Intramolecular carboxylic group-assisted cleavage of N-(2-hydroxyphenyl)phthalamic acid (7) and N-(2-methoxyphenyl)phthalamic acid (8): Absence of intramolecular general acid catalysis due to 2-OH in 7
✍ Scribed by Yoke-Leng Sim; Azhar Ariffin; M. Niyaz Khan
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 249 KB
- Volume
- 38
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
Abstract
The values of pseudo first‐order rate constants (k~obs~) for the cleavage of N‐(2‐hydroxyphenyl)phthalamic acid (7), obtained at 4.9 × 10^−2^ M HCl, 35°C, and within CH~3~CN content range 2–80% (v/v) in mixed aqueous solvent are smaller than k~obs~ for the cleavage of N‐(2‐methoxyphenyl)phthalamic acid (8), obtained under almost similar experimental conditions, by nearly 1.5‐ to 2‐fold. These observations show the absence of expected intramolecular general acid catalysis due to 2‐OH group in 7. The values of k~obs~ for the cleavage of 7 and 8 decrease by more than 20‐fold with the increase in the content of CH~3~CN from 2 to 80–82% (v/v) in mixed aqueous solvent. The kinetic data reveal that in acidic aqueous cleavage of 7, N‐cyclization (leading to the formation of imide) and O‐cyclization (leading to the formation of phthalic anhydride) vary from ∼10 to 15% and ∼90 to 85%, respectively, with the increase in CH~3~CN content from 2 to 80% (v/v). Similar increase in CH~3~CN content causes increase in N‐cyclization from ∼0 to 5% and decrease in O‐cyclization from ∼100 to 95% in the acidic aqueous cleavage of 8. Some speculative, yet conceivable, reasons for nearly 10 and 0% N‐cyclization in the cleavage of respective 7 and 8 at low content of CH~3~CN have been described. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 746–758, 2006