Carbonyl compound , acidity general reactions

In general, the xanthenes are synthesized by the reaction of two moles of a nucleophilic / -substituted phenol (10) with an electrophilic carbonyl compound (11), the reaction occurring most readily with an acid catalyst at temperatures of 100—200°C. 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

The hydration reaction of alkynes leading to carbonyl compounds is generally carried out in dilute acidic conditions with mercuric 1on salts (often the sulfate) as catalysts (ref. 5). Only very reactive alkynes (phenylacety-lene and derivatives) can be hydrated in strong acidic conditions (HgSO ) without mercury salts (ref. 6). Mercury exchanged or impregnated sulfonic resins have also been used in such reactions (ref. 7). Nevertheless, the loss of the catalyst during the reaction and environmental problems due to the use of mercury make this reaction method not as convenient as it should be for the preparation of carbonyl compounds. 

Phenylhydrazones of aromatic carbonyl compounds are generally reducible in acid solution in a four-electron reduction, which leads to the formation of aniline and another amine in alkaline solutions only a, j6-unsaturated phenylhydrazones are reducible, and the reduction leads in a two-electron reaction to a saturated phenylhydrazone [1]. Scheme 1 shows the reduction route in aqueous solution that has been suggested [1,58]. 

Review. Seebach and Corey have published a general paper on the preparation and metalation of 1,3-dithianes and examples of the reaction of 2-lithio-l,3-dithianes with electrophilic reagents (alkyl halides, carbonyl compounds, acids, and oxides). The value of these sulfur-stabilized anionic reagents is that they are equivalent to acyl anions (a), in which the normal polarity of the carbonyl group is reversed (reversible umpolung). 

The addition of nitrogen bases to carbonyl compounds has been found to occur through a general acid-catalyzed or an apparently uncatalyzed pathway depending on the basicity of the nucleophile. The addition of weak bases such as aromatic amines and semicarba-zide to carbonyl compounds exhibits general acid catalysis. A substantial body of kinetic evidence and chemical intuition now indicates that the addition reaction proceeds via a transition state (equation 30) where HB represents a general acid and B a general base. 

Equation (40) is known as a transimination or a trans-SchifSza-tion reaction. It probably proceeds via a two-step mechanism involving a m-diamine intermediate, analogous to the carbinolamine intermediate observed in imine formation fix>m carbonyl compounds. The transimination reactions of Schiff bases derived from either aliphatic or aromatic amines with semicarbazide, hydroxylamine or methoxyamine, of oximes with semicarbazide and of semicarbazones with hydroxylamine all appear general base catalyzed in terms of one protonated and one free base reactant or general acid catalyzed in terms of two reactants of free base. These results may be accounted for by a nearly symmetrical mechanism for transimination (equations 41 and 42). Simple proton transfers are considered to be rapid and... 

Dicationic Carbonyl Compound In general, stable carbocations do not react with non activated benzene. In 1991, Shudo et al demonstrated that multiply positively charged (dicationic and tricationic) species have sufficient electrophilicity toward benzene. In the presence of super Bronsted acids (TfOH, TfOH-SbFs), Friedel-Crafts type reaction of benzene was carried out with cinnamaldehyde, its derivatives, and nitromethane (Scheme 2.13) [34]. 

Oxaziridines are powerful oxidizing agents. Free halogen is formed from hydrobromic acid (B-67MI50800). Reduction by iodide in acidic media generally yields a carbonyl compound, an amine and two equivalents of iodine from an oxaziridine (1). With 2-alkyl-, 2-acyl and with N-unsubstituted oxaziridines the reaction proceeds practically quantitatively and has been used in characterization. Owing to fast competing reactions, iodide reduction of 2-aryloxaziridines does not proceed quantitatively but may serve as a hint to their presence. 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

For the reaction of carbonyl compounds with conjugated dienes two mechanistic pathways have generally been taken into account when Lewis acid-catalyzed reactions are considered ... 

If the initially formed /3-hydroxy carbonyl compound 3 still has an a-hydrogen, a subsequent elimination of water can take place, leading to an o ,/3-unsaturated aldehyde or ketone 4. In some cases the dehydration occurs already under the aldol reaction conditions in general it can be carried out by heating in the presence of acid ... 

Generally the desired substituted carbonyl compound 3 is obtained after hydrolytic workup under acidic conditions. With simple alkyl halides an irreversible A-alkylation may take place as a side-reaction to give a quaternary ammonium salt 7 ... 

As a general rule, nucleophilic addition reactions are characteristic only of aldehydes and ketones, not of carboxylic acid derivatives. The reason for the difference is structural. As discussed previously in A Preview of Carbonyl Compounds and shown in Figure 19.14, the tetrahedral intermediate produced by addition of a nucleophile to a carboxylic acid derivative can eliminate a leaving group, leading to a net nucleophilic acyl substitution reaction. The tetrahedral intermediate... 

Decarboxylation is not a general reaction of carboxylic acids. Rather, it is unique to compounds that have a second carbonyl group two atoms away from the —COoH. That is, only substituted malonic acids and /3-keto acids undergo loss of CC>2 on heating. The decarboxylation reaction occurs by a cyclic mechanism and involves initial formation of an enol, thereby accounting for the need to have a second carbonyl group appropriately positioned. 

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