The C12 materials thus made are quite unusual. They are relatively high boiling, and
thus hardly qualify as favored gasoline fractions, but the hydrogenated C12’s have
RONs in the 95–97 range, which is indicative of a highly branched structure. Many
different isomers are made and a precise characterization of the product is difficult.
The operation can be conducted in the “dimer” mode with n-butenes only (1-butene
or 2-butene) reacting among themselves, but the end product is not as attractive as
the “codimer” that is obtained by reacting isobutene with n-butenes, or essentially
by reacting isobutene with 2-butene since 1-butene rapidly isomerizes to 2-butene
as the reaction proceeds. Figure 9.2.4 plots the octane number of the hydrogenated
dimer product (in this case after only 90% hydrogenation), versus the weight ratio
of isobutene reacted relative to the total weight of butenes reacted. The reciprocal
of this value is sometimes called the “reaction ratio.” It was soon learned during
World War II that a low reaction ratio is desirable in order to obtain a high octane
product.