T-Butyl halide

Kennedy and co-workers10 studied the kinetics of the reaction between Me3Al and t-butyl halides using methyl halide solvents as a model for initiation and termination in cationic polymerization. Neopentane was generated rapidly, without side reactions and rates were determined by NMR spectroscopy. The major conclusions were ... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

An analogous mechanism is capable of explaining similar reactions, for example (90), which occur in systems involving protonated esters and t-butyl halides (Riveros et al., 1979). 

Structural modifications of the reactive intermediates also alter selectivity. The alkylating agent in the isopropylation of toluene, approximating the i-propyl cation, yields 28.5% ro-i-propyltoluene (Condon, 1948, 1949). The reaction of toluene with t-butyl halides under Friedel-Crafts conditions results in the formation of only 7% ro-t-butyltoluene (Schlatter and Clark, 1953). More precisely, the parajmeta ratio is greater for the more selective tertiary ion than for the more reactive secondary species. These results are in agreement with the expectation of a depressed reactivity for the t-butyl cation as compared to the less stable i-propyl cation. 

The ionic product can either rapidly collapse into the t-butyl halide by reaction with the anion (termination) or propagate with the monomer. When X = F, the anion is stable and the sustained ionisation favours polymerisation. When X = Cl, the anion is bulky and tends strongly to release a chloride ion to the carbenium ion, unless its stability is enhanced by a hi er polarity of the medium. When X = Br, the instability of the anion is so marked, that formation of BBraOH and Br is practically instantaneous and the carbenium ion pairs collapses without any chance to propagate, even in a polar surrounding. 

Kennedy and coworkers (7) further studied the effect of solvent (MeCl, MeBr, Mel and cyclopentane) and the nature of the halogen (Cl, Br or I) in the t-butyl halide on the rate of neopentane formation in the reaction between f-butyl halide and MejAl. The findings of this work are 1. The rate of neopentane formation with different t-butyl halides follows the order t-BuCl > t-BuBr > t-BuI. 2. The nature of the solvent exerts a significant influence on the rate of alkylation, i.e., rate decreases in the order MeCl > MeBr > Mel > cyclopentane. 3. The rate of neopentane formation is first order in [t-BuX] and in [MejAl]. 4. The activation energies in the range — 20° to 80° are 11 kcal/mole for all the methyl halide solvents and 16 kcal/mole for cyclopentane. 

Trimethylaluminum exists as a dimer in soli4 liquid and vapor phase and in nonpolar solvents. However, the active species is the monomeric form. The methyl halide solvent helps to break up the methyl bridge of the MejAl dimer enabling the t-butyl halide to interact with the alkylaluminum to generate the initiating t-Bu ion (ii). Kinetic analysis... 

Isobutylene polymerizations were carried out by charging isobutylene, methyl chloride, and the alkylaluminum compound in methyl chloride solution and adding the t-butyl halide initiator in methyl chloride rapidly. Polymerizations ensued immediately and were over in 5-10 min. The reactions were terminated after 15 min by the addition of prechilled methanol. The polymers were dried in vacuum at 40° to constant weight and were characterized by number average and viscosity average molecular weights. All reactions were carried out in duplicate. 

The nature of the counter anion is determined by the alkylalumi-num coinitiator and by the halogen in the t-butyl halide initiator. 

These conclusions derived from model experiments lead to the following predictions for isobutylene polymerizations induced by alkylaluminum/t-butyl halide systems. 

A decrease in temperature telow —70° reduces polyisobutylene molecular weight produced by the Me2AlCl/t-BuX (X = Cl or Br) system. It is possible that below — 70° the rate of initiation is diminished to such an extent that the unused t-butyl halide starts to function as a chain transfer agent and thus reduces the moleoilar weights. 

Similar observations have been reported by Saveant et al. in their studies on the electrochemical reduction of simple aliphatic halides [132g] (/ -.. s-, and t-butyl halides) at a glassy carbon electrode. Cyclic voltammetry of the butyl halides showed one or two irre crsible waves, depending on the relative reducibility of the alkyl halide RX and of the radical R. All transfer coefficients reported were smaller than 0.5 (between 0.2 and 0.32). The fact that the transfer coefficient was small was taken as further evidence that the reduction pathways do not involve the RX anion radical as an intermediate. Our observations from the electrochemical reduction of 18, 158, 19, and 131 at a glassy carbon electrode are in agreement with this. The cyclic voltammetric shape, the peak width tp 2 — p = (180-150) mV, and the value of a =0.25-0.32 at different scan rates [137] showed, without question, that electron transfer and decomposition of the anion radical... 

Substitution reactions of t-butyl halides, you will recall from Chapter 17, invariably follow the S l mechanism. In other words, the rate-determining step of their substitution reactions is unimolecu-lar—it involves only the alkyl halide. And this means that, no matter what the nucleophile is, the reaction goes at the same rate. You can t speed this S l reaction up, for example, by using hydroxide instead of water, or even by increasing the concentration of hydroxide. You d be wasting your time, we said (p. 000). 

Efficiency of different t-butyl halides Inn the polymerization of THF t-butyl chloride ( ) t-butyl bromide... 

Organic compounds with covalent bonds to electronegative elements may dissociate to form carbocations, especially if the cation is stabilized by the inductive effect, as in the f-butyl cation, or by resonance, as in the cumyl (2-phenylprop-2-yl) cation. This can happen slowly in polar solvents such as water. The SN1 reaction of t-butyl halides is an example (reaction 5.6). The slow and rate-determining heterolysis of the halide is followed by a rapid reaction of the f-butyl cation with water to give the alcohol product. -Cl HiO... 

A sterically hindered carbon center, such as the tertiary carbon in a t-butyl halide, is generally not conducive to an S 2 displacement. 

Explain why f-butyl alcohol reacts at equal rates with HCl, HBr, and HI (to form, in each case, the corresponding t-butyl halide). 

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