Retention of configuration is an indication of neighbouring group participation

Enantiomerically pure (R)-2-bromopropanoic acid reacts with concentrated sodium hydrox ide to give (S)-lactic acid. The reaction goes with inversion and is a typical SN2 reaction—and a good one too, since the reaction centre is adjacent to a carbonyl group. If, on the other hand, the reaction is run using Ag₂O and a low concentration of sodium hydrox ide, (R)-lactic acid is obtained—there is overall retention of stereochemistry

Nucleophilic substitution reactions that go with retention of stereochemistry are rather rare and mostly go through two successive inversions with neighbouring group participation, like the example you saw in the last section. This time the neighbouring group is carboxylate: the silver oxide is important because it encourages the ionization of the starting material by act ing as a halogen-selective Lewis acid. A three-membered ring intermediate forms, which then gets opened by hydroxide in a second SN2 step.

●Retention suggests participation If you see a substitution reaction at a stereogenic saturated carbon atom that goes with retention of stereochemistry, look for neighbouring group participation!

Why does the carboxylate group participate only at low HO− concentration and in the pres ence of Ag+? You can think of the situation in these two reactions in terms of the factors that favour SN1 and SN2 reactions. In the fi rst, we have conditions suited to an SN2 reaction: a very good nucleophile (HO−) and a good leaving group (Br−). Improve the leaving group by adding Ag+ (Ag+ assists Br −’s departure much as H+ assists the departure of OH− by allowing it to leave as H₂O) and worsen the nucleophile (H₂O instead of HO−, of which there is now only a low concentration), and we have the sorts of conditions that would favour an SN1 reaction. The trouble is, without neighbouring group participation, the cation here would be rather un stable—right next to a carbonyl group. The carboxylate saves the day by participating in the departure of the Br− and forming the lactone. The key thing to remember is that a reaction always goes by the mechanism with the fastest rate.

 ●Neighbouring groups participate only if they speed up the reaction.

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المزيد

مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct