Phenanthrene-2-and 3-Sulfonates

Phenanthrene. Studies in the sulfonation of phenanthrene 52 indicated a positional order of reactivity of 3 > 2 > 9 > 1 This is different from the theoretical order of 9 > 1 > 4 > 3 > 2, because of steric hindrance which accounts for the complete absence of the 4-isomer. Reaction of phenanthrene 52 with chlorosulfonic acid (one equivalent) in boiling chloroform affords the 2- and 3-sulfonic acids 53 and 54 (Equation 16) (85% yield of the mixture of isomers was isolated as the lead sulfonates). ... 

Upon sulfonation, phenanthrene-3-sulfonic acid gives mainly (but not exclusively) the 3,6-disulfonic acid (Fieser, L. F. J. Am. Chem. Soe. 1929, 51, 2471), but the starting phenanthrene-3-sulfonic acid can be obtained from phenanthrene in only ca. 25% yield, and then isolated only from a mixture including a comparable amount of the 2-sulfonic acid (Fieser, L. F. Org. Synth., Coll. Vol. II1943, 482). 

Phenanthrene (Table XII, p. 189). Phenanthrene is as readily sul-fonated as anthracene when the reaction is carried out for three hours at 120-125° with concentrated sulfuric acid, more than 40% of the phenanthrene is converted into disulfonic acids. Two monosulfonic acids are isolated under these experimental conditions phenanthrene-2-sulfonic acid (25% yield) and phenanthrene-3-sulfonic acid (27% yield). The same acids are obtained by sulfonating at 100.° for eight hours (2-acid, 7% 3deld 3-acid, 9% yield), and in addition a third acid, phenanthrene-9-sulfonic acid (6% 3deld) is formed. The 9-acid is formed in larger amounts at lower temperatures (14.5% at 20°, twenty days). A fourth monosulfonic acid, the 1-acid, is isolated from the sulfonation reaction conducted at 60° for several days. The 3uelds of the four monosulfonic acids isolated under these conditions are 4% of the 1-acid, 18% of the 2-acid, 19% of the 3-acid, and 13% of the 9-acid. There is no indication that the only other possible monosulfonic acid, the 4-isomer, is ever formed, probably because the 4-position of phenanthrene is particularly hindered. 

The reactions of the higher hydrocarbons with electrophilic reagents are more complex than of naphthalene. For example, phenanthrene can be nitrated and sulfonated, and the products are mixtures of 1-, 2-, 3-, 4-, and 9-substituted phenanthrenes ... 

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