Acid anhydrides resonance

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

Theoretically, this resonance could also take place in acid chlorides, acid anhydrides, and esters to give resonance structures (Fig.F). However, the process is much less important because oxygen and chlorine are less nucleophilic than nitrogen. In these structures, the positive charge ends up on an oxygen or a chlorine atom. 

These atoms are more electronegative than nitrogen and less able to stabilise a positive charge. These resonance structures might occur to a small extent with esters and acid anhydrides, but are far less likely in acid chlorides. This tend also matches the trend in reactivity. 

Fig.F. Resonance structures for (a) an acid chloride (b) an acid anhydride (c) an ester...

Although the resonance effect is weak in esters and acid anhydrides, it explain why acid anhydrides are more reactive than esters. Acid anhydrides have two carbonyl groups and so resonance can occur with either carbonyl group (Following fig.). Due to this, the lone pair of the central oxygen is split between both groups that means that the resonance effect is split between both carbonyl groups. 

This means that the effect of resonance at any one carbonyl group is diminished and it will remain strongly electrophilic. With an ester, there is only one carbonyl group and so it experiences the full impact of the resonance effect. Therefore, its electrophilic strength will be diminished relative to an acid anhydride. 

Acylation can be achieved using either acyl halides or acid anhydrides. The product is a ketone. Acyl halides are more reactive than the anhydrides, but still require a Lewis acid catalyst to promote the reaction (Scheme 2.6). The attacking species is the resonance-stabilized acylium ion or the complex. 

Acid anhydrides are better stabilized by electron delocalization than are acyl chlorides. The lone-pair electrons of oxygen are delocalized more effectively into the carbonyl gronp. Resonance involves both carbonyl gronps of an acid anhydride. 

This section deals with the chemical shifts of the carboxylic anhydrides. As a group, the anhydrides are very reactive and readily decompose to the corresponding carboxylic acid in the presence of the traces of water found in DMSO-d6, polysol and acetone-d6. In general, the carbonyl chemical shift of the anhydride resonates at a higher field than that of the carboxylic acid. 

Consequently, aldehydes and ketones are not as reactive as carbonyl compounds in which Y is a very weak base (acyl halides and acid anhydrides), but are more reactive than carbonyl compounds in which Y is a relatively strong base (carboxylic acids, esters, and amides). A molecular orbital explanation of why resonance electron donation decreases the reactivity of the carbonyl group is given in Section 17.15. 

An ester is hydrolyzed with aqueous acid as well as with aqueous hydroxide. Under acidic conditions, the mechanism of ester hydrolysis is similar to that of acid chlorides and acid anhydrides, as shown previously. When ethyl butanoate (29) is treated with an acid catalyst in water, the products are carboxylic acid (butanoic acid, 6) and ethanol. The OH unit in this acid is clearly derived from the water, and the OEt rmit in ethanol is derived from the OEt unit of the ester. It is known that water does not react directly with 29 to give 7, so the acid catalyst must facilitate the reaction. This, of course, indicates that 29 reacts as a base with the acid catalyst to give the resonance-stabilized oxocarbenium ion 30. 

We have seen throughout the past several sections that acid chlorides are most reactive toward nucleophilic acyl substitution, followed by acid anhydrides and esters the least reactive are amides. Carboxylate anions are negatively charged and therefore repel nucleophiles the resonance in these species is quite stabilizing. Both of these factors make carbo>qrlate anions essentially inert to nucleophilic acyl substitution (hence, we have not examined them to this point in the chapter). Another useful way to think about the reactions of the functional derivatives of carboxylic acids is summarized in Figure 18.2. 

Stability imparted by resonance increases in the order of add halide < acid anhydride < ester < amide. 

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