![]() The reaction of an acyl chloride with an alcohol also gives an ester, but this conversion cannot be reversed by adding HCl to the reaction mixture.Įsters are one of the most common carboxylic derivatives. The acid-catalyzed formation of esters from carboxylic acids and alcohols, described earlier, is a good example of a reversible acylation reaction, the products being determined by the addition or removal of water from the system. In principle all steps are reversible, but in practice many reactions of this kind are irreversible unless changes in the reactants and conditions are made. Acid catalysts act to increase the electrophilicity of the acyl reactant whereas, base catalysts act on the nucleophilic reactant to increase its reactivity. The number of individual steps in these mechanisms vary, but the essential characteristic of the overall transformation is that of addition followed by elimination. Also, a specific example of acyl chloride formation from the reaction of a carboxylic acid with thionyl chloride will be shown. The facility with which nucleophilic reagents add to a carbonyl group was noted earlier for aldehydes and ketones.Īcid and base-catalyzed variations of this mechanism are shown above. In this two-stage mechanism bond formation occurs before bond cleavage, and the carbonyl carbon atom undergoes a hybridization change from sp 2 to sp 3 and back again. This tetrahedral intermediate then undergoes an elimination to yield the products. On the other hand, for most cases of electrophilic aromatic substitution bond-making preceded bond-breaking.Īs illustrated in the following diagram, acylation reactions generally take place by an addition-elimination process in which a nucleophilic reactant bonds to the electrophilic carbonyl carbon atom to create a tetrahedral intermediate. In nucleophilic substitution reactions of alkyl compounds examples of bond-breaking preceding bond-making (the S N1 mechanism), and of bond-breaking and bond-making occurring simultaneously (the S N2 mechanism) were observed. The timing of these events may vary with the reacting system. The bond from the substrate to the leaving group must be broken, and a bond to the replacement group must be formed. In any substitution reaction two things must happen. Clearly, the mechanism by which acylation reactions occur must be different from the S N1 and S N2 procedures described earlier. Furthermore, such substitution reactions of alcohols and ethers are rare, except in the presence of strong mineral acids. After all, it was previously noted that halogens bonded to sp 2 or sp hybridized carbon atoms do not usually undergo substitution reactions with nucleophilic reagents. Indeed, an alert reader may well be puzzled by the facility of these nucleophilic substitution reactions. Base catalyzed hydrolysis produces carboxylate salts.īefore proceeding further, it is important to review the general mechanism by means of which all these acyl transfer or acylation reactions take place. The carboxylic acids themselves are not an essential part of this diagram, although all the derivatives shown can be hydrolyzed to the carboxylic acid state (light blue formulas and reaction arrows). Specific examples of these conversions will be displayed by clicking on the product formula. A better and more general anhydride synthesis can be achieved from acyl chlorides, and amides are easily made from any of the more reactive derivatives. ![]() Methods for converting carboxylic acids into these derivatives were shown in a previous section, but the amide and anhydride preparations were not general and required strong heating. Reactivity: acyl halides > anhydrides > esters ≈ acids > amidesīecause of these differences, the conversion of one type of acid derivative into another is generally restricted to those outlined in the following diagram. ![]() Amides, on the other hand, react with water only in the presence of strong acid or base catalysts and external heating. In homogeneous solvent systems, reaction of acyl chlorides with water occurs rapidly, and does not require heating or catalysts. If Nuc-H is water the reaction is often called hydrolysis, if Nuc–H is an alcohol the reaction is called alcoholysis, and for ammonia and amines it is called aminolysis.ĭifferent carboxylic acid derivatives have very different reactivities, acyl chlorides and bromides being the most reactive and amides the least reactive, as noted in the following qualitatively ordered list. For certain nucleophilic reagents the reaction may assume other names as well. ![]() The overall transformation is defined by the following equation, and may be classified either as nucleophilic substitution at an acyl group or as acylation of a nucleophile. This is probably the single most important reaction of carboxylic acid derivatives. ![]()
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