Reactions of Sulfur-Based Nucleophiles with Halogenated Aliphatics

Table 2.8 compares hydrolysis half-lives with half-lives for reaction with sulfur-based nucleophiles for several halogenated aliphatics. These data show that the environmental half-lives for substrates such as 1-bromohexane and 1,2-dibromoethane can be substantially reduced in the presence of HS and polysulfides. Enhanced degradation of 2-bromopropane and 1,1,1-trichloroethane, as well as chloroform and carbon tetrachloride (results not shown) was not observed, suggesting that steric hindrance significantly impedes reaction with the sulfur based nucleophiles (Haag and Mill, 1988a). 

Based on laboratory studies, Schwarzenbach et al. (1985) calculated that the reaction product distribution for primary alkyl bromides at an initial concentration of 1 X 10 M in an aquatic environment containing a constant concentration of total H2S at 5 X 10 M (at pH 8.0) will result in the distribution of alcohol, thiol and dialkyl sulfide of 75%, 25%, and 0.2%, respectively. Formation of even small amounts of the dialkyl sulfides is of concern because of their persistence in ground-water systems. Schwarzenbach et al. (1985) have identified 24 dialkyl sulfides in groundwater samples from the site of a chemical plant that manufactures alkyl halides. 

Neighboring Group Participation (intramolecuiar Nucleophiiic Dispiacement) 

This is an example of an intramolecular nucleophilic substitution reaction. A cyclic intermediate is formed that undergoes attack by an externed nucleophile, typically H2O, to form products. These reactions typically involve 3-, 5-, or 6-membered ring intermediates. 

A comparison of the hydrolysis rate for 2-ethylthio-l-chloroethane (EtSCH2-CH2CI) and 2-hydroxy-1-chloroethane (HOCH2CH2CI) illustrates the dramatic effect that neighboring group participation can have on reactivity. Hydrolysis of 

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