Substituent effects on carbocation stability: the pK(R) for p-quinone methide

Area of Science:

• Physical Organic Chemistry
• Reaction Kinetics
• Thermodynamics

Background:

• p-Quinone methides are reactive intermediates with an "extended" carbonyl group.
• Understanding their reactivity with water is crucial for organic reaction mechanisms.
• Previous studies have investigated related carbocation chemistry.

Purpose of the Study:

• To determine the Lewis acidity constants (pK(R)) of O-protonated and neutral p-quinone methide.
• To quantify the rate constant for p-quinone methide generation and water addition.
• To assess the thermodynamic driving force and Marcus intrinsic barriers for water addition reactions.

Main Methods:

• Acid-catalyzed cleavage of 4-hydroxybenzyl alcohol in aqueous solution.
• Kinetic measurements to determine rate constants (k(H) and k(s)).
• Thermodynamic calculations and Marcus theory analysis.

Main Results:

• Determined k(H) = 1.5 x 10(-)(3) M(-)(1) s(-)(1) for p-quinone methide generation.
• Calculated pK(R) = -9.6 for O-protonated and pK(R) = 2.3 for neutral p-quinone methide.
• Found a thermodynamic driving force of 6 kcal/mol for water transfer from formaldehyde hydrate to p-quinone methide.
• Estimated Marcus intrinsic barrier for water addition to p-quinone methide is 4.5 kcal/mol higher than for formaldehyde.

Conclusions:

• p-Quinone methide exhibits significant Lewis acidity.
• The reaction with water is influenced by a balance between bond strengths and product aromatization.
• O-Alkylation minimally affects the relative Marcus intrinsic barriers for water addition.