Hydrolysis, of amides

Amides are the least reactive carboxylic acid derivative, and the only nucleophilic acyl substitution reaction they undergo is hydrolysis. Amides are fairly stable in water, but the amide bond is cleaved on heating in the presence of strong acids or bases. Nominally, this cleavage produces an amine and a carboxylic acid. In acid, however, the amine is proto-nated, giving an ammonium ion  

In base the carboxylic acid is deprotonated, giving a carboxylate ion 0 O 

The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible in both cases. The amine product is protonated in acid the carboxylic acid is deprotonated in base. 

Mechanistically, amide hydrolysis is similar to the hydrolysis of other carboxylic acid derivatives. The mechanism of hydrolysis in acid is presented in Mechanism 19.4. It proceeds in two stages a tetrahedral intermediate is formed in the first stage and dissociates in the second. 

The hydrolysis of amide bonds in protein and simpler entities by enzymes has been known for many years and explored extensively. The degradation of protein by a proteinase such as subtilisin is the basis of action of biological washing powders. Obviously the commercial importance of 

The lipase is lyophilized (most of the water is removed from around the protein) before use in such a system, to avoid a pronounced back-reaction (hydrolysis). If the enzyme shows different catalytic properties at different pH values, then these differences will be reflected in the reactions taking place in the organic solvent. The enzyme memorizes the pH of the solution from which it was lyophilized  

Similarly, an amine and a carboxylic acid can be condensed together to form an amide using a hydrolase enzyme working backwards . The method has been used widely to prepare small peptides. The preparation of the sub-sequence of dynorphin (33) was accomplished using a medley of enzymes, namely a-chymotrypsin (a-C), papain (P) and trypsin (T) 

It has also been demonstrated that lipases can be used to make amide bonds. Thus both porcine pancreatic lipase and Candida cylindracea lipase, in an organic solvent, proved to be effective catalysts for the synthesis of a wide range of dipeptides. 

KINETIC METHODS USED IN STUDIES OF THE SOLVOLYSIS OF ACID HALIDES 

The hydrolyses of carboxylic acid amides, RCONR R , are catalysed by both acids and bases, and generally do not take place in neutral media. Ost-wald176 made the first quantitative measurements of the catalytic activity of 

Kinetic investigations of amide hydrolysis showed that the rate of hydrolysis in basic media is proportional to the concentration of amide and hydroxide ion. Similarly, early work180-183 on the acid-catalysed hydrolysis of amides showed that the rate of acidic hydrolysis is, in general, proportional to the concentration of amide and hydroxonium ion. In several acidic hydrolyses, however, a maximum is observed in the pH-rate profile at 3-6 pH units, a phenomenon first reported by Benrath184 and since supported by other workers185 190. This behaviour of amides is in contrast to the hydrolysis of nitriles whose rate constant increases continuously with the hydrogen ion concentration191. 

Amides are reasonably strong bases, with pKa s of the order of — 1 to — 3189, and therefore are appreciably protonated in solutions of mineral acids192-194. The rate maxima are obtained in strongly acid conditions and the hydrolysis may be formulated 

A reasonably complete discussion of the role of acid in amide hydrolysis requires the inclusion of the Hammett acidity function (//0)l99,200i and the controversial topic of the significance of this function in the determination of mechanism201,202. 

In the hydrolysis reaction of an amide in the presence of acids or bases, the products formed are carboxylic acid and amine. This is a nucleophilic acyl substitution reaction. 

Symmetric starting materials have often been applied, not only in the form of symmetric diacids. The strategy has often been used to prepare chiral diols [65-67] this has been reviewed very recently [68]. 

Due to the delocalization of electrons the amide group is normally planar and is significantly more stable than esters [72]. Nonetheless amides can be hydrolysed enzymatically under very mild conditions. Initially it might be expected that only enzymes that were evolved for this function by nature could hydrolyse this stable bond, but by now many examples are known where lipases and esterases hydrolyse amides, too [2, 34, 73]. A recent review discusses the mechanisms, modes of action and enantioselectivities of all the important enzymes [74]. 

The most prominent green example, the regioselective hydrolysis of an amide on an industrial scale, is the production of penicillin. PenG acylase selectively hydrolyses the more stable amide bond, leaving the /Mactam ring intact [75, 76]. For a full discussion of this example see Chapter 1 (Fig. 1.37) and Chapter 8. Since the starting material is already enantiopure the enzyme induces no stereoinformation. In other industrial processes the enantioselectivity of the enzymes is used. This is, in particular, the case in the production of natural and unnatural amino acids. 

In acid, however, the amine is protonated, giving an ammonium ion, R2NH2 O OH 

The amide is activated toward nucleophilic attack by protonation of its carbonyl oxygen. The cation produced in this step is stabilized by resonance involving the nitrogen lone pair and is more stable than the intermediate in which the amide nitrogen is protonated. 

In practice, both steps may be combined in a single operation by simply heating a carboxylic acid and an amine together  

These thermal methods for preparing amides are limited in their generality. Most often amides are prepared in the laboratory from acyl chlorides, acid anhydrides, or esters, and these are the methods that you should apply to solving synthetic problems. 

Slebocka-Tilk, H. Bennet, A. J. Hogg, H. J. Brown, R. S. /. Am. Chem. Soc. 1991,113,1288. Slebocka-Tilk, H. Brown, R. S. Olekszyk, J. /. Am. Chem. Soc. 1987,109,4620. The compounds studied in this investigation were acetanilide and N-cyclohexylacetamide. 

Calculated energy profiles for the hydrolysis of formamide in the gas phase ( ) and in aqueous solution (o). (Adapted from reference 215.) 

Alternative pathways for the formation of the tetrahedral intermediate in the basic hydrolysis of formamide. 

Kinetic isotope effects observed in the acidic hydrolysis of formamide. 

Mechanism proposed for the hydrolysis of formamide in acidic solution. 

---

As an example, experimental kinetic data on the hydrolysis of amides under basic conditions as well as under acid catalysis were correlated with quantitative data on charge distribution and the resonance effect [13]. Thus, the values on the free energy of activation, AG , for the acid catalyzed hydrolysis of amides could be modeled quite well by Eq. (5)... 

In base the tetrahedral intermediate is formed m a manner analogous to that pro posed for ester saponification Steps 1 and 2 m Figure 20 8 show the formation of the tetrahedral intermediate m the basic hydrolysis of amides In step 3 the basic ammo group of the tetrahedral intermediate abstracts a proton from water and m step 4 the derived ammonium ion dissociates Conversion of the carboxylic acid to its corresponding carboxylate anion m step 5 completes the process and renders the overall reaction irreversible... 

Nitriles are classified as carboxylic acid derivafives because fhey are convened fo car boxylic acids on hydrolysis The condifions required are similar fo fhose for fhe hydrol ysis of amides namely healing m aqueous acid or base for several hours Like fhe hydrolysis of amides nilrile hydrolysis is irreversible m fhe presence of acids or bases Acid hydrolysis yields ammonium ion and a carboxylic acid... 

We already discussed bolh Ihe acidic and basic hydrolysis of amides (see Seclion 20 17) All lhal remains to complete Ihe mechamslic piclure of nilrile hydrolysis is to examine Ihe conversion of Ihe nilnle to Ihe conespondmg amide... 

Insight into the factors that govern breakdown of tetrahedral intermediates has also been gained by studying the hydrolysis of amide acetals. If the amine is expelled, an ester is formed, whereas elimination of an alcohol gives an amide ... 

The hydrolysis of amides to carboxylic acids and amines requires considerably more vigorous conditions than ester hydrolysis. The reason is that the electron-releasing... 

The mechanism for acid-catalyzed hydrolysis of amides involves attack by water on the protonated amide. An inqjortant feature of the chemistry of amides is that the most basic site in an amide is the carbonyl oxygen. Very little of the N-protonated form is present. The major factor that contributes to the stability of the O-protonated form is the... 

Nitriles are classified as carboxylic acid derivatives because they are converted to carboxylic acids on hydrolysis. The conditions required are similar- to those for the hydrolysis of amides, namely, heating in aqueous acid or base for several hours. Like the hydrolysis of amides, nitrile hydrolysis is ineversible in the presence of acids or bases. Acid hydrolysis yields fflnmonium ion and a carboxylic acid. 

In these papers, the carboxylic acid to be protected was a stable, unsubstituted compound. Harsh conditions were acceptable for both formation and cleavage of the amide. Typically, a simple secondary amide is very difficult to cleave. As the pKa of the conjugate acid of an amide decreases, the rate of hydrolysis of amides derived from these amines increases. The dimethylamide of a cephalosporin was prepared as follows using 2,2 -dipyridyl disulfide. ... 

Amide hydrolysis is common in biological chemistry. Just as the hydrolysis of esters is the initial step in the digestion of dietary fats, the hydrolysis of amides is the initial step in the digestion of dietary proteins. The reaction is catalyzed by protease enzymes and occurs by a mechanism almost identical to that we just saw for fat hydrolysis. That is, an initial nucleophilic acyl substitution of an alcohol group in the enzyme on an amide linkage in the protein gives an acyl enzyme intermediate that then undergoes hydrolysis. 

The value of (Ag ) can be estimated from experimental studies of methoxy-catalyzed hydrolysis of amides. That is, after some literature search you may find (Ref. 6) that the rate constant for an attack of CH3 - O on an amide is around 0.3 sec-1. The corresponding Ag is found in the exercise below. 

As in the case of esters, hydrolysis of amides is also a fundamental reaction in organic chemistry and plays a key role in biological systems. The reaction has been covered extensively in organic chemistry and biochemistry textbooks. 

Selective cleavage of peptides and proteins is an important procedure in biochemistry and molecular biology. The half-life for the uncatalyzed hydrolysis of amide bonds is 350 500 years at room temperature and pH 4 8. Clearly, efficient methods of cleavage are needed. Despite their great catalytic power and selectivity to sequence, proteinases have some disadvantages. Peptides 420,423,424,426 an(j proteins428 429 can be hydrolytically cleaved near histidine and methionine residues with several palladium(II) aqua complexes, often with catalytic turnover. 

Hydrolysis of amide groups to carboxylate is a major cause of instability in acrylamide-based polymers, especially at alkaline pH and high temperatures. The performance of oil-recovery polymers may be adversely affected by excessive hydrolysis, which can promote precipitation from sea water solution. This work has studied the effects of the sodium salts of acrylic acid and AMPS, 2-acrylamido-2-methylpropanesulfonic acid, as comonomers, on the rate of hydrolysis of polyacrylamides in alkaline solution at high temperatures. Copolymers were prepared containing from 0-53 mole % of the anionic comonomers, and hydrolyzed in aqueous solution at pH 8.5 at 90°C, 108°C and 120°C. The extent of hydrolysis was measured by a conductometric method, analyzing for the total carboxylate content. 

Figure 1. Hydrolysis of amide in sodium acrylate (A) copolymers at 108 ° C, pH 8.5, 0.025 M amide.

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

Hydrolysis of epoxides and arene oxides Hydrolysis of amides and esters Hydrolysis of glycosides... 

---