Dissociation of chlorine

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

Cf. the heat of dissociation of chlorine, 57,000 calories, with the values of E in the table on page 96. 

At the present time a number of gaseous unimolecular reactions are known. The view that none exist, although it appeared plausible for a time, has now been definitely abandoned. Nevertheless, unimolecular reactions are rather exceptional and appear to be confined to molecules of rather complex structure. It is possible that the decomposition of diatomic molecules into atoms at high temperatures is unimolecular but more probable that it is bimolecular, the reverse reaction of recombination being termolecular. Thus the rate of dissociation of chlorine would be k1 [Cl2]2 while the rate of recombination of the atoms would be 2 [Cl]2 [Clg], according to the Herzfeld theory (p. 111). 

Reactions 1 and 2 involve dissociation of chlorine into chlorine atoms and the breaking of a C-H bond of methane to give a methyl radical and a hydrogen atom. The methyl radical, like chlorine and hydrogen atoms, has one electron not involved in bonding. Atoms and radicals usually are highly reactive, so... 

The chlorination of saturated hydrocarbons can be induced by light, but also can be carried out at temperatures of about 300° in the dark. Under such circumstances the mechanism is similar to that of light-induced chlorination, except that the chlorine atoms are formed by thermal dissociation of chlorine molecules. Solid carbon surfaces catalyze thermal chlorination, possibly by aiding in the cleavage of the chlorine molecules. 

The initiation step is dissociation of chlorine to two chlorine atoms. 

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). 

The method has been used to determine the thermal dissociation of chlorine and bromine. 3 The hot-wire manometer (Pirani gauge, 6.VII J) has also been used to measure small vapour pressures. ... 

A system which encloses, simultaneously, an aqueous solution of a gas, a mixture of this gas with water vapor, and a definite solid compound formed by the union of the gas and water is in equilibrium, at each temperature, when the pressure has a definite value the liquid mixture and the gaseous mixture has, at the same time, a definite composition the total mass of gas and the total mass of water contained in the system do not influence either this tension nor this composition this law was first recognized by Isambert in studying the dissociation of chlorine hydrate the curves of transformation tension of a great number... 

Dissociation of chlorine hydrate.— Analogous facts, which caught the attention of Isambert and Le Chatelier, are observed in the study of systems having for independent components water and chlorine at low temperatures one may observe such a system divided into three phases chlorine hydrate crystals, a liquid... 

The significance and use of standard affinities may be illustrated by considering the dissociation of chlorine molecules according to the equation... 

The cathodic dissolution of chlorine is limited by diffusion of chlorine in the melt to the electrode surface under high current conditions, whereas at low currents the process of dissociation of chlorine on the surface is followed by the charge transfer process [83]. 

Note that both reactants and products are gaseous.) The amount of energy required to break a particular bond in a gaseous molecule under standard conditions is called the (standard) bond dissociation enthaipy of that bond, symbolised AHJ-g. For example, the standard enthalpy change for the dissociation of chlorine molecules at 298 K,... 

The first stage, chain initiation, is the dissociation of chlorine molecules into atoms this is followed by two chain propagation reactions. Two molecules of hydrogen chloride are produced and the ejected chlorine atom is ready to react with more hydrogen. The final steps, chain termination, stop the reaction. 

It is found that the dissociation of chlorine and bromine is greatly accelerated by the presence of He, Ar and Xe Since the increase in the dissociation rate was associated with a decrease of E/p it cannot be explained by an increase of the average electron energy. Consequently, it has been proposed that the noble gas atoms in their metastable states behave as energy catalysts. Energy catalysis is fairly effective only when the metastable atom X gives up almost all its energy on collision with the molecule M2 ... 

Chain Initiation Chain initiation involves formation of radicals from NBS by light-induced homolytic cleavage of the N—Br bond in NBS. This step is analogous to the homolytic dissociation of chlorine to chlorine radicals. 

Dissociation of chlorine. Chlorine molecules are dissociated to atoms. The enthalpy change for this equals the Cl—Cl bond dissociation energy, which is 240 kJ per mole of bonds, or 120 kJ per mole of Cl atoms. 

The energy of activation of a gas-phase reaction where bonds are broken homolytically but no bonds are formed is equal to AH° An example of this type of reaction is the chain-initiating step in the chlorination of methane—the dissociation of chlorine molecules into chlorine atoms ... 

Sharpe RG, Dixon warren S, Durston PJ, Palmer RE (1995) The electronic cattilyst dissociation of chlorinated hydrocarbons by metal-insulator-metal electron emitters. Chem Phys Lett 234 354-358... 

---