radical substitution of alkanes by chlorine in sunlight... 

This reaction is not really suitable for preparing specific halogenoalkanes, because we get a mixture of substitution products. In the reaction between methane and chlorine, the products can include dichloromethane, trichloromethane and tetrachloromethane as well as chloromethane. These other products result from propagation steps in which a chlorine free radical attacks a halogenoalkane already formed. For example ... 

Notice that the methyl side-chain is not affected under the conditions used in the reaction above. However, we learnt on page 206 that chlorine will react with alkanes in the presence of ultraviolet (UV) light or strong sunlight. This is a free radical substitution reaction. So if the chlorine gas is passed into boiling methylbenzene, in the presence of UV light, the following reaction takes place ... 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

---