Abstract
Rates of cleavage, krel, relative to that of phenyltrimethylsilane have been measured spectrophotometrically at 50·0° for some X·C6H4·SiMe3 compounds. In methanol–aqueous perchloric acid (5 : 2, v/v) values of krel for various substituents (X) are: m-But 3·86, m-SiMe3 1·63 (the observed relative rate has been halved to allow for the availability of two equivalent aryl–SiMe3 bonds), m-OH 0·87, m-OPh 0·36, m-SMe 0·19, and p-SMe 78. In acetic acid–aqueous sulphuric acid (4 : 3, v/v), values of 104krel are m-Cl 119, m-Br 117, m-CO2Me 11, m-CF3 21, p-CO2Me 18·8, m-SO3H 14·5, p-SO3H 9·0, m-NO2 3·7, and p-NO2 1·4. The value of krel for the p-CO2H group varies with the strength of the sulphuric acid, possibly because of hydrogen bonding between the group and the acidic medium. For both media, values of log krel show an excellent linear correlation with the quantity [σ+ 0·65 (σ+–σ)].The order of deactivation by strongly electron-withdrawing substituents, reported for the first time for an electrophilic aromatic substitution, is the same for meta- and para-substituents, viz. NO2 > Me3N+ > SO3H > CF3 > p-CO2H > CO2Me > Cl.
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