Reactivity effects sulphonation

Kinetic isotope effects also show a dependence upon the reactivity of the electrophile. Thus some reactions, e.g. positive chlorination, show no isotope effect whereas others, e.g. sulphonation, do show an isotope effect. There are two ways of visualising the reasons for this and they are closely related. Very... 

A small isotope effect has been observed in nitration of benzene by nitronium borofluoride in tetramethylene sulphone at 30 °C (kH/kD = 0.86) and this has been attributed to a secondary effect of the change in hybridisation from sp2 to sp3 of the ring carbon during the course of the reaction109. However, naphthalene gives an isotope effect of 1.15 under the same conditions, and anthracene a value of 2.6115. It does not seem at all clear why these relatively unhindered and normally more reactive molecules should give rise to an isotope effect when benzene does not. 

Kinetic isotope effects have not been observed in the reaction of 1-naphthol-4-sulphonic-2-acid with 2-methoxydiazobenzene127, imidazole-2,4,5-d3 with 4-diazobenzene sulphonic acid128, or indole-3- / with 4-nitrodiazobenzene12S, nor has base catalysis been observed in those cases where it has been measured in each of these reactions one or both of the reagents is relatively reactive. 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

Data are also available with a-acetylenic aliphatic sulphones, which involve only two steps i.e., saturation of the triple bond without subsequent cleavage of the Caliphalic—S bond, since it is not reactive. However, the introduction of an aromatic ring to the S02 group does not lead, contrary to what is observed with enones, to a potential shift toward less reducing potential values. Thus, the aromatic moiety introduced apparently does not bring any additional conjugation effect but even seems to decrease the activation of the unsaturated bond, as shown by data in Tables 6 and 7 where most of the potentials refer to the same saturated calomel electrode under similar experimental conditions. 

Selecting a sulphonated dye molecule containing an amino group as the nucleophile leads directly to a dichlorotriazine dye. In certain cases a suitable intermediate may be condensed with cyanuric chloride and then the chromogenic grouping is synthesised from this reaction product. Both of these routes are illustrated in a simple way for Cl Reactive Red 1 (7.1) in Scheme 7.7. In these dyes the electronic effects responsible for the lability of the chloro substituents are muted by feedback of electrons from the electron-donating imino bridging... 

One hydrophilic anthraquinone dye (Al) and one hydrophobic anthraquinone dye (A2) were selected to explore the effect of solubility to the UV-irradiation. From the experimental data, the hydrophilic anthraquinone dye (sulphonate anthraquinone reactive dye) has about twice the dye removal rate than hydrophobic anthraquinone dyes at high pH. Since all the anthraquinone dyes have similar chemical structures and the pH did not show any distinguish effect in dye removal for A2, it is obvious that the rate difference is mainly due to the difference of solubility. The effect of dye s solubility to the photodegradation may be interpreted by the photo-ionization mechanisms proposed by Leaver I.H. (1980). The water can carry the charged intermediates (Al only), which lower the photo-ionization energy and therefore facilitate the dye degradation. 

Figure 5.5 Effect of reaction medium on the relative reactivity of hydroxyl and bromide ions with a lipophilic sulphonate ester.

---