Related Experiment Videos
Conformation and Steric Effects in Mono- and Dimethoxybenzoic Acids
Otto Exner1, Pavel Fiedler, Milos Budesínský
1Department of Organic Chemistry, University of Pardubice, 53210 Pardubice, Czech Republic.
The Journal of Organic Chemistry
|October 25, 2001
Summary
Methoxybenzoic acids exhibit strong intramolecular hydrogen bonds, influencing their acidity. These hydrogen bonds, not molecular conformation, explain acidity differences in various solvents and compared to methylbenzoic acids.
Area of Science:
- Organic Chemistry
- Physical Chemistry
Background:
- Understanding substituent effects on molecular properties is crucial in organic chemistry.
- The influence of methoxy groups on benzoic acid conformation and acidity requires detailed investigation.
Purpose of the Study:
- To determine the solution conformation of mono- and dimethoxybenzoic acids.
- To elucidate the role of intramolecular hydrogen bonding and steric effects on their acidity.
- To differentiate polar and steric contributions to acidity in various solvents.
Main Methods:
- Infrared (IR) spectroscopy.
- Carbon-13 ((13)C) and Proton (1H) Nuclear Magnetic Resonance (NMR) spectroscopy.
- Acidity measurements (pK) in dimethyl sulfoxide and methanol.
Main Results:
- A strong intramolecular hydrogen bond was identified in all 2-methoxy derivatives, persisting in polar aprotic solvents but not methanol.
- Molecular conformation was found not to be responsible for enhanced enthalpy of formation values.
- Hydrogen bonding and potential dimerization explain differences in relative acidities (deltapK) across solvents and compared to methylbenzoic acids.
Conclusions:
- Intramolecular hydrogen bonding is a key factor influencing methoxybenzoic acid acidity.
- Steric effects on acidity are solvent-dependent, being more regular in methanol than in dimethyl sulfoxide.
- Acidity enhancement by ortho substituents is better explained by electrostatic induction in the anion than steric hindrance.