Inversion (chemistry)

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Inversion is a term used to describe several effects of chemical reactions or physical processes, which means a reversal of certain properties of the reaction products compared to the starting materials.

Inversion of the configuration of a stereocenter

Inversion is when a chemical reaction at a stereogenic center of stereoisomers results in a change in configuration of a stereocenter

  • D → L

or

  • L → D

or.

  • ( R ) → ( S )

or

  • ( S ) → ( R ).

If a chiral compound is e.g. B. contains two different stereocenters in the 1-position and 2-position and an inversion occurs as a result of the reaction at both stereocenters

  • (1 R , 2 S ) in (1 S , 2 R ),
  • (1 S , 2 R ) in (1 R , 2 S ) or
  • (1 R , 2 R ) in (1 S , 2 S )

convert.

In contrast, with retention, the configuration is retained.

If, in a reaction from a stereoisomer, both configurations arise to the same extent, one speaks of a racemization :

  • ( R ) → ( RS ) or
  • ( S ) → ( RS ).

An inversion does not necessarily have to reverse the sign of the specific rotation of optically active chemical compounds, since ( R ) -configuration does not mean clockwise and ( S ) -configuration does not mean left-handed. Inversions take place, for example, in bimolecular nucleophilic substitution reactions (S N 2), retentions in internal nucleophilic substitution reactions (S N i) or in S N reactions involving neighboring groups. If the nucleophile and leaving group are identical in S N 2 reactions, the reaction corresponds to the inversion of the chiral center. The inversion of configuration in S N 2 reactions has various effects on stereochemistry. The optical activity is retained as long as the leaving group and nucleophile are not identical or meso compounds are not formed. In cyclic systems, cis and trans stereoisomers can be converted into one another. In the case of substrates with more than one stereocenter, the inversion occurs only at the carbon atoms that react with the incoming nucleophile. Due to orbital and charge effects, a transition state with apical entry of a nucleophile and apical exit of the leaving group (and thus an inversion) is energetically most favorable in S N 2 reactions at carbon atoms. In silicon chemistry, on the other hand, substitutions often take place with retention of configuration if the leaving group is not clearly electronegative and its apical position is therefore not clearly favored.

A well-known inversion is the Walden reversal in nucleophilic substitutions that proceed according to the S N 2 mechanism. The observation of the Walden reversal in nucleophilic substitutions on enantiomerically pure substrates is thus an experimental indication of the S N 2 mechanism. So z. B. levorotatory enantiomerically pure ( R ) - (-) - 2-bromooctane with complete inversion (S N 2) at the stereogenic center (formerly often called "asymmetric carbon atom") with concentrated sodium hydroxide solution into dextrorotatory, enantiomerically pure ( S ) - (+) - Transfer 2-octanol . It should be noted that nucleophilic substitutions can be associated with both configuration reversal (inversion) and configuration retention (retention). The probability of the occurrence of one or the other case depends in particular on the existing substituents and the substituents already present and on the reaction conditions. The retention of the configuration observed in substitution reactions can also be the result of two successive inversions. In some cases, sigmatropic rearrangements can also be associated with inversion.

With certain reactions, both inversion and retention can occur, i.e. the inversion can be incomplete. If this happens to the same extent, a racemate is obtained (racemization).

Inversion of the direction of rotation of polarized light

If the strength of the change in the direction of rotation of stereoisomers ( polarization plane of light) is different and if this ratio changes due to a chemical reaction, this is also called an inversion . An example of this is cane sugar inversion (see invert sugar ), when D - (+) - sucrose is split into D - (+) - glucose and D - (-) - fructose by hydrolysis . In the resulting glucose-fructose mixture, the original (+) - rotation of the D - (+) - sucrose is transformed into a (-) by the stronger left-hand rotation of the D - (-) - fructose compared to the D - (+) - glucose in the reaction product. ) -Rotation reversed. The speed of inversion, which is one of the technically important hydrolysis reactions, can be easily followed with the polarimeter of a given aqueous sucrose solution by means of the rotation progressing from approx. + 66.5 ° to approx. -19.9 °.

Pyramidal inversion

In the case of compounds with trigonal-pyramidal coordinated atoms, one speaks of an inversion when the atom at the top of the pyramid swings through the plane spanned by the three substituents. This changes the direction of the bonds emanating from this atom. If the three substituents are different, isomers are converted into one another by the pyramidal inversion, which are sometimes also called invertomers. These are enantiomers if the compound has no other stereogenic unit. The energy barrier for pyramidal inversion can be very different.

Stereogenic center on a nitrogen atom

It is usually very low on the nitrogen atom of amines . The ammonia molecule in particular is not rigid, as the hydrogen atoms can fold over to the other side of the pyramid via a planar transition state. The energy barrier for the pyramidal inversion in ammonia is so small at 24.2 kJ / mol that at room temperature of ammonia and amines derived from it, NR 1 R 2 R 3 (R 1 , R 2 and R 3 : three different organic residues ) do not allow any enantiomers to be isolated. At room temperature, for example, around 20 billion inversions per second ( 14 NH 3 with 23.870 GHz and 15 NH 3 with 22.789 GHz) take place in ammonia .

In certain bridged N -heterocyclic compounds, pyramidal inversion at the nitrogen atom is not possible. One example of this is the Trögersche Base . There are therefore two stable enantiomers of Tröger's base that can be separated, for example, by chromatography on a chiral stationary phase.

Stereogenic center on a phosphorus atom

In the case of phosphorus compounds of the type PR 1 R 2 R 3 ( phosphanes ), the stereogenic center is located on the phosphorus atom. At about 113 kJ / mol, the inversion barrier is much higher than that of the analogous amines NR 1 R 2 R 3 . This is why inversions are only observed in phosphines at high temperatures.

Cyclohexane and cyclohexane derivatives

Inversion is also the mutual transformation of the two chair conformations of a six-membered ring. In the ring inversion of cyclohexane , all axial hydrogen atoms become equatorial and all equatorial become axial:

Ring-Flip.png

See also

Individual evidence

  1. ↑ a b Eberhard Breitmaier, Günther Jung: Organic Chemistry, 7th complete revision. u. exp. Edition 2012 Basics, classes of compounds, reactions, concepts, molecular structure, natural substances, synthesis planning, sustainability . Georg Thieme Verlag, 2014, ISBN 3-13-159987-1 , p. 285 ( limited preview in Google Book search).
  2. ^ A b Karl-Heinz Hellwich: Stereochemistry Basic Concepts . Springer-Verlag, 2013, ISBN 978-3-662-10051-6 , pp. 47 ( limited preview in Google Book search).
  3. ^ Kurt Peter C. Vollhardt, Neil E. Schore: Organic chemistry . John Wiley & Sons, 2011, ISBN 3-527-32754-1 , pp. 254 ( limited preview in Google Book search).
  4. ^ Ian Fleming: Molecular Orbitals and Reactions of Organic Compounds . John Wiley & Sons, 2012, ISBN 3-527-33069-0 , pp. 110 ( limited preview in Google Book search).
  5. ↑ a b Entry on inversion. In: Römpp Online . Georg Thieme Verlag, accessed on January 9, 2017.
  6. ↑ Reinhard Brückner: reaction mechanisms, organic reactions, stereochemistry, modern synthesis methods . Springer-Verlag, 2014, ISBN 978-3-662-45684-2 , pp. 76 ( limited preview in Google Book search).
  7. ^ Inversion - Lexicon of Chemistry . ( Spektrum.de ).
  8. ↑ Christoph Kölmel, Christian Oehsenfeld, Reinhart Ahlrichs: An ab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines . In: Theor. Chim. Acta , 1991, 82, pp. 271-284 ( doi : 10.1007 / BF01113258 ).
  9. ↑ Pk: RÖMPP Lexikon Chemie, 10th edition, 1996-1999 Volume 4: M - Pk (Maser) . Georg Thieme Verlag, 2014, ISBN 3-13-200021-3 , p. 2540 ( limited preview in Google Book search).
  10. ^ Siegfried Hauptmann : Organic Chemistry , 2nd Edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 96, ISBN 3-342-00280-8 .
  11. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds , John Wiles & Sons, 1994, p. 360, ISBN 0-471-05446-1 .
  12. ^ Sheila R. Buxton, Stanley M. Roberts: Introduction to Organic Stereochemistry . Springer-Verlag, 2013, ISBN 978-3-663-09876-8 , pp. 5 ( limited preview in Google Book search).
  13. ^ Karl-Heinz Hellwich, Carsten Siebert: Exercises for stereochemistry . Springer-Verlag, 2013, ISBN 978-3-662-10659-4 , pp. 65 ( limited preview in Google Book search).