Microwave Chemistry in Liquid Media

© G. Whittaker, 1994 & 2007. This work, or extracts from this work may be reproduced only with the written permission of the author.

 General Remarks on Microwave Syntheses in Liquid Media

 Any dipolar solvent with relatively low molecular weight will tend to display a capacity for heating under microwave irradiation, as illustrated in Table 1. Indeed, many solvents are not only heated, but display a capacity for superheating. This has been investigated by a number of authors,¹⁹ and a model proposed for the behaviour based on the mechanism of nucleate bubble formation which is required for boiling.²⁰

Solvent Temp.  / oC B.P.  /oC Solvent Temp.  /oC B.P.  /oC Water 81 100 Acetic acid 110 119 Methanol 65 65 Ethyl acetate 73 77 Ethanol 78 78 Chloroform 49 61 1-Propanol 97 97 Acetone 56 56 1-Butanol 109 117 DMF 131 153 1-Pentanol 106 137 Diethyl ether 32 35 1-Hexanol 92 158 Hexane 25 68 1-Chlorobutane 76 78 Heptane 26 98 1-Bromobutane 95 101 CCl4 28 77

 Table 1 Heating of organic solvents by microwaves (50cm³, 560Watts for 1 minute)

 Superheating, which may result in boiling points being raised by up to 20^oC above their conventional value under microwave irradiation (see Table 2 below), is widely believed to be responsible for the rate and yield increases which accompany many liquid phase reactions. Reports by a number of workers have suggested that other mechanisms peculiar to microwaves are responsible for some enhanced reactions, and these will be considered in other sections.

Solvent MW B.P.  /oC B.P.  /oC Solvent MW B.P.  /oC B.P.  /oC Water 104 100 Ethanol 103 79 Methanol 84 65 Dichloromethane 55 40 THF 81 66 Acetonitrile 107 81 2-Propanol 100 82 Acetone 81 56 1-Butanol 132 118 Dimethoxyethane 106 85 Diglyme 175 162 Ethyl acetate 95 78 Acetic anhydride 155 140 iso-amyl alcohol 149 130 2-Butanone 97 80 Chlorobenzene 150 132 Trichloroethylene 108 87 1-Chlorobutane 100 78

 Table 2 Superheating of organic solvents by microwaves

 19. G. Bond, R. Moyes, S. Pollington & D. Whan. Chemistry & Industry 686-687 (1991).

 20. D.R. Baghurst & D.M.P. Mingos. J Chem Soc Chem Commun 674-677 (1992).