# Problem: When (S)-2-bromopropanoic acid [(S)-CH3CHBrCO2H] reacts with concentrated sodium hydroxide, the product formed (after acidification) is (R)-2-hydroxypropanoic acid [(R)-CH3CHOHCO2H, commonly known as (R)-lactic acid]. This is, of course, the normal stereochemical result for an SN2 reaction. However, when the same reaction is carried out with a low concentration of hydroxide ion in the presence of Ag2O (where Ag+ acts as a Lewis acid), it takes place with overall retention of configuration to produce (S)-2-hydroxypropanoic acid. The mechanism of this reaction involves a phenomenon called neighboring-group participation. Write a detailed mechanism for this reaction that accounts for the net retention of configuration when Ag+ and a low concentration of hydroxide are used.

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When (S)-2-bromopropanoic acid [(S)-CH3CHBrCO2H] reacts with concentrated sodium hydroxide, the product formed (after acidification) is (R)-2-hydroxypropanoic acid [(R)-CH3CHOHCO2H, commonly known as (R)-lactic acid]. This is, of course, the normal stereochemical result for an SN2 reaction. However, when the same reaction is carried out with a low concentration of hydroxide ion in the presence of Ag2O (where Ag+ acts as a Lewis acid), it takes place with overall retention of configuration to produce (S)-2-hydroxypropanoic acid. The mechanism of this reaction involves a phenomenon called neighboring-group participation. Write a detailed mechanism for this reaction that accounts for the net retention of configuration when Ag+ and a low concentration of hydroxide are used.