Haloalkanes unimolecular elimination

Further Reactions of Haloalkanes Unimolecular Substitution and Pathways of Elimination... 

Chapter 7 FURTHER REACTIONS OF HALOALKANES UNIMOLECULAR SUBSTITUTION AND PATHWAYS OF ELIMINATION... 

The first step is the rate-determining one for this reaction, and we will follow the velocity of this reaction. In this step, the haloalkane ionises to produce an intermediate species, in this case a carbocation. Subsequently, the carbocation can be attacked by water to give either 2-methylpropanol or 2-methylpropene. The overall reaction is known as unimolecu-lar nucleophilic substitution, S l, when the nucleophilic attack predominates. Alternatively, if after initially forming the same carbocation elimination of a proton occurs to give 2-methylpropene, the process is termed unimolecular elimination, El. At room temperature, the predominant reaction is S l, but the rate-determining step is the same for both reactions, and both lead to formation of the same amount of HCI. It is not uncommon in chemical kinetics for different pathways to occur via the same intermediate. As the only ionic species formed in either case is HCI, the progress of the reaction can be followed by the increase of the conductance of the solution. If Aq is the conductance of the solvent. A, the conductance of the solution at time t and A the final conductance of the solution (r = t ), then... 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

At one extreme, breaking of the C—Lv bond to give a carbocation is complete before any reaction occurs with the base to lose a hydrogen and form the carbon-carbon double bond. This mechanism is designated an El reaction, where E stands for Elimination and 1 stands for unimolecular. One species (in this case, the haloalkane) is involved in the rate-determining step. The mechanism of an El reaction is illustrated here by the reaction of 2-bromo-2-methylpropane to form 2-methylpropene. 

Unimolecular HX elimination from haloalkanes can occur via either three- or four-centered processes. " The threshold energy for the four-centered process is usually smaller than for the three-centered process and in competitive situations the four-centered pathway is favored unless substituents lower the threshold for the three-centered pathway. A five-centered elimination process was inferred from the HF emission observed in the flash photolysis of 3,3,3-trifluoropropyne however, such processes are not well characterized. Activation of most of the molecules listed in Table 2.17 was accomplished by radical recombination the chloroolefins were activated by photon absorption and excited electronic states may contribute to the elimination of HCl. The energy available is the average excitation energy of the activated molecule less the endoergicity for HX elimination. The fluoroalkane molecules have 50-60 kcal mole more energy... 

As is true for substitution reactions, elimination reactions of haloalkanes can occur by different mechanisms. If the rate-determining step is unimolecular, then the mechanism is called El. If the rate-determining step is bimolecular, the mechanism is called E2. In this section, we will explore these mechanisms in... 

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