An in situ NMR study of tert-butanol dehydration in 95 and 85% sulfuri
c acid did not indicate the formation of tert-butyl cations as active
reaction intermediates. Instead, only NMR lines from oxonium ions unde
r the condition of fast exchange with water and the acid were observed
. It is most likely that, in the concentrated acid, the active interme
diates of this reaction are represented by tert-butyl sulfuric ester w
hich probably is a precursor of invisible tert-butyl carbenium ions re
presenting short-lived excited or transition states.