Laser flash photolysis of the hydrogen abstraction reaction of triplet
benzophenone ((BP)-B-3) from 2-propanol and 1,4-cyclohexadiene in sup
ercritical ethane and fluoroform was investigated. Bimolecular rate co
nstants based on bulk concentrations decrease with an increase in pres
sure along both isotherms studied. These results corroborate our previ
ous studies in CO2 that show increased reaction rates due to enhanced
local compositions of cosolvent around the (BP)-B-3 solute. Analysis o
f the results includes prediction of the thermodynamic pressure effect
on the rate constant, which suggests an increase in the rate constant
with pressure, as well as the effects of increased local cosolvent co
ncentrations about (BP)-B-3. Spectroscopic measurements of the local c
omposition of 2-propanol about a solute in supercritical CO2 are used
to explain the apparent discrepancy between experiment and prediction,
providing reasonable evidence that the local environment can influenc
e kinetically controlled reactions in supercritical fluids.