Anion-π interactions influence pK(a) values.

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• 1,8-disubstituted arene naphthols are a class of organic compounds with potential applications.
• Understanding the factors influencing their acidity (pK(a)') is crucial for predicting their behavior and designing new molecules.

Purpose of the Study:

• To synthesize five 8-(4-R-phenyl)-1-naphthol derivatives.
• To determine the pK(a)' values of these compounds in an acetonitrile/water mixture.
• To investigate the relationship between the electronic nature of the arene substituent and the measured acidity.

Main Methods:

• Synthesis of naphthol derivatives via Palladium(II) chloride (PdCl(2))-catalyzed electrophilic aromatic substitution.
• Measurement of pK(a)' values in a mixed solvent system (acetonitrile/water).
• Linear Free Energy Relationship (LFER) analysis to correlate substituent effects with acidity.
• Density Functional Theory (DFT) calculations (M06-2X) to interpret bonding contributions.

Main Results:

• Five 8-(4-R-phenyl)-1-naphthol derivatives were successfully synthesized.
• Measured pK(a)' values ranged from 8.42 (for R=NO(2)) to 8.71 (for R=OMe).
• A clear correlation was observed between the electronic properties of the arene substituent (R) and the pK(a)' values.
• DFT calculations revealed that both initial OH-π bonding and, more significantly, product anion-π interactions contribute to the observed acidity trends.

Conclusions:

• The acidity of these 1,8-disubstituted arene naphthols is directly influenced by the electronic nature of the substituent on the pendant phenyl ring.
• Anion-π interactions play a dominant role in stabilizing the conjugate base, thereby affecting the measured pK(a)' values.
• The findings provide insights into structure-property relationships in substituted naphthols and have broader implications for related chemical systems.