Rate determining step radical halogenation

We know that bromine is less reactive than chlorine in the rate-determining step of halogenation of methane. We also recall that bromine is more selective than chlorine. Both the rate of reaction and the selectivity of free radical halogenation are related to the first propagation step, so let s look at the relationship between reactivity and selectivity in terms of the structure of the transition states for chlorination and bromination. 

Kosower s radical (66) reacts with alkyl halides by a mechanism which transfers a halogen atom to the pyridinyl radical in the rate determining step (Py is 66) ... 

To explain the difference between chlorination and bromination, we return to the Hammond postulate (Section 7.15) to estimate the relative energy of the transition states of the rate-determining steps of these reactions. The rate-determining step is the abstraction of a hydrogen atom by the halogen radical, so we must compare these steps for bromination and chlorination. Keep in mind ... 

Conclusion Because the rate-determining step in bromination is endothermic, the more stable radical is formed faster, and often a single radical halogenation product predominates. 

Binuclear oxidative additions, because they involve le rather than 2e changes at the metals, often go via radicals. One of the best known examples is shown in Eq. 6.21. The rate determining step is abstraction of a halogen atom from RX by the d Co(II) the resulting R combines with a second Co(II) center ... 

The procedure can be extended to achieve selective a-bromination and iodination of carboxylic acids and the general mechanism of the a-halogenation is outlined in Chapter 5, p 170. The autocatalytic effects in the selective a-chlorination of propionic acid to the 2-chloro and 2,2-dichloro acids have been studied in a semibatch reaction at 90-130 °C. The reaction was performed in the presence of chlorosulfonic acid and dichloroacetic acid as catalysts and oxygen as the radical scavenger. Kinetic experiments indicated autocatalytic formation of both products and that the selectivity was independent of the chlorine concentration in the liquid phase. The results confirmed the validity of the proposed reaction scheme which involved formation of the reaction intermediate, propanoyl chloride from propionic acid and chlorosulfonic acid, the acid-catalysed enolization of the acid, and a hydroxyl-chlorine exchange reaction. The acid-catalysed enolization was the rate-determining step in the reaction sequence. ... 

Halogen abstraction by the stable free radical 1 -ethyl-4-(methoxycarbonyl)pyridinyl (Py ) proceeds by the mechanism shown in Eq. (5-66) [214, 570]. The first step, which is rate-determining, is a transfer of the halogen atom to the pyridinyl radical. 

Group VIA -Cr, Mo, W. The stable, water-soluble metal alkyl [(H20)5-Cr( y -<7-CH2C6H5)] is obtained by one-electron oxidative additions to [Cr-(H20)J with reactive alkyl halides. The rate-determining, halogen-transfer step generates an alkyl free radical, which then rapidly reacts with a second molecule of Cr(ll) ion or undergoes radical coupling and reduction to alkane ... 

Similarly the metal-cyclopentadiene bonding is directly involved in the reaction of cobaltocene with benzyl, allyl, or propargyl halides to give the respective compounds (45). These reactions go by a two-step radical mechanism. The conversion of (45), with R = CHaX and X = Q, Br, or I, to the salt (46) follows a first-order rate law rate constants and activation parameters have been determined. The high rates observed are attributed to assistance by the cyclopentadienylcobalt of the heterolysis of the carbon-halogen bond. A plot of logarithms of rate constants... 

We know that the enthalpy change for the first propagation step in halogenation is always less exothermic than the enthalpy change for the second step. This step is rate-determining. The second step, in which the methyl radical reacts with the halogen, is quite exothermic and has a very low activation energy (less than 1 kj mole ). 

It says that the substitution product R—X is produced at a rate that is determined by two constants and two concentration terms. For given initial concentrations of the substrate R—H and the halogen and for a given reaction temperature, the rate of formation of the substitution product is directly proportional to the rate constant ky k being the rate constant of the propagation step in which the radical R is produced from the hydrocarbon R—H. 

In ATRP, perhaps the most important kinetic parameters are the rate constants for the activation ( act) and deactivation ( deact) steps (see Fig. 11.16), which determine the magnitude of the equihbrium constant (Xeq = act / deact)- In the absence of any side reactions other than radical termination by coupUng or disproportionation, K q determines the polymerization rate. While ATRP does not occur or occurs very slowly if Xeq is too small, too large an equilibrium constant, as it has been shown earlier, may actually lead to an apparently slower polymerization. The magnitudes of act and deact depend on the structure of the monomer, on the halogens, and on the transition metal complexes. The measured values of these rate constants for some model systems resembling the structure of the dormant/active species, are shown in Tables 11.3 and 11.4. 

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