Neighbouring group participation or NGP in
organic chemistry has been defined by
IUPAC as the interaction of a reaction centre with a
lone pair of electrons in an atom or the electrons present in a
sigma bond or
pi bond . When NGP is in operation it is normal for the
reaction rate to be increased. It is also possible for the
stereochemistry of the reaction to be abnormal (or unexpected) when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry (For instance the reaction of a
diene such as cyclohex-1,3-diene with
maleic anhydride normally gives the
exo isomer because of a secondary effect {overlap of the carbonyl group π orbitals with the transition state in the
Diels-Alder reaction} this page is limited to neighbouring group effects seen with
carbocations and SN2 reactions.
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