Chloride Ion Catalysed Rearrangement, and Isotope Exchange of the Isomeric Methylallyl Chlorides in Acetonitrile

Abstract

The bromide ion catalysed rearrangement reaction of α -methylallyl bromide in acetone was studied by England and Hughes and assigned an SN2' mechanism, and this was the only isomeric rearrangement reaction proceeding by this mechanism which had been thoroughly investigated. 79,120 The kinetics and mechanism of the chloride ion catalysed isomeric rearrangement reactions of α-and γ-methylallyl chloride in acetonitrile have now been investigated and an SN2’ mechanism has been assigned to these reactions, various other possible mechanisms having been excluded. The kinetics and mechanics of the isotope exchange reactions of the two isomers in acetonitrile have also bean studied and these reactions have been assigned en SN2 mechanism. The isotope exchange reactions of both the methylallyl bromides and the methylallyl chlorides in acetone were studied by England and Hughes, the reaction medium being unfavourable for a study of the SN2' reactions of the latter substrates. A comparison of the kinetic data for the rearrangement end Isotope exchange reactions of the methylallyl chlorides in acetonitrile with that for the corresponding reactions of the methylallyl bromides in acetone la made. The two systems are similar in behaviour as may be seen from the data tabulated below. Substrate Mechanism K2(25˚C) ∆H˚ ∆S˚ (M-1 sec.-1) (kcal.mole-1) (cal.deg.-1 mole-1) α-methylallyl chloride SN2 2.87X10-6 20.8 -13.9 α-methylallyl chloride SN2’ 1.33X10-8 24.2 -13.4 γ-methylallyl chloride SN2 3.15X10-4 18.8 -11.6 γ-methylallyl chloride SN2’ 5.31X10-9 24.3 -14.9 α-methylallyl bromide SN2 8.79X10-4 15.9 -19.1 α-methylallyl bromide SN2’ 1.49X10-5 18.8 -17.7 γ-methylallyl bromide SN2 1.41X10-1 14.1 -15.0 α-methylallyl chloride SN2 2.30X10-7 19.9 -22.1 γ-methylallyl chloride SN2 2.88X10-5 15.8 -26.2 A comparison of the kinetic data for the isotope exchange reactions of the methylallyl chlorides in acetonitrile and acetone is also made. England and Hughes' data for the latter solvent is shown above. Relative rates of nucleophilic attack at saturated carbon, π-bonded carbon and hydrogen are discussed for these substrates, a minor concurrent elimination reaction accompanying the SN2’ reaction of α-methylallyl chloride in acetonitrile providing information on the latter. The nucleophilic reactivity of the halide ions is much greater for attack at a saturated carbon atom than for attack at a π-bonded carton atom, while toward hydrogen these ions are very weak nucleophiles as shown by the rate ratios given below for the chloride ion catalysed reactions of α-methylallyl chloride in acetonitrile. In the absence of chloride ions a slow surface-catalysed elimination reaction takes place which promotes hydrogen chloride catalysed rearrangement. The addition of 0.05M Et4 NCl prevents the latter reaction from occurring and gives a rata ratio of 5.5 for the chloride ion catalysed and surface-catalysed elimination reactions. By comparison with other dipolar aprotic solvents, acetonitrile appears to be particularly suitable for slow reactions requiring prolonged reaction times at elevated temperatures. Acetone, as mentioned above, and both dimethyl sulphoxide and dimethylformamide were found to be unsuitable for SN2’ studies, the latter two being: Investigated during the search for a suitable solvent in which to study the reactions of the methylallyl chlorides.