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Methylation of tin(II) by methyl iodide: influences of different environmental factors on the efficiency and reaction kinetics

The methylation reaction of Sn(II) with methyl iodide (MeI) in water has been studied using sensitive GC-QSIL-FPD technology. The pH value, amount of MeI and salinity (S) are the three important factors that influence the methylation reaction in an aquatic environment. In all experiments, monomethyltin (MMT) is the only methylation product of the tin(II) reacting with MeI observed. At the 95% confidence level, the pH, MeI and S are significant for the MMT yield. The concentration of MMT in the reactor increases with increase in pH within the selected pH range of 4-9 because four different species of Sn(II)-Sn2+, SnOH+, Sn(OH)(2)(0) and Sn(OH)(3)(-)-have different reaction activities with MeI. The methylation activity of Sn(II) was found to be highest at a salinity of 0.1 M at three different pH levels: 5, 7 and 9. Higher concentration of Cl- (as a relatively weak nucleophilic ion) will obstruct nucleophilic attack of Sn(II) on MeI. MMT production also increases with rising volume of MeI. Moreover, first-order reaction rates have been calculated at different pH, salinity and MeI, and found to be in the range 0.0018-0.0199 h(-1). The reaction rate also varies largely under different reaction conditions. One probable mechanism for the methylation reaction of Sn(II) with MeI is a S(N)2 nucleophilic attack on the methyl group of MeI by Sn(II), via a process of oxidative methyl-transfer. Copyright (c) 2006 John Wiley & Sons, Ltd.