The elimination of water from a conformationally complex alcohol: A computational study of the gas phase dehydration of n-butanol

Relative stabilities of the 14 conformers of n-butanol were calculated at the CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level. The three most stable structures found, TGt, TGg and TGg′ (trans (T or t) and gauche (G or g) with respect to the CC–CC, CC–CO and CC–OH dihedral angles, respectively) lie within 0.14 kcal/mol, ΔH(0 K), with the TGt being favoured thermodynamically (the ZPVE corrections were found at the MP2 level). The rotational isomerizations to the other conformers were examined, in particular those involved in the elimination of water. The elimination of H2O from n-butanol involving the formation of the corresponding carbene, 1-butene, methylcyclopropane and cyclobutane, 1,1-, 1,2-, 1,3- and 1,4-elimination, respectively, was systematically investigated. The 1,2-H2O loss occurring from the TGg′ conformer and involving a four-center transition state has been found to be thermodynamically and kinetically the most favoured route. This elimination which leads to the 1-butene olefin isomer is endothermic by 8.25 kcal/mol, ΔH(0 K), with an associated activation enthalpy at 0 K of 67.26 kcal/mol at the CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level (relative to the TGt conformer). The respective CBS-QB3 estimates are 7.84 and 67.88 kcal/mol.