TABLE OF CONTENTS The sulfonation of aromatic nitro compounds is not of great synthetic importance because it is generally easier to nitrate an aromatic sulfonic acid.1 However, in some cases, this may cause nitrodesulfonation5 and also sulfonation, unlike nitration, often gives mainly one product rather than a mixture of isomers, consequently sulfonation of aromatic nitro compounds is sometimes a useful procedure.
In the aryl nitro compounds, the powerful electron-withdrawing (-I, -M) effect of the nitro group results in the sulfonation of nitrobenzene being more difficult than that of benzene. Early work1 claimed that heating nitrobenzene with excess chlorosulfonic acid (two equivalents) afforded m-nitrobenzenesulfonyl chloride, but under drastic conditions (150 C, 20 hours) the only product was reported8 to be tetrachloro- p-benzoquinone (chloranil).
In more recent work,156 nitrobenzene 89, by heating with chlorosulfonic acid (four equivalents) at 100 120 C (8 hours), afforded the m-sulfonyl chloride. The yield of the product was claimed to be increased to 96.3% by carrying out the reaction with chlorosulfonic acid at 112 C followed by cooling the mixture to 70 C and the addition of thionyl chloride.157
Nitrobenzene ( 89, 184.5g) by reaction with a mixture of iodine (1.1 g) and 98% sulfuric acid (57 g) in chlorosulfonic acid at 30 C afforded pentachloronitrobenzene 90 (87% yield)158 (Equation 28). The product is useful as a soil fungicide and the synthesis provides another example of the use of a mixture of chlorosulfonic acid and iodine as a chlorinating reagent (see Section...
