The continuously rotated cellular reactor (CRCR): A high dilution reactor for allylic bromination
Abstract
The Continuously Rotated Cellular Reactor (CRCR)$\sp1$ has been developed as a high dilution reactor for the allylic bromination of cyclohexene. The CRCR, based on Taylor vortex flow,$\sp2$ allows for spatial and temporal control of reactions. Reactants, injected into opposite ends of the column, are spatially separated with reaction occurring along a well defined reaction boundary near the center of the reactor. A maximum in the time dependent bromine concentration profile is observed during the reaction. The concentration maximum increases with increasing rotation frequency of the inner cylinder and occurs when the rate of increase in concentration of bromine due to vortex driven diffusion equals the rate of loss by reaction. A linear dependence of the relative rates of addition and substitution versus the maximum bromine concentration is found. This is in accord with theory which predicts a linear relationship between the rate ratio and the bromine concentration at high dilution ($<$10$\sp{-4}$ M). The kinetic ratio of addition to substitution, determined from the slope, gave a value of 1.88 $\times$ 10$\sp2$ for the bromination of cyclohexene in the presence of the HBr scavenger propylene oxide. At 1600 rpm a reaction front was not observed as the rate of transport exceeded the reaction rate. Fitting of the concentration profile gave the kinetic ratio as 1.65 $\times$ 10$\sp3$. High dilution experiments showed bromine atom selectivity for hydrogen removal of cyclohexene, 3-bromocyclohexene and 3,6-dibromocyclohexene as 1.0:0.45:$<$ 0.05. The bromine addition selectivity of cyclohexene, 3-bromocyclohexene and 3,6-dibromocyclohexene was 1.0:0.18:$<$ 0.01. In the absence of a scavenger, reaction rates were more than tripled. The rate was three times faster than expected based on diffusion of the reactants to the reaction interface. A proliferation in products was also observed. In the presence of a scavenger only trans-1,2-dibromocyclohexane, 3-bromocyclohexene and 3,6-dibromocyclohexene were isolated. Additional products--bromocyclohexane, 1r,2t,3c,4t-tetrabromocyclohexane and 1,3,4-tribromocyclohexene--were isolated in the absence of a scavenger. The presence of bromocyclohexane implies a rapid radical chain reaction involving HBr.
Degree
Ph.D.
Advisors
Grutzner, Purdue University.
Subject Area
Organic chemistry
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