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Susquehanna Valley Local Section

October 2004 Meeting

The three hundred and fifty second meeting of the Section will be held on Wednesday, October 20, 2004 at Wilkes University. The guest speaker, Dr. Paul Rablen, will present a talk entitled "How Come Vinegar Is Sour, But Wine Is Not? Resonance and Electrostatic Stabilization of the Acetate Anion."

Dinner: will begin at 6:30 p.m. in the Miller Conference Room (2nd floor) of the Henry Student Center
Lecture: will start at 8:00 p.m. in Room 270 of the Stark Learning Center

The dinner will be buffet style with the following items: Chicken Marsala, Portabella Shitake Stroganoff (a vegetarian dish), green beans with red peppers, roasted red potatoes, a green salad, coffee tea, iced tea, and a fall desert served with coffee. Cost: $12.00. Please RSVP: to Mary Lou Gillespie (570-408-4750) or mgilles@wilkes.edu no later than noon on Friday 15 October 2004.

Directions: Take I-81 North to Route 309 North (Exit 170B, Old Exit 47B). Follow Route 309 North to Exit 3 (Plains/River Street) and make a left onto River Street at the traffic light at the bottom of the exit ramp. Follow River Street south to Wilkes University (located on the left side of River St). Parking is available behind the Henry Student Union Building (the SUB). There is an entrance to this lot immediately past the SUB on South St. (which is one long block past Northampton St.) You may also park in any of the other campus lots.

Stark Learning Center is on River St. mid-block between Northampton and South Streets.

Detailed directions and maps can be found at: http://www.wilkes.edu/about/campus/directions.asp.

How Come Vinegar Is Sour, But Wine Is Not? Resonance and Electrostatic Stabilization of the Acetate Anion -
Using ab initio MO theory, a series of isodesmic reactions were studied in which nitrogen-, oxygen-, and fluorine-containing species acted as proton donors and acceptors. Comparison of protonation and deprotonation energies suggests that approximately three quarters of the enhanced acidity of acetic acid comes from electrostatic stabilization, while the remaining quarter results from › resonance. Similar logic shows that only one third of the enhanced acidity of the nitrogen analog acetamidine is electrostatic in nature, and that the remaining two thirds derives from resonance. The primary importance of electrostatics for oxygen and of › resonance for nitrogen is further supported by the behavior of carbonic acid and guanidine. The contributions of hydroxy, amino, and fluorine substituents to the acidity and basicity of a series of alcohols and amines are well described by a simple electrostatic model with a single adjustable parameter. The model requires the electrostatic contributions of polar bonds to be additive; to be of equal magnitude but opposite sign for anions and cations; and to be strictly proportional to the electronegativity differences between the atoms comprising the bonds. Application of this model to a series of reactions lacking › bonds results in a correlation coefficient of 0.99, and indicates that on average C-F bonds contribute 15 kcal/mol, C-O bonds 9 kcal/mol, and C-N bonds 4 kcal/mol to differential acidity and basicity. Further application of the model allows estimation of › resonance contributions to the acidity and basicity of acetic acid and a series of related compounds. These › resonance contributions are found to be much greater for nitrogen than for oxygen, and significantly greater for acidity (anions) than for basicity (cations).

Paul Rablen got his B.A. in Chemistry at Haverford College; M.A. at Columbia University; Ph.D. at Yale with Ken Wiberg; and was a postdoctoral associate at Yale with Bill Jorgensen. He's been on the faculty of Swarthmore College ever since then. He is an associate professor now and just became the chair of the department of chemistry and biochemistry. His research concerns varied topics in physical organic chemistry, studied primarily via ab initio MO calculations.