Carboxylation Groups, esters

Campanelli et al. [13] obtained the initial hydrolysis rate constant of molten PET around 250°C from the concentrations of carboxyl groups, ester bonds and water after the reaction. The results are shown in Table 25.5, together with the results at 100°C [14] and 186°C [15] for solid PET. 

Carbonyl group Ketones Aldehydes Carboxylic acids Carboxyl group Ester Polymer... 

C-Terminal carboxyl group. Esters of 3-mercaptopropanoyl-L-proline are 100-200 fold less potent than their carboxylic acid derivatives (see Table 6.9), indicating enhanced binding by a free C-terminal carboxyl group. 

Polymer with carboxylate group (ester linkage) T-C-OrI... 

CHEMICAL REACTIONS Reactions of the carboxyl group Esters formation and reactions Acyl chlorides formation and reactions Decarboxylation Miscellaneous reactions Reactions of the pyrone carbonyl group Reactions of the pyrone ring Reactions of the benzene ring Conversion to other heterocycles... 

Reactions of the Carboxyl Group. Ester and amide formation may be mentioned first one H atom of an alcohol or an amine is replaced by the acyl group. But C-acylation ( ester condensation in organic chemistry) is also known, along with the reverse reaction, thioclastic cleavage ... 

Although two isomeric esters are theoretically possible, the main product is the 2-ester (formulated above) traces of the isomeric l-ester are eliminated during purification. These derivatives possess a free carboxyl group their equivalent weights may therefore be determined by titration with standard alkali and thus serve as an additional check upon the identity of the compound. 

In synthetic target molecules esters, lactones, amides, and lactams are the most common carboxylic acid derivatives. In order to synthesize them from carboxylic acids one has generally to produce an activated acid derivative, and an enormous variety of activating reagents is known, mostly developed for peptide syntheses (M. Bodanszky, 1976). In actual syntheses of complex esters and amides, however, only a small selection of these remedies is used, and we shall mention only generally applicable methods. The classic means of activating carboxyl groups arc the acyl azide method of Curtius and the acyl chloride method of Emil Fischer. 

The blocking and deblocking of carboxyl groups occurs by reactions similar to those described for hydroxyl and amino groups. The most important protected derivatives are /-butyl, benzyl, and methyl esters. These may be cleaved in this order by trifluoroacetic acid, hydrogenolysis, and strong acid or base (J.F.W. McOmie, 1973). 2,2,2-Trihaloethyl esters are cleaved electro-lytically (M.F. Semmelhack, 1972) or by zinc in acetic acid like the Tbeoc- and Tceoc-protected hydroxyl and amino groups. 

Many compounds contain more than one functional group Prostaglandin Ei a hormone that regulates the relaxation of smooth muscles con tains two different kinds of carbonyl groups Classify each one (aldehyde ketone carboxylic acid ester amide acyl chloride or acid anhydride) Identify the most acidic proton in prostaglandin Ei and use Table 1 7 to estimate its pK ... 

Section 19 15 An intramolecular esterification can occur when a molecule contains both a hydroxyl and a carboxyl group Cyclic esters are called lactones and are most stable when the nng is five or six membered... 

Conversions of acid anhydrides to other carboxylic acid derivatives are illustrated m Table 20 2 Because a more highly stabilized carbonyl group must result m order for nucleophilic acyl substitution to be effective acid anhydrides are readily converted to carboxylic acids esters and amides but not to acyl chlorides... 

Hydrolysis removes Ihe acelyl group from nilrogen and converls Ihe Iwo ester funclions lo carboxyl groups Decarboxylalion gives Ihe desired producl... 

Ester formation is a typical reaction of the carboxyl group... 

Carboxyl groups of ammo acids and peptides are normally protected as esters Methyl and ethyl esters are prepared by Fischer esterification Deprotection of methyl and ethyl esters is accomplished by hydrolysis m base Benzyl esters are a popular choice because they can also be removed by hydrogenolysis Thus a synthetic peptide protected at both... 

Section 27 16 Carboxyl groups are normally protected as benzyl methyl or ethyl esters Hydrolysis m dilute base is normally used to deprotect methyl and ethyl esters Benzyl protecting groups are removed by hydrogenolysis... 

Carboxylic acid esters Carboxylic acid groups... 

Pectins are generally classed according to their ester content as high methoxyl pectins (>50% of the carboxyl groups esterified) or low methoxyl pectins (<50% of carboxyl groups esterified) (pectic acid, methyl ester [9049-34-1]). Low methoxyl pectins, like algins, require calcium for gelation. 

The chemistry of benzenecarboxyUc acids generally is the same as that of other carboxyUc acids, which can be converted into esters, salts, acid chlorides, and anhydrides. Each carboxyl group can react separately, so that compounds in which carboxyl groups are converted into different derivatives can be prepared. Because there are aromatic hydrogens available in most of these acids, they also undergo reactions characteristic of the benzene nucleus. Some of the anhydrides have characteristic reactions. 

Reactions of the Carboxyl Groups. Carboxyl groups in the ortho position spontaneously form a strainless ftve-membered ring when heated to give anhydrides as shown for (8). Salts and esters (4) are readily formed as shown for (4) and (5), respectively. 

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