Reactions aryl halides, kinetics

These reactions follow first-order kinetics and proceed with racemisalion if the reaction site is an optically active centre. For alkyl halides nucleophilic substitution proceeds easily primary halides favour Sn2 mechanisms and tertiary halides favour S 1 mechanisms. Aryl halides undergo nucleophilic substitution with difficulty and sometimes involve aryne intermediates. 

Kinetic studies of these reactions reveal that they follow a second order rate law Rate = [Aryl halide] [Nucleophile]... 

Ketones, coupling reactions of aryl halides with, 487-489 k-factor, 244 Kinetic studies, 73 Kumada coupling, 467, 489, 491... 

The reactions involving either benzophenone hydrazone or w-hexylamine have been studied by reaction calorimetry. The benzophenone hydrazone reaction presents zero order kinetics, while the hexylamine reaction is first order in the aryl halide and zero order in the amine. Under synthetically relevant conditions, at 90°C, the rate of the hexylamine reaction is about 30-fold higher than the rate of the benzophenone reaction. 

The heat profile shows that the reaction has zero order kinetics at first, and then switches to positive order kinetics. The benzophenone hydrazone reacts first only when it is completely consumed, the reaction involving hexylamine begins. Samples were taken and analyzed by and NMR. One sample was taken when the aryl halide conversion was low, at about 5%, and the profile was overall zero order the second when the profile had switched to positive order and the conversion of the halide was greater than 50%. 

We studied the competitive amination of two amines (benzophenone hydrazone and -hexylamine) and one aryl halide (3-bromobenzotrifluoride), catalyzed by Pd(BlNAP). We showed that, when reacting alone at the same conditions, n-hexylamine is considerably more reactive and shows positive order kinetics benzophenone hydrazone shows zero order kinetics and forms a very stable intermediate, the BlNAP(Pd)Ar(amine) we also observed by NMR. During the competitive reaction of the two amines, the benzophenone hydrazone reacts first and only when it is completely consumed, the hexylamine starts to react. In this case it is the stability of the major intermediate, and not the relative reactivity, which dictates the selectivity. 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutations isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phenylnitrenes, Kinetics and spectroscopy of substituted, 36, 255 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25 Photochemistry, of aryl halides and related compounds, 20, 191 Photochemistry, of carbonium ions, 9, 129... 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

At this point, it can be concluded that the direct and indirect electrochemical approach of the reaction in the case of aryl halides has provided a quantitative kinetic demonstration of the mechanism and the establishment of the nature of the side-reactions (termination steps in the chain process). In poor H-atom donor solvents, the latter involve electron-transfer reduction of the aryl radical. 

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