Sulphonic acids, abundance

Oxygen-17 is quadrupolar, I = f, and has very low natural abundance (0.037%). Despite these limitations 170 NMR is now a very valuable tool for use in studies on structure and bonding. Table 31 gives some 170 chemical shifts of some sulphonic acid derivatives. 

Bourns (1963) investigated the bromination of anisole-4-sulphonic-2,6- 2 acid. In the presence of 0 0 to 2-Om bromide ions the ratio kgjkjy increases from 1-00 to 1-31. As a competitive reaction bromode-sulphonation takes place, bromine substituting the sulphonic group in the 4-position. Using anisole-4-sulphonic acid containing the natural abundance of the sulphur isotopes. Bourns measured sulphur isotope effects k jk is 1-003, 1-013 and 1-017 for initial bromide ion concentrations of 0-0, 0-05 and 0-50m respectively. This confirms, that bromodesulphonation also follows mechanism (6-2). 

If even after long standing an abundant separation of crystals does not take place, the cause is probably due to the large percentage of anhydride in the fuming sulphuric acid. Under these conditions it is diluted with concentrated acid, and the experiment repeated. If on the other hand the acid is too weak, the benzene will not dissolve in it. In this case, during the sulphonation the mixture is not cooled, and the reaction is allowed to take place at 40-50°. 

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