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How does isomerization convert straight-chain butane into branched isobutane?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

Isomerization is the chemical process that converts a compound into an isomeric form—a molecule with the same chemical composition but a different structure. In the case of butane, which consists of four carbon atoms in a straight chain, the conversion to its branched-chain isomer, isobutane, is achieved by heating the butane to 100 °C or higher in the presence of a catalyst.1

Industrially, this process is often performed using a catalyst of aluminum chloride supported on alumina and promoted by hydrogen chloride gas. The reaction is typically carried out in the presence of hydrogen, which is used to inhibit undesirable side reactions.23

This transformation is commercially significant because branched-chain hydrocarbons are more effective as motor fuels than their straight-chain counterparts. The resulting isobutane serves as a vital starting material in the production of 2,2,4-trimethylpentane (isooctane), a key component in high-octane gasoline.13