Nucleophilicity

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In chemistry, nucleophilicity (from the Greek nucleos = core, philos = friend) is a measure of the ability of an atom with a free electron pair to attack a (partially) positively charged atom while forming a covalent bond . Typical nucleophiles are often negatively charged, have a strongly negative partial charge or have a free electron pair in a relatively high- energy atomic orbital .

The concept is related to that of the Lewis bases , but is measured using rate constants rather than equilibrium constants . Conversely, the ability of a reactant to be attacked by a nucleophilic particle is known as electrophilicity .

Examples of nucleophilic particles

Typical anionic nucleophiles are:

Important neutral nucleophiles are:

Estimation of the nucleophilicity of a particle

The nucleophilicity of a molecule is usually equated with the nucleophilicity of the most nucleophilic atom.

Basicity

The relationship between nucleophilicity and basicity depends heavily on the type of solvent used (see below). In aprotic solvents, the nucleophilicity of a substance correlates well with its basicity. In other words, the more basic the substance, the greater the nucleophilic character. In protic solvents, this relationship is no longer applicable. Since protic solvents mainly form hydrogen bonds with hard Lewis bases , the nucleophilic character is significantly weakened ( HSAB concept ). Therefore, soft Lewis bases are the better nucleophiles than hard ones. Both hardness and basicity are closely related to the polarizability of the particles. Polarizable particles are generally more nucleophilic than comparably charged, less polarizable molecules.

Steric

The nucleophilicity is also strongly influenced by the sterics . Very extensive substituents on the nucleophilic atom shield it well and thus prevent a nucleophilic attack. The basicity is increased by the presence of several alkyl groups , but the sterics is of much greater importance. Potential electrophiles can also be shielded by bulky residues.

Choice of solvent

Furthermore, reactions of nucleophiles are sensitively influenced by the choice of solvent . A high degree of solvation of the attacking particle significantly reduces the nucleophilicity. Conversely, the nucleophilicity increases in polar aprotic solvents such as acetone , since hydrogen bonds do not form here. The hydrogen bonds ensure the formation of stable hydration shells in water, for example. In non-polar solvents, the nucleophile and counterion (mostly metal cation ) sometimes do not dissolve at all. If they do, they exist as associated ion pairs and are only moderately reactive.

Taking all these factors into account, the nucleophilic properties of many molecules and thus the reaction behavior as an attacking nucleophile or as a leaving group can be estimated very precisely.

With preparative tricks, however, reactions that are not favored by a lack of nucleophilicity can also be forced. The Finkelstein reaction is an example .

Examples

The following table shows the nucleophilicity of some molecules with methanol as the solvent:

Relative nucleophilicity Molecules
Very strong I⁻, HS⁻, RS⁻
Strong Br⁻, OH⁻, RO⁻, CN⁻, N 3
medium NH 3 , Cl⁻, F⁻, RCO 2
Weak H 2 O, RAW
Very weak RCO 2 H

Nucleophilic reactions

A nucleophilic reaction connects two reaction partners via a covalent bond . Sometimes another bond is broken, so that a less nucleophilic atom group is split off, i.e. acts as a leaving group . The concept of nucleophilicity can therefore be used to predict the course of reactions. It is characteristic of nucleophiles that they alone provide both electrons required for binding, while the electrophile “only” contributes its ability to stabilize the electron pair. Similar to redox reactions , where every oxidation also means the reduction of the other reaction partner, nucleophiles are immediately followed by the associated electrophilic reaction.

Important reactions involving electrophiles and nucleophiles are:

Individual evidence

  1. Entry on nucleophilicity . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.N04251 Version: 2.3.1.
  2. Dr. Ian Hunt: Chapter 8: Nucleophiles. University of Calgary, accessed November 19, 2018 .