|
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons.〔Organic Chemistry - Robert Thornton Morrison, Robert Neilson Boyd〕 The carbanion exists in a trigonal pyramidal geometry. Formally, a carbanion is the conjugate base of a carbon acid. :R3C-H + B− → R3C− + H-B where B stands for the base. A carbanion is one of several reactive intermediates in organic chemistry. ==Theory== A carbanion is a nucleophile. The stability and reactivity of a carbanion is determined by several factors. These include #The inductive effect. Electronegative atoms adjacent to the charge will stabilize the charge; #Hybridization of the charge-bearing atom. The greater the s-character of the charge-bearing atom, the more stable the anion; #The extent of conjugation of the anion. Resonance effects can stabilize the anion. This is especially true when the anion is stabilized as a result of aromaticity. A carbanion is a reactive intermediate and is encountered in organic chemistry for instance in the E1cB elimination reaction and in organometallic chemistry in for instance a Grignard reaction or in alkyl lithium chemistry. Stable carbanions do however exist. In 1984 Olmstead presented the lithium crown ether salt of the triphenylmethyl carbanion from triphenylmethane, n-butyllithium and 12-crown-4 at low temperatures:〔''The isolation and x-ray structures of lithium crown ether salts of the free phenyl carbanions ()- and ()-'' Marilyn M. Olmstead, Philip P. Power; J. Am. Chem. Soc.; 1985; 107(7); 2174-2175. 〕 Adding ''n''-butyllithium to triphenylmethane in THF at low temperatures followed by 12-crown-4 results in a red solution and the salt complex precipitates at −20 °C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2°. This propeller shape is less pronounced with a tetramethylammonium counterion . One tool for the detection of carbanions in solution is proton NMR.〔''A Simple and Convenient Method for Generation and NMR Observation of Stable Carbanions.'' Hamid S. Kasmai Journal of Chemical Education • Vol. 76 No. 6 June 1999〕 A spectrum of cyclopentadiene in DMSO shows four vinylic protons at 6.5 ppm and two methylene bridge protons at 3 ppm whereas the cyclopentadienyl anion has a single resonance at 5.50 ppm. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbanion」の詳細全文を読む スポンサード リンク
|