Supercritical Carbon Dioxide as a Medium for Conducting Free

Chapter 8 Supercritical Carbon Dioxide as a Medium for Conducting Free-Radical Reactions

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 18, 2016 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0577.ch008

James M . Tanko, Joseph F. Blackert, and Mitra Sadeghipour Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212

Supercritical carbon dioxide (SC-CO ) is found to be an excellent solvent for free radical brominations. Reaction yields, times, and selectivities are analogous to what is observed in conventional organic solvents (CCI , CFC's, and benzene). SC-CO thus appears to be a suitable, "environmentally-benign" alternative solvent for free radical reactions. 2

4

2

There is an increased awareness of the need to reduce or eliminate toxic chemical waste and/or by-products which arise in the course of chemical synthesis and manufacture (1-3). A fundamental change in the philosophy of synthetic design is needed wherein the priority is placed upon health and environmental impact rather than just the efficiency (%-yield) associated with a chemical transformation. Certainly one aspect of synthesis in which dramatic advances in pollution prevention can be realized entails replacing many of the toxic, environmentally-threatening solvents utilized in most chemical processes with non-toxic, "environmentallybenign" alternatives. The challenge from the chemical perspective is to identify suitable alternatives. Our contribution to this effort has involved examining the potential use of supercritical carbon dioxide (SC-CO ) as a solvent for free radical reactions. In this paper, we report our results pertaining to the free radical bromination of alkylaromatics in SC-CO (4). Generally, solvents suitable for free radical reactions do not possess reactive functionalities (e.g., abstractable hydrogens and reactive double bonds). Unfortunately, many of the solvents which meet this criteria are either carcinogenic (e.g., benzene) or damaging to the environment. Chlorofluorocarbons and carbon tetrachloride illustrate this latter problem as these materials are believed to be major culprits in the depletion of the earth's ozone layer (5). Ozone in the earth's atmosphere shields the surface from damaging ultraviolet irradiation in the 280 - 320 nm region (equation 1). Nature maintains a delicate balance between this natural process which depletes ozone, and other natural 2

2

0097-6156/94/0577-0098$08.00/0 © 1994 American Chemical Society

Anastas and Farris; Benign by Design ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

8.

TANKOETAL.

99

Supercritical Carbon Dioxide

processes which produce ozone. Freons and CCI4 are believed to destroy this delicate balance. Chlorine atoms generated from these compounds (equation 2) introduce an unnatural mechanism for ozone depletion (5). O3

CC1 F


2

2

+ hv

->

O2

+ hv

->

*CC1F

+ Ο 2

(1)

+ Cl*

(2)

At high altitudes (40 km), a process catalytic in CI* is believed to operate (Scheme 1), while at lower altitudes where high concentrations of atomic oxygen are not available, a slightly different mechanism is suggested (Scheme 2). This latter process is believed to be responsible for the seasonal hole in the ozone layer over Antarctica (5). Scheme 1 CI* + O3 -> CIO* + 0

CIO* + O2

->

O3 + Ο

CI* + O2

2 02 (overall)

Scheme 2 2 CI* + 2 0

-»

2 CIO* + 2 0

CIO*

->

C100C1

C100C1 + hv

->

Cl* + C100*

2

3

C100*

->

CI* +

0

2 O3 + hv

->

3 O2 (overall)

2

2

To solve this problem, in accordance with international agreement (Montreal Protocol, 1987; London Amendment, 1990) the production of CFC's and CCI4 is to be phased out in 1995. This "solution" of course generates a new problem: The chemical industry must find suitable "environmentally benign" alternative solvents for extractions, separations, processing and synthesis. SC-CO2 as a Reaction Solvent. The supercritical state is achieved when a substance is taken above its critical temperature and pressure. The bulk properties of a supercritical fluid are intermediate between those of a gas and a liquid. Because of the unique properties of supercritical fluids, analytical methods based upon their use


100

BENIGN BY DESIGN

are becoming increasingly popular on both a laboratory and industrial scale (chromatography, extractions, etc..) (6). In a similar vein, there are several potential advantages which may be realized with the use of SC-CO2 solvent for chemical reactions:


a s a

• Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure (7,8). This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. • The supercritical state of CO2 is relatively easily attained (T = 31 °C, P = 74 bar) (7). This feature of SC-CO2 implies that the "cost" (e.g., energy, apparatus, and materials) will not be prohibitive with the conversion to c

c

SC-C0 . 2

• Finally, CO2 is non-toxic and "environmentally benign." Another factor to be considered in the use of SC-CO2 as a reaction solvent deals with the possible effect of pressure on reaction rate (and selectivity) (9). For the hypothetical reaction A + Β —» C, the effect of pressure on reaction rate is expressed by equation 3, where k is the rate constant for the reaction and AV* (the volume of activation) is the difference in molar volume between the transition state and reactants (AV* = V - V - V ) (10). Reactions exhibiting negative volumes of activation imply a transition state which is smaller and more compact than the reactants (e.g., an associative process). Similarly, a positive volume of activation implies expansion as the reactants progress to the transition state (e.g., a dissociative process). t e

A

B

[δΐη k I δΡ] = - A V Ί

act

/ RT

(3)

The Diels-Alder reaction is characterized by substantially negative volumes of activation (ca. -20 -> - 50 cm /mol, depending on the substrates) (77), and has been extensively studied in SC-CO2 (12-16). At high CO2 pressures, the measured volume of activation is analogous to that observed in conventional solvents. However, at pressures just above the critical pressure of CO2, there is a surprisingly large variation of k with pressure (AV* is on the order of -500 cm /mol) (72-76). Similar variations of rate over a narrow range of pressures just above the critical pressure have also been noted for other reactions in supercritical fluid solvent (77). This dramatic variation of rate with pressure just above the critcal point is attributed to a phenomenon referred to as "clustering" in which solvent molecules are believed to aggregate (cluster) about the solute (12-17). 3

3

Free Radical Reactions in SC-C0 . Literature Precedent. The issue as to whether CO2 is inert to free radicals must be addressed. Since CO2 formally possesses a carbon-oxygen double bond, there exists a possibility of radical addition 2


8. TANKO ET AL.

101

Supercritical Carbon Dioxide

to CO2 (equation 4). In conventional solvents, the equilibrium depicted in equation 4 favors reactants. (Decarboxylation of carboxyl radicals is facile, e.g., the Kolbe electrolysis) (18). What effect the high pressure associated with the SC-CO2 medium might have on this equilibrium is unclear.


R- + CQ2

RCQ2-

(4)

Literature precedent suggests that SC-CO2 is inert to stabilized carboncentered radicals (e.g., benzyl). For example, McHugh reported the autooxidation of cumene in SC-CO2 via the free radical chain process outlined in Scheme 3 (79). This report is significant because it demonstrates for the first time that it is possible to conduct free radical chain reactions in SC-CO2. Scheme 3 CH3 H

+ 0

I CH

2

SC-CO2 • 110OC

j . / / ^ \ \

Supercritical Carbon Dioxide as a Medium for Conducting Free

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