Mechanism phenolic acid esters formed

Imidazolides of aromatic sulfonic acids react much more slowly in alcoholysis reactions than the carboxylic acid imidazolides. Although the reaction with phenols is quantitative when a melt is heated to 100 °C for several hours, with alcohols under these conditions only very slight alcoholysis is observed. In the presence of 0.05 equivalents (catalytic amount) of sodium ethoxide, imidazole sodium, of NaNH2, however, imidazolides of sulfonic acids react with alcohols almost quantitatively and exothermically at room temperature in a very short time to form sulfonic acid esters (sulfonates). (If the ratio of sulfonic acid imidazolide to alcoholate is 1 2, ethers are formed see Chapter 17). The mechanism of catalysis by base corresponds to that operative in the synthesis of carboxylic esters by the imidazolide method. Because of the more pronounced nucleophilic character of alkoxide ions, sulfonates can also be prepared in good yield by alcoholysis of their imidazolides in the presence of hydroxide ions i.e., with alcoholic sodium hydroxide. 45 Examples of syntheses of sulfonates are presented below. 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

The mechanism of carbodiimide-mediated amide formation is outlined in Figure 13.4. The first intermediate formed during the reaction of a carbodiimide with a carboxylic acid (or an ammonium carboxylate [56]) is an O-acylisourea [1]. Under acidic conditions [57,58], this intermediate reacts with acids to yield anhydrides, and with alcohols, phenols, HOBt, or other compounds containing hydroxyl groups, to yield the corresponding esters. The carbodiimide is thereby transformed into a urea. 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenolic compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcoholic media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

When treated with acid chlorides and acid anhydrides, phenols form esters. Under Friedel-Crafts conditions, phenolic esters undergo a Fries rearrangement in which the acyl group migrates to the 2- and 4-positions. Thus, treatment of the ester 11 with aluminium chloride in an inert solvent gives a mixture of 2- and 4-hydroxyacetophenones [(hydroxy-phenyl)ethanones] C-acylation has occurred (Scheme 4.7). The two isomers are separable and this is a useful method for the production of phenolic ketones. The mechanism remains uncertain, but it would appear that the acylium ion (RCO" ) is generated and that a Friedel-Crafts mechanism operates. 

The reaction of phenols with nitrous acid gives the ortho- and para-nitroso products, which are formed through a neutral dienone intermediate, the proton loss from the latter being the rate-limiting step" " . It has been shown that the nitrous acid can act as a catalyst for the formation of the nitro derivatives. Thus the conventional preparation of nitro compounds by the oxidation of nitroso compounds may be replaced by methods using an electron-transfer pathway in certain cases. In the latter method, the phenoxide reacts with nitrosonium ion to give the phenoxy radical and nitric oxide radical. The nitric oxide radical is in equilibrium with the nitronium radical by reaction with nitronium ion. The reaction of the phenoxy radical with the nitroninm radical resnlts in the formation of the ortho- and para-mixo prodncts" . Leis and coworkers carried ont kinetic stndies on the reaction of phenolate ions with alkyl nitrites and fonnd that the initially formed product is the 0-nitrite ester, which evolves by a complex mechanism to give the ortho-and the para-nitro products". ... 

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