Effect of substituents on secondary deuterium isotope effects in the solvolysis of aryl diazonium salts

CHED 1395

Ronald Benning Jr., sheats@rider.edu1, Labe A. Black2, Robin Lefkowitz, lefkowitz@rider.edu1, Grant McSurdy, sheats@rider.edu1, and John E. Sheats, sheats@rider.edu1. (1) Department of Chemistry, Biochemistry and Physics, Rider University, 2083 Lawrenceville Rd., Lawrenceville, NJ 08648, (2) Department of Chemistry, Bucknell University, Lewisburg, PA 17837
Benzenediazonium salts, C6H5N2+X- solvolyze in aqueous solution to form C6H5+ which subsequently forms C6H5OH and a small amount of C6H5X. Solvolysis of Benzenediazonium-2,4,6-d3 salts is retarded by a large secondary deuterium isotope effect kH/kD = 1.54 +/- 0.02. This isotope effect is attributed to decreased stabilization of the phenyl cation by hyperconjugation in the deuterated analog. 3-Methoxybenzenediazonium salts, which decompose 5 times as rapidly, show a reduced isotope effect kH/kD =1.40, whereas, 3-chlorobenzenediazonium salts, which decompose 0.033 times as rapidly show an increased isotope effect kH/kD = 1.60. We conclude that substituents which enhance the stability of the aryl cation, either by inductive or resonance effects, decrease the importance of hyperconjugation,lower the activation energy and decrease the kinetic isotope effect; whereas, substituents which decrease the stability of the aryl cation increase the activation energy and increase the kinetic isotope effect. Correlation of these results with the Hammett and Swain-Lupton Equations will be discussed.
 

Undergraduate Research Poster Session: Physical Chemistry
2:00 PM-4:00 PM, Monday, March 26, 2007 Hyatt Regency Chicago -- Riverside Center, Poster

Division of Chemical Education

The 233rd ACS National Meeting, Chicago, IL, March 25-29, 2007