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Published on: October 18, 2018
Complex formation between alpha-cyclodextrin and 4-substituted phenols studied by potentiometric and competitive
Journal of Pharmaceutical Sciences
|November 1, 1983
Summary
Alpha-cyclodextrin forms complexes with phenols. Complex stability is influenced by the phenol's 4-substituent, with the conjugate base form generally showing higher stability constants than the conjugate acid form.
Area of Science:
- Supramolecular Chemistry
- Physical Organic Chemistry
Background:
- Alpha-cyclodextrin (α-CD) is a cyclic oligosaccharide known for its ability to form inclusion complexes.
- Phenols are versatile organic compounds with acidic properties, capable of undergoing various chemical interactions.
Purpose of the Study:
- To quantify the binding affinity between alpha-cyclodextrin and both the acid and base forms of nine different phenols.
- To investigate the influence of phenol structure, specifically the 4-substituent, on complex stability.
Main Methods:
- Potentiometric titration to measure apparent acid dissociation constants in the presence of varying cyclodextrin concentrations.
- Modified competitive spectrophotometric method using methyl orange to supplement potentiometric data.
Main Results:
- Stability constants (K11a and K11b) were determined for alpha-cyclodextrin complexation with nine phenols in aqueous solution at 25°C.
- For all phenols studied, the stability constant for the conjugate base (K11b) was consistently higher than that for the conjugate acid (K11a).
- Complex stability increased with electron density and polarizability at the phenol's 4-substituent, indicating this position is crucial for binding.
Conclusions:
- The 4-substituent of the phenol is the primary site of interaction with alpha-cyclodextrin for both neutral and anionic forms.
- The greater stability of the conjugate base complexes is attributed to the more effective delocalization of charge in the phenoxide anion compared to the neutral phenol.
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