ANCHIMERIC ASSISTANCE PDF

Anchimeric Assistance Neighboring Group Participation The participation of neighboring groups in an SN reaction is revealed by unique stereochemical results retention in the substitution process and also usually by obvious rate enhancements in comparison to a model in which neighboring group participation would be stereoelectronically impossible. Consider the case of transiodocyclohexyl brosylate remember, brosylate is p-bromobenzenesulfonate, a very good leaving group, better even than tosylate why? In comparison to the corresponding cis isomer, the rate of solvolysis in acetic acetic is more than a million fold faster. This is because in the cis isomer, the iodo substituent cannot approach from the backside of the carbon which is undergoing substitution.

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For instance the unsaturated tosylate will react more quickly times faster for aqueous solvolysis with a nucleophile than the saturated tosylate. The carbocationic intermediate will be stabilized by resonance where the positive charge is spread over several atoms, in the diagram below this is shown.

Here is a different view of the same intermediates. Even if the alkene is more remote from the reacting center the alkene can still act in this way. For instance in the following alkyl benzenesulfonate the alkene is able to delocalise the carbocation.

In the allyl system the alkene orbitals overlap with the orbitals of a SN2 transition state. This is because the carbocationic intermediate is delocalised onto many different carbons through a reversible ring opening. NGP by an aromatic ring[ edit ] In the case of a benzyl halide the reactivity is higher because the SN2 transition state enjoys a similar overlap effect to that in the allyl system.

An aromatic ring can assist in the formation of a carbocationic intermediate called a phenonium ion by delocalising the positive charge. The mechanism which forms A and B is shown below. Corresponding intermediates are referred to a nonclassical ions , with the 2-norbornyl system as the most well known case.

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30.5: Anchimeric Assistance by Other Neighboring Groups

Ryan Spoering on March 11, Intramolecular reactions Nucleophilic catalysis 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. In essence, the iodine lowers the activation energy. Anchimeric assistance Similar phenomena as above can occur on an intramolecular level. Anchimeric assistance is when a neighboring group participates in a reaction.

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30.4: Anchimeric Assistance

For instance the unsaturated tosylate will react more quickly times faster for aqueous solvolysis with a nucleophile than the saturated tosylate. The carbocationic intermediate will be stabilized by resonance where the positive charge is spread over several atoms, in the diagram below this is shown. Here is a different view of the same intermediates. Even if the alkene is more remote from the reacting center the alkene can still act in this way. For instance in the following alkyl benzenesulfonate the alkene is able to delocalise the carbocation. In the allyl system the alkene orbitals overlap with the orbitals of a SN2 transition state. This is because the carbocationic intermediate is delocalised onto many different carbons through a reversible ring opening.

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