Neighbouring group participation NGP also known as anchimeric assistance 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 contained within the parent molecule but not conjugated with the reaction centre. 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 e. A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile , the rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary alkyl chloride without a heteroatom. For instance the unsaturated tosylate will react more quickly 10 11 times faster for aqueous solvolysis with a nucleophile than the saturated tosylate.
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Organic chemistry 17: Anchimeric assistance, epoxide transformations
When the solvolysis rates of alkyl halides and sulfonate esters are measured, some curious influences of neighboring substituents are observed. The reaction rates for ethyl chloride and neopentyl chloride are nearly identical, but the triphenyl compound reacts 60, times faster. Equations for the latter two solvolyses are shown in the following diagram. However, the increased rate of the phenyl substituted compound is perplexing, especially in view of the greater electronegativity of phenyl groups relative to methyl note that diphenylacetic acid is over nine times more acidic than isobutyric acid. This intramolecular interaction corresponds to the last example in the previous section, and is similar to an intramolecular S N 2 reaction.
30.4: Anchimeric Assistance
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Neighbouring group participation
Ryan Spoering on March 11, Consider the reaction below. The I - initiates a nucleophilic attack on the carbon to which the Cl is bound, thus replacing it. The I is then much more vulnerable to replacement by the acetyl group. Similar phenomena as above can occur on an intramolecular level. Anchimeric assistance is when a neighboring group participates in a reaction.