Acid catalysis esterification

In typical processes, the gaseous effluent from the second-stage oxidation is cooled and fed to an absorber to isolate the MAA as a 20—40% aqueous solution. The MAA may then be concentrated by extraction into a suitable organic solvent such as butyl acetate, toluene, or dibutyl ketone. Azeotropic dehydration and solvent recovery, followed by fractional distillation, is used to obtain the pure product. Water, solvent, and low boiling by-products are removed in a first-stage column. The column bottoms are then fed to a second column where MAA is taken overhead. Esterification to MMA or other esters is readily achieved using acid catalysis. 

In the esterification of organic acids with alcohols, it has been shown that in most cases under acid catalysis, the union is between acyl and alkoxy groups. Acid hydrolysis of acetoxysuccinic acid gives malic acid with retention of configuration at the asymmetric carbon atom (11) ... 

The traditional catalyst used for esterification of acids to methyl esters is sulfuric acid. Homogeneous sulfuric acid catalysis has many downsides. When using sulfuric acid, much capital expense is required for Hastalloy and/or other specialty metals of construction. Homogeneous catalysis results in the contamination of the product by sulfur containing species. Therefore, neutralization and removal of acid is required to meet biodiesel specifications and to protect the downstream transesterification reactor. Inevitably, when using sulfuric acid, organic sulfur compounds will be produced. These products will cause the resultant biodiesel to fail specification tests. 

It is difficult to effect attack on the carbonyl carbon atom of RC02H, (171), with nucleophiles of the general type Y , as they commonly remove proton instead, and the resultant RCO20 is then insusceptible to nucleophilic attack. Weaker nucleophiles of the form YH, e.g. ROH, do not suffer this inability, but their reactions with the relatively unreactive carbonyl carbon atom of RC02H are slow. The carbonyl character may be enhanced by protonation, however, i.e. by acid catalysis in, for example, esterification [(171) — (172)] ... 

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

DMT can be made from crude TA or from />-xylene directly. Esterification of TA with methanol occurs under sulfiiric acid catalysis. Direct oxidation of /7-xylene with methanol present utilizes copper and manganese salt catalysis. 

Perhaps, a more important reason for the little research in this particular area is the slow reaction rate associated with acid catalysis in general. However, the ability of solid acids to catalyze both esterification and transesterification reactions simultaneously and the possibility for employing catalysts that are reusable and green, meaning that they do not pose a great environmental threat, are attractive aspects that make the study of these materials imperative. 

Following extraction/cleanup, quinoxaline-2-carboxylic acid can be detected by electron capture, or mass spectrometric techniques, after gas chromatographic separation on capillary or conventional columns. A prerequisite of quin-oxaline-2-carboxylic acid analysis by gas chromatography is the derivatization of the molecule by means of esterification. Esterification has been accomplished with methanol (419, 420, 422), ethanol (421), or propanol (423) under sulfuric acid catalysis. Further purification of the alkyl ester derivative with solid-phase extraction on a silica gel column (422), thin-layer chromatography on silica gel plate (420), or liquid chromatography on Hypersil-ODS, 3 m, column (423), has been reported. 

The original Garner preparation3 of 5 involves the conversion of serine into the protected methyl ester 3 and controlled reduction of the latter by DIBAL. The reaction sequence employed for the preparation of 3 involves the protection of the amino acid as N-Boc derivative using di-tert-butyl dicarbonate, esterification with methyl iodide or diazomethane, and acetonization with 2,2-dimethoxypropane under acid catalysis. The N-Boc methyl serinate and the ester 3 require purification by vacuum distillation or chromatography. In a modification to this procedure reported by McKillop,2 the esterification reaction of serine is carried out first by methanol/acetyl chloride. The resulting ester is then converted into the N-Boc derivative 2 with di-tert-butyl dicarbonate and the latter transformed into 3 by acetonization. This procedure avoids... 

The size of the cavity plays a crucial role on the selectivity of the reaction. For example, when the esterification was performed with p-sulfonatocalix [4]arenes, the kcaiixarene/k HBS value for histidine was indeed increased from 24 to 86. 2H NMR studies supported the formation of an inclusion complex of caiixarenes with basic amino acids, and the reaction followed Michaelis-Menten kinetics. The specific rate enhancement observed for basic amino acids His 33, Lys 34 and Arg 35 is the result of a stabilization by the anionic sulfonate groups of the cationic intermediate, which can undergo esterification (see Scheme 13.8). In contrast, the formation of Phe 36, might proceed via simple acid catalysis. 

As ethanol and methanol are common laboratory solvents, their application in extraction and reaction chemistry is not be discussed at length here details on many procedures using these solvents can be found in chemistry textbooks and the primary literature. However, exciting new procedures using acid catalysis in aqueous ethanol for the esterification of platform molecules have recently been reported.This reaction also highlights the reactivity of alcohols, as ethanol is one of the substrates in the reaction (Figure 5.4). It is likely that ethanol and water will continue to play a prominent role as solvents in the new transformation chemistries being developed. 

Substituted oxazolidin-5-one derivatives, which are prepared from N -protected a-annino dicarboxyhc acids and paraformaldehyde, are employed for dual protection of the a-annino and a-carboxy groups in the synthesis of P-aspartyl and y-glutamyl esters (Scheme 4).Py For this purpose the oxazolidinone derivatives are synthesized by treatment of the Z amino acids with paraformaldehyde in a nnixture of acetic anhydride, acetic acid, and traces of thionyl chloride or by azeotropic distillation of the Z amino acids with paraformaldehyde and 4-toluenesulfonic acid in benzene. The resulting heterocychc compounds are readily converted into the tert-butyl esters with isobutene under acid catalysis. Esterification is achieved with tert-butyl bromidet or with Boc-F.P l Finally, the oxazolidinone ring is opened by alkaline hydrolysis or catalytic hydrogenolysis to yield the tert-butyl esters. 

The simplest esterification reagents are the corresponding alcohols ROH themselves. Different esters have been used as the analytical derivatives of carboxylic adds Me, Et, Pr, iso-Pr, isomeric Bu (excluding tert-B i esters, owing to their poorer synthetic yields), and so forth. This method requires the use of excess of dry alcohol and acid catalysis by BCI3, BF3, CH3COCI, SOCI2,... 

The esterification of carboxylic acids can be provided, also, by synthetic equivalents of alcanols acetals RCH(0R )2 (with acid catalysis), ortho-esters RC(0R )3 (acid catalysis), and dialkylcarbonates C0(0R)2 (base catalysis). The series of bifunctional reagents of this type [dimethylformamide dialkylac-etals (CH3)2N-CH(0R)2] is commercially available. Besides the esterification of carboxyl groups, these compounds react with primary amino groups and, thus, are used for GC analysis of amino acids (see the entry Derivatization of Amines, Amino Acids, and Related Compounds for GC Analysis ) ... 

The widely accepted mechanism of the acid-catalyzed esterification is designated as the Aac2 process (A = acid catalysis, AC = acyl transfer, 2 = bimolecular). As shown in Schemes la and lb the rate-deter-... 

The Fischer esterification is one of the oldest and most reliable methods of converting an acid to an ester. The reaction requires acid catalysis, elevated temperatures, and frequently the removal of the water by-product. As long as the remainder of the starting acid can withstand these requirements, the Fischer esterification deserves consideration. COgH COgMe... 

Esters react by both acid and nucleophile initiated mechanisms. Hydrolysis of esters by acid catalysis is exactly the reverse of the mechanism for the acid-catalyzed esterification of a carboxylic acid. Base-catalyzed hydrolysis of esters is called saponification. Hydroxide attacks to form a tetrahedral intermediate. Loss of alkoxide ion then occurs. The alkoxide neutralizes the resulting carboxylic acid to form the salt. 

Another interesting HMF-derived compound is levulinic acid formed together with formic acid by solid acid catalysis. A one-pot cascade route from c.g. inulin seems feasible. Manzer et al. [37] give examples in which an esterification step with an alkene is coupled as well. The... 

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