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Chapter 34: Problem 759

Show the probable mechanistic steps involved in the preparation of coumarin by the condensation of acetic anhydride with salicylaldehyde in the presence of sodium acetate as catalyst.

Short Answer

Expert verified
The mechanism of coumarin synthesis by Pechmann condensation involves five key steps: (1) Formation of acetyloxonium ion from acetic anhydride and sodium acetate, (2) nucleophilic attack on acetyloxonium ion by salicylaldehyde, forming an ester intermediate, (3) intramolecular cyclization of ester intermediate, forming a cyclic compound, (4) rearrangement and tautomerization of the cyclic compound to form an enol ester, and (5) de-esterification of the enol ester to form the final product, coumarin.

Step by step solution

01

Formation of acetyloxonium ion from acetic anhydride and sodium acetate

Acetic anhydride reacts with sodium acetate to form acetyloxonium ion, which is a reactive species and acts as an electrophile. The reaction mechanism is as follows: \[CH_3CO_2Na + (CH_3CO)_2O \rightarrow CH_3CO_2CH_3 + CH_3CO_2^-\] This equilibrium reaction results in the formation of the acetyloxonium ion, which will participate in the subsequent nucleophilic attack by salicylaldehyde.
02

Nucleophilic attack on acetyloxonium ion by salicylaldehyde

The phenolic oxygen of salicylaldehyde acts as a nucleophile and attacks the electrophilic carbon of the acetyloxonium ion. The reaction is as follows: \[C_6H_4CHO(OH) + CH_3CO_2CH_3 \rightarrow C_6H_4CH(OC(O)CH_3)OH\] This reaction leads to the formation of an ester intermediate product.
03

Intramolecular cyclization of ester intermediate

In this step, the ester intermediate undergoes intramolecular cyclization. The phenolic oxygen attacks the electrophilic carbonyl carbon of the ester group, leading to the formation of a cyclic compound. \[C_6H_4CH(OC(O)CH_3)OH \rightarrow C_6H_4CH(O)OC(O)CH_3\] This cyclic compound has a five-membered ring which includes the oxygen atom.
04

Rearrangement and tautomerization of the cyclic compound

The cyclic compound undergoes rearrangement and tautomerization through proton shifts. The enol form of the cyclical compound is converted to a more stable keto form. \[C_6H_4CH(O)OC(O)CH_3 \rightarrow C_6H_4C(O)CH(OC(O)CH_3)\] This rearrangement and tautomerization result in the formation of an enol ester.
05

De-esterification to form coumarin

In the final step, the enol ester undergoes de-esterification with the help of sodium acetate. The ester bond is cleaved, and the reaction is as follows: \[C_6H_4C(O)CH(OC(O)CH_3) + H_2O \rightarrow C_6H_4C(O)CH(OH) + CH_3CO_2H\] The resulting product is coumarin. In conclusion, the preparation of coumarin from acetic anhydride and salicylaldehyde in the presence of sodium acetate as a catalyst involves a series of nucleophilic attacks, intramolecular cyclization, rearrangement, tautomerization, and de-esterification reactions. These mechanistic steps play crucial roles in the efficient synthesis of coumarin through Pechmann condensation.

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Most popular questions from this chapter

(a) Nerol and geraniol cyclize under the influence of acid to yield \(\alpha\) -terpineol. How could the relative ease of cyclization of these alcohols, coupled with other reactions, be used to establish the configurations of the double bond of geraniol and nerol? What is the mechanism of this reaction? (b) The cyclization of optically active linalool produces optically active \(\alpha\) -terpineol. Explain.

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Explain how use of ultraviolet, infrared, or n.m.r. spectroscopy could be used to distinguish between the following possible structures for civetone. (a) 9-cycloheptadecenone and 2-cycloheptadecenone (b) cis-9-cycloheptadecenone and trans-9-cycloheptadecenone (c) 8-methyl-8-cyclohexadecenone and 9-cycloheptadecenone (d) 8-cycloheptadecenone and 9-cycloheptadecenone

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