Acyl chlorides 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... 

Acyl chlorides Although chlorine has unshared electron pairs it is a poor electron pair donor m resonance of the type... 

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

There are large differences in reactivity among the various carboxylic acid derivatives, such as amides, esters, and acyl chlorides. One important factor is the resonance stabilization provided by the heteroatom. This decreases in the order N > O > Cl. Electron donation reduces the electrophilicity of the carbonyl group, and the corresponding stabilization is lost in the tetrahedral intermediate. 

In a first step, the carboxylic acid 1 is converted into the corresponding acyl chloride 2 by treatment with thionyl chloride or phosphorous trichloride. The acyl chloride is then treated with diazomethane to give the diazo ketone 3, which is stabilized by resonance, and hydrogen chloride ... 

In Friedel-Crafts acylations, an acyl halide, almost always the chloride, in the presence of a Lewis acid is employed to acylate an aromatic ring. The process is initiated by polarization of the carbon-chlorine bond of the acyl chloride, resulting in formation of a resonance-stabilized acylium ion. 

You may perhaps remember that the acylium cation is the acylating species or, alternatively, you can deduce that the acyl chloride interacts with the Lewis acid AICI3, and is most likely to give AlCU. The acylium cation then emerges as the other part. This is resonance stabilized, as shown. 

The carbon-halogen bond distances in acyl halides increase in the direction F < Cl < Br < I, and are similar, but slightly larger than, those of the alkyl halides (Table 7). Nuclear quadrupole resonance frequencies of halogen compounds suggest that the charge density on the chlorine atom of an acyl chloride is greater than that on an alkyl chloride (Table 8). 

The C—Cl bond is so long that the lone-pair orbital (3p) of chlorine is too far away to overlap effectively with the it orbital of the carbonyl group. The carbonyl group of an acyl chloride feels the normal electron-withdrawing effect of an electronegative substituent without much compensating electron donation by resonance. This destabilizes the carbonyl group and makes it more reactive. 

The electrophile, an acyl cation, is generated in a manner similar to that outlined in Figure 17.4 for the generation of the carbocation electrophile from an alkyl halide. First the Lewis acid, aluminum trichloride, complexes with the chlorine of the acyl chloride. Then A1C14 leaves, generating an acyl cation. The acyl cation is actually more stable than most other carbocations that we have encountered because it has a resonance structure that has the octet rule satisfied for all of the atoms ... 

Consider the case of an acyl chloride. The chlorine is an inductive electron with-drawer and a resonance electron donor. As we saw in Chapter 17, the inductive effect is stronger. (Recall that chlorine is not a very strong resonance electron donor because the long C—Cl bond and the size difference between the 3p AO on the Cl and the 2p AO on the C result in poor overlap of these orbitals.) In addition, chloride anion is a very weak base. Overall, acyl chlorides are the most reactive of the carboxylic acid derivatives discussed here and are the least favored at equilibrium. 

Because of the contribution of structures such as the one on the right to the resonance hybrid, the a-carbon of an enamine is nucleophilic. However, an enamine is a much weaker nucleophile than an enolate anion. For it to react in the SN2 reaction, the alkyl halide electrophile must be very reactive (see Table 8.1). An enamine can also be used as a nucleophile in substitution reactions with acyl chlorides. The reactive electrophiles commonly used in reactions with enamines are ... 

The thermodynamic stabilities of carbonyl compounds are directly related to the stabilities of their R2C 0 resonance structures. The order of thermodynamic stabilities of the common types of carbonyl compounds is RCOC1 (acyl chlorides) <... 

The carbocation that is formed upon protonation of a carbonyl compound can lose H+ from the a-carbon to give an enol. Enols are good nucleophiles. Thus, under acidic conditions, carbonyl compounds are electrophilic at the carbonyl C and nucleophilic at the a-carbon and on oxygen, just like they are under basic conditions. Resonance-stabilized carbonyl compounds such as amides and esters are much less prone to enolize under acidic conditions than less stable carbonyl compounds such as ketones, aldehydes, and acyl chlorides in fact, esters and amides rarely undergo reactions at the a-carbon under acidic conditions. 

Resonance stabilization in acyl chlorides is not nearly as prononnced as in other derivatives of carboxylic acids ... 

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. 

Esters, carboxylic acids, and amides each have two major resonance contributors. The second resonance contributor for acyl chlorides and acid anyhdrides is much less important. 

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