HF‐SbF5 catalysed isomerization of 2‐methylpentane: Kinetics and mechanism of rearrangement and hydride‐ion transfer steps in alkylcarbonium ion reactions

Abstract

AbstractA kinetic investigation has been made of the isomerization of 2‐methylpentane with liquid HF‐SbF5 as catalyst at temperatures of from −20 to +20 °C. The isomerization proceeds in three distinct stages: first 3‐methylpentane is formed, then the equilibrium mixture of 2‐ and 3‐methylpentane isomerizes to equilibrium with 2,3‐dimethylbutane, and finally that mixture reacts to give n‐hexane and neohexane, respectively, until thermodynamic equilibrium is reached. Rate constants at 0 ° for these four reactions are 0.42, 0.030, 0.00015, and 0.0007 s−1 · (mole hexane) · (mole SbF5)−1, respectively. In the first reaction, mass transfer between acid and hydrocarbon phases is rate limiting; the rate‐determining step of the second reaction is the intramolecular rearrangement of the intermediate tertiary carbonium ions, and of the last two reactions the intermolecular hydride‐ion transfer between secondary and tertiary carbons. The rate constant for hydrideion transfer from a secondary carbon to a tertiary carbonium ion is about 0.06 1 · mole−1 · s−1 at 0 ° (EA = 14−15 kcal · mole−1), and for that between two tertiary carbons is greater than 10 1 · mole−1 · s−1. The rate of rearrangement of a tertiary ion to a secondary or tertiary ion with different degree of chain branching is about 0.1 s−1 at 0 ° (EA = 16−17 kcal · mole−1). A mechanism for that type of rearrangement via a (protonated) cyclopropane ring is consistent with the data. In particular, it explains why n‐butane does not isomerize to isobutane under conditions where n‐pentane and n‐hexane are rapidly converted.