Free electron pair

A lone pair of electrons (also non-binding or a literal translation of the English lone pair also lone pair of electrons called) consists of two electrons on one atom which an opposite spin and have the same atomic and molecular orbital filled. In chemistry , these electrons are also known as valence electrons . As a valence electron pair, the free electron pair is generally not involved in any bond with other atoms , but only belongs to one atom - an exception is e.g. B. Ozone. In a valence line formula , a lone pair of electrons is represented either by two dots ( IUPAC recommendation) or by a line on the atom concerned. There is also the representation of the free electrons as electron clouds. The following figures show molecules with lone pairs marked in blue :

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  Water with two electron pairs as points.

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  Water with two electron pairs as lines.

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  Water with two electron pairs as electron clouds.

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  Carbon dioxide with four free electron pairs.

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  Hydrocyanic acid with one free electron pair.

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  Ammonia with a lone pair of electrons.

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  Nitrogen molecule with two free electron pairs.

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  Nitrogen molecule with two electron clouds.

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  Ozone with six free electron pairs.

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  Hydrogen chloride with three free electron pairs.

Lone pairs of electrons contribute to the spatial structure of molecules , the shape of which can be predicted using the electron pair repulsion model (VSEPR model) for simple compounds . The best known example is the angled shape of the water molecule , which is decisive for some properties of water .

In contrast to a free electron pair, a binding electron pair represents the connection between two atoms. This is known as a covalent bond.

Chirality due to a lone pair of electrons

A free electron pair behaves at a stereocenter like another substituent.

Amines

If a tertiary amine NR ¹ R ² R ^{3 has} three different organic radicals (R ¹  ≠ R ²  ≠ R ³ ) and a lone pair of electrons on the nitrogen atom, one might expect such amines to be chiral . At room temperature, however, because of the rapid inversion (“swinging through” of the lone pair of electrons), no enantiomers can usually be isolated. However, this does not apply to special tertiary amines in which the nitrogen atom is prevented from inversion by a “bridgehead” position (e.g. Tröger's base ). There are therefore two stable enantiomers of Tröger's base that can be separated, for example, by chromatography on a chiral stationary phase.

Sulphoxides are chiral if the radicals R ¹ and R ^{2 are} different. The image shows a pair of enantiomers of sulfoxides.

Sulfoxides

With two different organic radicals (R ¹  ≠ R ² ), sulfoxides of the type O = SR ¹ R ² are chiral due to the lone pair of electrons on the sulfur atom.

Individual evidence

1. ↑ Entry on lone (electron) pair . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.L03618 .
2. ^ Siegfried Hauptmann : Organic chemistry. 2nd Edition. VEB Deutscher Verlag für Grundstofftindustrie, Leipzig 1985, ISBN 3-342-00280-8 , p. 96.
3. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds. John Wiles & Sons, 1994, ISBN 0-471-05446-1 , p. 360.
4. ↑ Albert Gossauer: Structure and reactivity of biomolecules. Verlag Helvetica Chimica Acta, Zurich 2006, ISBN 3-906390-29-2 , pp. 235-236.