Reactivity Differences of Octenes over Solid Phosphoric Acid

Abstract

Reactivity differences between 1-octene and 2,4,4-trimethylpentene have been studied over solid phosphoric acid in the temperature range 75−220 °C. No 1-octene oligomerization occurred unless it was preceded by skeletal isomerization, while 2,4,4-trimethylpentene readily oligomerized even at 100 °C. The stability of the carbocation intermediate could explain the reactivity difference between linear and branched olefins, but not why 1-octene was almost impervious to oligomerization. This could be explained in terms of the stability of the phosphoric acid ester and the time period it is able to polarize the olefin, which implied that solid phosphoric acid followed a nonclassic carbocation mechanism. This could also account for the limited carbon number distribution of the product and the cis-favoring double-bond isomerization that was observed. Comparative classic carbocation data was obtained in the temperature range 75−125 °C over Amberlyst 15 to illustrate the difference.