Kolbe’s reaction

The presence of -OH group In phenols activates the aromatic ring towards electrophilic substitution and directs the Incoming group to ortho and para positions due to resonance effect. Reimer-Tiemann reaction of phenol 5delds sallcylaldehyde. In presence of sodium hydroxide, phenol generates phenoxlde Ion which Is even more reactive than phenol. Thus, In alkaline medium, phenol undergoes Kolbe s reaction. 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

Nitromethane is the most easily accessible aliphatic nitro-com-pound Kolbe s method of preparation is much less satisfactory when applied to higher members of the series. The course of the reaction is clear, and the reasons for the decomposition which takes place are similar... 

Since, in Kolbe s synthesis, as here described, the mono-sodium salicylate reacts to some extent with unchanged sodium phenoxide, producing the di-sodium salt, part of the phenol is liberated and excluded from the reaction. The reaction proceeds to completion if the sodium phenoxide is heated to about 150° for a long time, with carbon dioxide under pressure in the autoclave. This is the technical method of Schmitt. 

Sandmeyer s synthesis of aromatic nitriles is far more elegant than the removal of water from the ammonium salts of carboxylic acids, which latter reaction is also applicable to benzene derivatives. In particular, the former synthesis permits of the preparation of carboxylic acids via the nitriles, and so provides a complete substitute for Kolbe s synthesis (alkyl halide and potassium cyanide), which is inapplicable to aromatic compounds. The simplest example is the conversion of aniline into benzoic add. The converse transformation is Hofmann s degradation (benzamide aniline, see p. 152). 

In many cases both Kolbe and non-Kolbe products are isolated from a reaction. Carboxylic acids with an a-alkyl substituent show a pronounced dual behaviour. In these cases, an increase in the acid concentration improves the yield of the Kolbe product. An example of the effect of increased substrate concentration is given in Kolbe s classical paper [47] where 2-methylbutyric acid in high concentration affords mostly a dimethylbexane whereas more recent workers [64], using more dilute solutions, obtained both this hydrocarbon and butan-2-ol. Some quantitative data is available (Table 9.2) for the products from oxidation of cyclohexanecar-boxylic acids to show the extent of Kolbe versus non-Kolbe reactions. The range of products is here increased through hydrogen atom abstraction by radical intermediates in the Kolbe reaction, which leads to some of the monomer hydrocarbon... 

In connection with his investigation of ketonic acids, Hofer 3 has used the electrosynthetic reaction, previously discovered with Miller,4 which consists in electrolyzing potassium salts of organic acids in mixture with potassium acetate and other lower fatty acids. The general nature of the reaction is that the two anions unite, as in Kolbe s synthesis, carbonic acid being split off, e.g.,... 

Another example of a Kolbe dimerization reaction is found in Corey s synthesis of a-onoceradiene (14), y6-onceradiene (15) and (+)-pentacyclosqualene (16) each is derived from either the ammonium salt 18, or the acetoxy ammonium salt derived in situ from 20 in the manner portrayed in Eqs. (4) and (5) [6,7]. 

B.S. Green and co-workers developed an improved preparation of the clathrate host compound tri-o-thymotide (TOT) and other trisalicylide derivatives. The synthesis began with the preparation of orfho-thymotic acid from thymol using the Kolbe-Schmitt reaction. The authors found that the yield of the product was dramatically increased when the reactants, solvents, and reagents were dried before use. Thus, thymol was dissolved in dry xylene, sodium metal was added and the temperature was kept at 130 °C for 20h in a dry carbon dioxide atmosphere. The desired carboxylated product was isolated in good yield. Finally, cyclodehydration with POCI3 afforded TOT in almost quantitative yield. 

Markovic, Z., Engelbrecht, J. P., Markovic, S. Theoretical study of the Kolbe-Schmitt reaction mechanism. Z Naturforsch., A Phys. Sci. 

The keto tautomers of monohydric phenols are frequently invoked as reactive intermediates in many reactions, such as the Reimer-Tiemann and Kolbe-Schmitt reactions , electrophilic substitution (e.g. bromination i) s well as the photo-Fries rearrangement (see Section IV.D and also Refs. 16-18 cited in Reference 7). In certain cases the keto... 

Until recently there were few cases in which an alkali nitrite could replace silver nitrite in the synthesis of nitro compounds. Such a case was Kolbe s nitroalkane synthesis, in which an alkali salt of a 2-haloalkanoic acid was treated with a boiling aqueous solution of an alkali nitrite the 2-nitroalkanoic acid formed lost carbon dioxide under the reaction conditions and the nitroalkane distilled over in the steam. The Kolbe synthesis is, however, of practical use only for the preparation of nitromethane (38% yield from chloro-acetic acid648) and nitroethane (about 50% yield from 2-bromopropionic acid649) it is substantially valueless for higher fatty acids.649,650... 

Salicylic acid is manufactured by what is known as Kolbe s synthesis, which consists in heating the sodium salt of phenol with carbon dioxide under pressure at 130°. The steps involved in the reaction are those indicated below —... 

Soon thereafter, Kolbe s comrade-in-arms, Frankland, submitted for publication a landmark paper that, similar to Brodie s and Hofmann s work, pointed toward serious anomalies in the copula theory. As a master (indeed, the founder) of organometallic chemistry, Frankland adduced examples of reactions that indicated that tin, zinc, mercury, antimony, arsenic, phosphorus, and nitrogen exhibit fixed maximum combining capacities with other atoms or radicals. Arsenic and antimony, for instance, seem to combine only with three or with five equivalents of other atoms or radicals. If the maximum capacity is reached, then only substitution, not addition, of other components can occur. Frankland had turned he now allied himself with the ammonia type theory of Hofmann and Wurtz, for the semimetals antimony and arsenic seemed to follow exactly the pattern established by the new organic nitrogen compounds of the latter chemists. The theory of copulas, he declared, could no longer be maintained. ... 

RCOO"Na+ + Na+OH- - Na2C03 + RH Other methods include the Wurtz reaction and Kolbe s method. Generally the alkanes are fairly unreactive. They form haloalkanes with halogens when irradiated with ultraviolet radiation. 

Kolbe claimed that his formulae represented real types, as contrasted with Gerhardt s imaginary types, but Gerhardt said that Kolbe s attribution of an absolute value to formulae was contrary to all logic . Acetic acid may be obtained from aldehyde or alcohol by oxidation, or decomposed into marsh gas and carbonic acid by heat the formula depends on the reaction. 

Couper says carbon has two highly distinctive properties (i) it combines with equal numbers of equivalents of hydrogen, chlorine, oxygen, sulphur, etc. (2) it enters into chemical union with itself. These two properties, in my opinion explain all that is characteristic of organic chemistry. The second property is, so far as I am aware, here signalised for the first time. When hydrogen and oxygen are replaced by equivalents of chlorine, etc., the carbon remains united to carbon. Substitution reactions prove beyond doubt that carbon enters into chemical union with carbon, and that in the most stable manner. Couper s formulae were used, but without bonds, by Wurtz. Couper s formula for alcohol may be compared with Kolbe s and the modern formula ... 

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