What is the structure of carbanions? What about its stability?

A carbanion is an anion where carbon has an unshared pair of electrons and has a negative charge with three substituents for a total of eight valence electrons. It appears in a trigonal pyramidal geometry. Thus, a carbanion is a base of a carbon acid.

Where B stands for the base. It is a reactive intermediate in organic chemistry.

Theory

A carbanion is a nucleophile. Its stability and reactivity are determined by many factors.

  • The inductive effect- Electronegative atoms which are next to the charge stabilize the charge.
  • Hybridization of the charge-bearing atom- When the s-character of the charge-bearing atom is more, then the anion will be more stable.
  • The extent of conjugation of the anion- Resonance effects stabilize the anion as a result of aromaticity.

A carbanion is a reactive intermediate. It is found in organic chemistry like in E1Cb elimination reaction. In organometallic chemistry as in Grignard reaction or in alkyl lithium chemistry. There are stable carbanions. In the year 1984, Olmstead showed the lithium crown ether salt of the triphenylmethyl carbanion from triphenylmethane, n-butyllithium and 12-crwon-4 at low temperatures.

When n-butyllithium is added to triphenylmethane in THF at low temperatures and then followed by 12-crwon-4, it results in a red solution and the salt dissolves at -20degree C. The lengths of the C-C bonds are 145 pm along with the phenyl ring which is propelled at an average angle of 31.2 degree. The propeller shape is not noticed much in tetramethylammonium counterion. To detect carbanions in solution, the tool is proton NMR. In DMSO, it is seen that a spectrum of cyclopentadiene has four vinylic protons at 6.5 ppm and two methylene bridge protons at 3 ppm. But the cyclopentadienyl anion has a single resonance at 5.50 ppm.

Carbon acids

Any molecule which contains a C-H can lose a proton which forms the carbanion. Therefore, any hydrocarbon which contains C-H bonds is an acid with a corresponding pKa value. Methane is not an acid but its estimated pKa  is 56. It can be compared to acetic acid with pKa 4.76. the conditions which determine the stability of the carbanion also determine the order in pKa in carbon acids.

In table 1, from methane, the acidity increases.

  • when the anion is aromatic due to added electron as in indene and cyclopentadiene, or the negative charge on carbon can be delocalized over aromatic rings such as in triphenylmethane or the carborane superacid.
  • When the carbanion is supported by electronegative groups, due to the partial neutralization of the negative charge such as in malononitrile.
  • When the carbanion is placed next to a carbonyl group. Since the negative charge present in the enolate can be distributed partially in the oxygen atom, the a-protons of carbonyl groups are acidic. The historically named acids such as Meldrum’s acid and barbituric acid are lactone and lactam respectively. The presence of acidic carbon protons makes them acidic. The acidity of carbonyl compounds is a vital factor in many organic reactions such as in the aldol reaction.

Chiral carbanions

The molecular geometry for a carbanion is a trigonal pyramid. But the main question is whether or not carbanions can show chirality. If the activation barrier for inversion is less, then if there is any attempt to introduce chirality, it will only put an end to inracemization like that of nitrogen inversion. However, there are proofs that carbanions can be chiral. For example, researches had been done with certain organolithium compounds.

The first proof that chiral organolithium compounds exists was obtained in the year 1950. Chiral 2-iodooctane reaction with sec-butyllithium in petroleum ether at -70degree was followed by reaction with dry ice yielded racemic 2-methylbutyric acid. Also, optically active 2-methyloctanoic acid which could have formed from similar optical active 2-methylheptyllithium with the carbon atom linked to lithium the carbonate.

When the reaction is heated to 0degree, the optical activity gets lost. There were more evidences which followed in the year 1960s. a reaction of the cis isomer of 2-methylcyclopropyl bromide with sec-butyllithium was again followed by carboxylation with dry ice resulting in cis-2-methylcyclopropylcarboxylic acid. The trans isomer that was formed indicates that the intermediate carbanion is not stable.

In the same manner, the reaction of (+)-(S)-l-bromo-l-methyl-2,2-diphenylcyclopropane with n-butyllithium which was followed by methanol yielded in product with retention of configuration.

Chiral methyllithium compounds

The phosphate 1 contains a chiral group along with the presence of a hydrogen and a deuterium substituent. The stannyl group gets replaced by lithium to intermediate 2 which undergoes a process of phosphate-phosphorane rearrangement to phosphorane 3 which on reaction with acetic acid results in alcohol 4. The chirality is preserved in this Reaction sequence in the range of -78degree to 0degree C.