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  Acyl Group Substitution Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Acyl Group Substitution

This is possibly the single very significant reaction of carboxylic acid derivatives. The overall transformation is described by the equation defined below, and might be categorized either as nucleophilic substitution at an acyl group or asacylation of a nucleophile. For specific nucleophilic reagents the reaction might assume other names as well. If Nuc-H is water the reaction is frequently called the hydrolysis, if Nuc-H is an alcohol the reaction is called alcoholysis and for amines and ammonia it is called aminolysis.

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Different carboxylic acid derivatives have very distinct reactivities, the acyl chlorides and bromides being the most amides and reactive the least reactive, as noticed within the following qualitatively ordered list. The alteration in reactivity is dramatic. Reaction of acyl chlorides with water takes place quickly, and does not need heating or catalysts, in homogeneous solvent systems. Alternatively, Amides, react with water only in the existence of strong acid or base catalysts and external heating.

Reactivity:   acyl halides > anhydrides >> esters ≈ acids >> amides

Due to these variations, the conversion of one type of acid derivative into another is usually restricted to those outlined in the diagram. Techniques for converting carboxylic acids into these derivatives were displayed in a earlier section, but the anhydride and amide preparations were not general and needed strong heating. A better and more common anhydride synthesis can be getting from acyl chlorides, and amides are simply made from any of the more reactive derivatives. Carboxylic acids themselves are not a necessary part of this diagram, Even though all the derivatives displayed can be hydrolyzed to the carboxylic acid state (light blue formulas and reaction arrows). Base catalyzed hydrolysis makes carboxylate salts.

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Before carry on further, it is significant to review the general technique by means of which all these acyl transfer or acylation reactions occur. Certainly, an alert reader may well be puzzled by the capability of these nucleophilic substitution reactions. Finally, it was earlier noticed that halogens bonded to sp2 or sp hybridized carbon atoms do not generally go through substitution reactions with nucleophilic reagents. Additionally, such type of substitution reactions ofalcohols and ethers are exceptional, except in the existence of strong mineral acids. Undoubtedly, the technique from which acylation reactions take place must be distinct from the SN1 and SN2 procedures explained previous.
In any substitution reaction two things have to occur. The bond from the substrate to the leaving group has to be broken, and a bond to the replacement group has to be formed. The timing of these events may change with the reacting system. In the nucleophilic substitution reactions of alkyl compounds instances of bond-breaking preceding bond-making (the SN1 mechanism), and of bond-breaking and bond-making taking place concurrently (the SN2 mechanism) were observed. Alternatively, for most examples of electrophilic aromatic substitution bond-making proceeded bond-breaking.

As demonstrated in the following diagram, acylation reactions generally occur by an addition-elimination process where a nucleophilic reactant bonds to the electrophilic carbonyl carbon atom to form a tetrahedral intermediate. This tetrahedral intermediate then goes through an elimination to yield the results. In this two-stage mechanism bond formation takes place before bond cleavage, and the carbonyl carbon atom goes through a hybridization change from sp2 to sp3 and back again. The capacity with which nucleophilic reagents add to a carbonyl group was noticed previous for aldehydes and ketones.

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Also, a particular instance of acyl chloride formation from the reaction of a carboxylic acid with thionyl chloride will be displayed. The number of individual steps in these techniques change, but the necessary feature of the whole transformation is that of addition followed by elimination. Acid catalysts act to increment the electrophilicity of the acyl reactant; where, base catalysts act on the nucleophilic reactant to increase its reactivity. Technically, all steps are reversible, but in practice several reactions of this type are irreversible unless changes in the conditions and reactants are made. The acid-catalyzed formation of esters from carboxylic alcohols and acids, explained previous, is a good instance of a reversible acylation reaction, the results being determined by the addition or removal of water from the system. The reaction of an acyl chloride by an alcohol also gives an ester, but this conversion can't be reversed by adding HCl to the reaction mixture.

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