Carbanion

from Wikipedia, the free encyclopedia
Carbanions are pyramidal if the radicals R 1 −R 3 stand for hydrogen atoms or alkyl groups. This can be explained by the s-components in the orbital carrying the negative charge. Such carbanions invert extremely quickly, the flipping is much easier than with analogous amines. The methyl anion (residues R 1 −R 3 = H) is more stable than the alkyl-substituted carbanions.
Carbanions mesomeric stabilized by –M substituents in (from left to right) an ester, an aldehyde (R 3 = H) or ketone (R 3 = alkyl or aryl radical ), a nitrile and a nitroalkane. Due to the conjugation with the –M substituent, these carbanions adopt a planar geometry.

In organic chemistry, a carbanion (read: carb-anion) is a negatively charged carbon atom to which one or more electron-withdrawing substituents ( carbonyl group in aldehydes , esters or ketones as well as a nitrile radical etc.) are bound.

In contrast to the carbenium ion , primary carbanions are more stable than secondary carbanions and these are more stable than tertiary carbanions.

The chemical behavior of Grignard compounds (RMgBr or RMgCl) and alkyllithium compounds (RLi) corresponds to that of a negatively charged alkyl radical (R - ), a carbanion. Isonitriles (R – NC) also react like carbanions, e.g. B. in the Passerini reaction or the Ugi reaction .

Carbanions often occur as reactive intermediates, which are often stabilized by mesomerism . Examples:

Carbanions were proposed as reactive intermediates in the condensation of benzoins in 1907 by RWL Clarke and A. Lapworth , and were found by W. Schlenk and E. Marcus in 1914 when studying the reaction of triphenylmethyl chloride with sodium (they examined the ionic structure of the compound via electrical conductivity) and the name coined in 1933 by ES Wallis and FH Adams.

See also

Positively charged alkyl radicals are called carbocation . An example of a carbocation is the carbenium ion.

Individual evidence

  1. ^ A b Ulrich Lüning: Organic reactions , 2nd edition, Elsevier GmbH, Munich, 2007, pp. 108–111, ISBN 978-3-8274-1834-0 .
  2. ^ Ivan Ernest: Binding, structure and reaction mechanisms in organic chemistry , Springer-Verlag, 1972, p. 128, ISBN 3-211-81060-9 .
  3. ↑ Paula Yurkanis Bruice: Organic Chemistry , Pearson Education Inc., 2007, 5th edition, p 1081, ISBN 978-3-8273-7190-4 .
  4. ↑ Clarke, Lapworth, J. Chem. Soc., Vol. 91, 1907, pp. 694-705.
  5. ↑ Schlenk, Marcus, Reports Deutsche Chem. Ges., Volume 47, 1914, pp. 164–1687.
  6. ^ Wallis, Adams, J. Am. Chem. Soc., Vol. 55, 1933, pp. 3838-3851. According to Thomas T. Tidwell, The first century of Ketenes (1905-2005): the birth of a family of reactive intermediates, Angewandte Chemie, Int. Edition, Volume 44, 2005, p. 5778.