DFT calculations of the alkylation reaction mechanisms of isobutane and 2-butene catalyzed by Brönsted acids

Density functional theory method was employed to determine the alkylation reaction course of 2-butene and isobutane. A carbonium ion mechanism is supported through this theoretical simulation. The first step is the formation of sec-C4H9+ from the protonation of 2-butene. The carbonium ion reacts with isobutane to form tert-butyl carbonium ion via hydride transfer. This carbonium ion reacts with 2-butene quickly to produce trimethylpentane carbonium ion (TMP+), which is finally converted to TMP at a high reaction rate via hydride transfer from isobutane. Their transition states are obtained by QST2 method and the transition states are verified by frequency analysis. The calculation results indicate that the energy barrier of each reaction is below zero, suggesting that each reaction step is fast because of the characteristics of carbonium ions. However, the additive reaction is considered to be the rate-limiting step.

Isobutane alkylation with 2-butene catalyzed by proton acids is simulated by DFT using Gaussian 03W programme. Carbonium ion mechanism is confirmed and two transition states of the hydride transfer reaction between isobutane and sec-butyl carbonium ion coming from 2-butene and the addition reaction between tert-butyl carbonium ion and 2-butene are displayed in the picture. Some kinetical parameters are calculated too. Figure optionsDownload as PowerPoint slide