The kinetics of the sulphonation of benzene have been measured at 5.0", 25.0" and 45.0" in aqueous sulphuric acid solutions, varying from 77.6 to 99.2 wt-% HzS04. In addition, sulphonation rate measurements have been made at 5.0" in 98.8 wt-% HzS04 containing various concentrations of sodium bisulphate. All reactions were found to be first order with respect to benzene. The kinetics are discussed in terms of the mechanism:It is suggested that sulphur trioxide or, though less probable, the sulphuric acid solvate of sulphur trioxide S03(H2S04) or disulphuric acid (H&07), is the sulphonating entity X and that kl < k2 1 B / + k3. No kinetic distinction can be made between the deprotonation by reaction [2] and the less probable intramolecular proton shift (reaction [3]).A comparison between the mechanisms of sulphonation in aqueous and fuming sulphuric acid furnishes additional evidence that so3, instead of S03(HzS04) or HzSz07, should be regarded as the sulphonating entity.
Rate measurements have been made on the homogeneous sulphonation of toluene in aqueous sulphuric acid varying from 70.0 to 95.9 wt-% at 5.0", 25.0" and 45.0'. In addition, rate constants have been determined for the homogeneous sulphonation of ethylbenzene and isopropylbenzene at 25.0" in aqueous sulphuric acid solutions varying from 72.4 to 89.1 wt-%. All reactions were found to be first order with respect to the aromatic compound. It is suggested that the differences in the overall rate constants for the sulphonation of toluene, ethylbenzene and isopropylbenzene originate predominantly from steric hindrance for ortho-substitution.
A recent publication by Eaborn and Taylor 1 on the sulphonation of toluene prompts us to report the results of a kinetic study on relative reactivities in the sulphonation of benzene and toluene.This work was undertaken in connection with a study on isomer distribution in the sulphonation of toluene under comparable conditions 2, since the available data on the sulphonation of toluene 3v4. 5 did not fit the selectivity relationship proposed by Brown 6.The sulphonation reactions were studied under homogeneous conditions a t 25.0" in 82.9 f 0.2 wt-% aqueous sulphuric acid. Experimental details will be given later. The reactions were found to be first order with respect to the aromatic hydrocarbons up to 98% conversion. The first order rate constants were calculated from In a/ao = -k t , and for benzene ( k b ) were found to be 3.31 f 0.06 x 10--6sec-l and for toluene ( A t ) 218 j , 4 x 10-6sec-1.A comparison c,f these data with data from earlier work (cf. Table I) shows that for toluene the agreement between our data and those of Eaborn and Taylor is satisfactory (interpolation from a semilog plot of k t uersz~s yo sulphuric acid leads to 82.9L.t = 350 x 10-6 sec-1 *). The discrepancy between our data and those of Gold and Satchel1 7 stands unexplained.
The ratio of the overall rate constants for the sulphonation of toluene and benzene in aqueous sulphuric acid at 25" decreases from 110 to 25 on varying the acid concentration from 78 to 92 wt-O/I. Similar trends are observed at 5 " and 45".The partial rate factors for orrho-, meta-and para-substitution in the sulphonation of toluene with aqueous sulphuric acid all decrease strongly with increasing acid concentration. The partial rate factor for para-substitution is temperature independent, indicating a negligible difference in activation enthalpy between substitution at the para-position in toluene and at a single position in benzene. The corresponding difference in activation entropiesThe latter difference is ascribed to differences in solvation of the starting hydrocarbons or/and of the transition states. These differences in solvation may also be responsible for the small but apparent deviations from the Selectivity Relationship observed in the present study. It is suggested that the sulphonation of toluene and benzene in aqueous sulphuric acid proceeds via a Wheland type of intermediate.
IntroductionElectrophilic aromatic substitution reactions, like nitration and halogenation, have been studied extensively in the last twenty years and partial rate factors for these types of substitution reactions have been obtained for a large number of compounds 172.The chemical literature is still lacking partial rate factors for aromatic sulphonation. We therefore thought it of interest to determine such partial rate factors. Toluene was regarded as an attractive compound to start with. Accordingly we set out to determine the rate of sulphonation of toluene
.r-Butylbenzene has been sulphonated homogeneously in aqueous sulphuric acid solutions varying from 72.4 to 91.0 wt-% at 5.0", 25.0" and 35.0". Under these conditions dealkylation of r-butylbenzene occurs. Pseudo first order rate constants for the sulphonation and the dealkylation reaction have been determined.The overall rate constant for sulphonation of r-butylbenzene is smaller than that of toluene. It is suggested that this difference arises predominantly from increased steric hindrance for ortho substitution in the former case.The overall rate constant for dealkylation increases rapidly with increasing acid
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