Nucleophile

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

The concept is related to the Lewis base is, however, with rate constants measured in place of the equilibrium constants. Conversely, the ability of a reactant to seek a nucleophilic attack of particles, referred to as electrophilicity.

Estimate the nucleophilicity of a particle

The nucleophilicity of a molecule is equivalent to a rule with the nucleophilicity of the nukleophilsten atom.

Basicity

The relation between the basicity and nucleophilicity depends greatly on the kind of the solvent used (see below). In aprotic solvents, the nucleophilicity of a substance correlates well with their basicity. That is the more basic the substance, the greater the nucleophilic character. In protic solvents, this relationship is no longer applicable. Since protic solvent to hard Lewis bases form hydrogen bonds mainly, in these nucleophilic character significantly attenuated ( HSAB concept). So here are soft Lewis bases, the better nucleophiles than hard. Both hardness and alkalinity are closely related to the polarizability of the particles. Polarizable particles are generally more nucleophilic than comparable charged, less polarizable molecules.

Steric

The nucleophilicity is also strongly influenced by the steric bulk. Very bulky substituents on the nucleophilic atom shield this well and thus prevent a nucleophilic attack. Although the basicity is defined by the presence of several alkyl groups increased, but the steric is of much greater significance. Likewise, potential electrophiles can be shielded by bulky residues. The reactivity of secondary, tertiary or cyclic hydrocarbons in nucleophilic reactions is usually less than at the primary reactants.

Choice of solvent

Further reactions of nucleophiles are sensitive influenced by the choice of solvent. A high degree of solvation of the attacking particle decreases the nucleophilicity considerably. Conversely, the nucleophilicity thus increases in polar aprotic solvents such as acetone, as it is not in the formation of hydrogen bonds here. The hydrogen bonds in water for example, provide for the formation of stable hydration shells. In nonpolar solvents, nucleophile and counter ion ( metal cation usually) do not solve partially. If they do, they are as associated ion pairs and are only moderately reactive.

Through an assessment of all these factors, the nucleophilic properties of many molecules can be estimated quite accurately and thus to estimate the reaction behavior as attacking nucleophile or a leaving group.

With preparative tricks but can also be due to lack of nucleophilicity not begüngstigte reactions force. Constitutes an example of Finkelstein reaction

Examples of nucleophilic particles

Typical anionic nucleophiles are

  • Hydroxide
  • Alkoxide
  • Thiolate
  • Carbanions
  • Halide
  • Peroxide
  • Cyanide
  • Azide

Important neutral nucleophiles are

  • Amines
  • Phosphines
  • Carbon monoxide
  • Alkenes
  • Alcohols
  • Aromatics
  • Water

Nucleophilic reactions

A nucleophilic reaction linking two reactants via a covalent bond. In this case, sometimes a different bond is broken, so that a less nucleophilic atom group is cleaved, thus functions as a leaving group. With the concept of nucleophilicity, therefore, the course of reactions predict. It is characteristic of nucleophiles that they alone represent both required for the bonding electrons available while the electrophile "only" brings its ability to stabilize the electron pair. Similar redox reactions, where each oxidation means the reduction of the other reactant at the same time, followed by nucleophilic directly the corresponding electrophilic reaction.

Important reactions involving electrophiles and nucleophiles are:

  • Nucleophilic substitution at saturated compounds
  • Nucleophilic addition at the unsaturated compounds
  • Addition-elimination reaction with carbonyls
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