The rate of the Diels-Alder reaction between
N-ethylmaleimide and 9-hydroxymethylanthracenein su
percritical carbon dioxide (scCO
2) was determined by following the disa
ppearance of 9-hydroxymethylanthracene with in situ UV/vis absor
ption s
pectrosco
py. The reaction conditions were 45-75
Cand 90-190 bar, which corres
pond to fluid densities (based on
pure carbon dioxide) ranging betweena
pproximately 340 and 730 kg m
-3. The measured reaction rate at low scCO
2 fluid densities was nearly25× faster than that re
ported in acetonitrile at the same tem
perature (45
C). An inverse relationshi
p betweenreaction rate and fluid density/
pressure was observed at all tem
peratures in scCO
2. The a
pparent activationvolumes were large and
positive (350 cm
3 mol
-1) and only a weak function of reduced tem
perature. Asolvo
phobic mechanism analogous to those observed in conventional solvents is
postulated to describe(a) the rate acceleration observed for this reaction in scCO
2 relative to that in acetonitrile, (b) the observedrelationshi
p between reaction rate and
pressure/tem
perature/density, and (c) the large,
positive activationvolumes. Solubility measurements in scCO
2, rate measurements in conventional solvents, and an em
piricalcorrelation are used to su
pport this theory. Our results advance the general understanding of reactivity insu
percritical fluids and
provide a rationale for selecting reactions which can be accelerated when conductedin scCO
2.