Neighboring Group Participation in Nucleophilic Substitution Reactions, Exams of Chemistry

Download Neighboring Group Participation in Nucleophilic Substitution Reactions and more Exams Chemistry in PDF only on Docsity! Neighboring Group Participation Dr Syed Gohar Taqi Kazimi Assistant Professor Department of Chemistry University of Sargodha Summary of SN reactions  In SN2 reaction bond making and breaking occurs simultaneously while in SN1 bond breaking occurs first.  During SN2 reaction, departure of LG is assisted by Nu- while in SN1, it is not.  SN2 is a 2nd order reaction while SN1 is 1st order.  In SN2 inversion of configuration takes place while in SN1 racemization occurs. Neighboring Group Participation NGP can occur from.  any group that have an unshared pair of electrons or  a double bond The electron density will accelerate the rate of departure of the leaving group  New cyclic reaction intermediate  Nucleophile must be in trans position to the leaving group Mustard gas is β, β’-dichloroethylenesulphide is a primary halide which undergo hydrolysis rapidly in H2O (a very poor nucleophile). This can not be explained on the basis of SN2 mechanism Alkoxy group is a weak nucleophile but it can function as neighboring nucleophile. For example solvolysis of isomeric p-bromobenzenesulphonate ester A and B produce the identical product mixtures, indicating the involvement of common cyclic oxonium ion as an intermediate. Practice Problems 1) Which will go faster and why? 2) Identify A and B in the following reaction Summary In general, any system that has a nucleophillic substituent situated properly for back side displacement of LG at another carbon atom can be expected to display NGP. Reactions proceeding through NGP can be identified by their enhanced rate of reaction and characteristic stereo chemical out come of retention of configuration