Meisenheimer complex
As Meisenheimer complexes of aromatic nitro compounds are labeled with nucleophiles in organic chemistry in the narrow sense adducts. It is also called Meisenheimer salts.
Discovery
Jackson and Boos (Harvard College, USA) discovered the late 19th century, that the action of sodium alkoxides to the almost colorless 2,4,6- trinitrochlorobenzene ( picryl chloride ) red salts arise. With sodium methoxide / methanol they got a connection for which they proposed the structural formula shown in the middle of the formula scheme. Your German name should be 3.5 -dinitro -4.4 -dimethoxy- chinolnitrosaures sodium. Chasing the salt with dilute mineral acid, 2,4,6- Trinitroanisol (right formula ) was formed.
Presumably, the researchers had expected that by replacing the chlorine substituents of Pikrylchlorids against the same methoxy 2,4,6- Trinitroanisol ( " Methylpikrat " ) would be created.
In the chemical laboratory of the Royal Academy of Sciences in Munich Jakob Meisenheimer examined at the beginning of the 20th century reactions of aromatic nitro compounds. Among other things, he established the suspected Alkylpikrate by letting alkyl iodides with the silver salt of picric acid react. In 1902, he announced that in the implementation of Alkylpikraten with " alcoholic potash " salts arise that stand out by their deep red color. Meisenheimer could confirm the now still valid constitution. In today's language Alkylpikrate are 1- alkoxy-2 ,4,6- trinitrobenzole, " alcoholic potash ", a solution of potassium hydroxide in the corresponding alcohol.
For this knowledge was crucial that the same salt originated from both Methylpikrat and potassium ethoxide as well as from Ethylpikrat and potassium (see formula picture).
The " Meisenheimer salts" decomposed when admitted dilute aqueous sulfuric acid. This from the " ethoxy -methoxy- salt " was both Ethylpikrat and Methylpikrat, consistent with Meis -heimer's or Jackson's constitutional formula.
Structure
Regarding the structure of the salts, the proposal of Jackson and Meisenheimer reality came very close. While we now expect that the negative charge is delocalized ( formula below left), but Meisenheimer chose the formula of a nitronate. X-ray analysis of the " 1,1- dimethoxy- complex " showed that the binding C-4/NO2 over C - 2 and C -6, and the nitro nitrogen is significantly shortened.
The bond C -2 and C -3 and C-5 and C-6 are shorter than the bonds C-3/C-4 or C-4/C-5. This shows that the para- nitro group to the most part of the negative charge takes, in terms of the average structural formula boundary in the image. Quantum chemical calculations (SCF method) are in good agreement with the results of X-ray structural analysis.
The intermediates occurring during the SNAr mechanism (Intermediate ) with tetragonal (sp3 -hybridized ) carbon atom are sometimes referred to as Meisenheimer complexes in the broader sense. After the discovery of history, it would be justified to call it the American chemist, that is, to speak of Jackson - Meisenheimer complexes.
Importance
The discovery stimulated further studies of benzenoid compounds with acceptor substituents. They culminated in the realization that compounds of this type by a nucleophilic association / dissociation mechanism - can react (also addition-elimination mechanism called SNAr ) as described in Article Nucleophilic aromatic substitution is discussed. Thus, for example, 1- chloro-2 ,4,6- trinitrobenzene ( picryl chloride ) react with many nucleophiles.