Class Summary          11/08/04          Term Paper Guidelines         

 

IMPORTANT DATES            Wed., Dec. 1st              First draught of paper due

                                                Wed., Dec. 8th             Review of other papers due

                                                Tues., Dec. 14th            Final Exam

                                                Fri., Dec. 17th               Final Paper Due

 

11/08   C10 Rings and Beyond – Strained Molecules               OLD NOTES

            Benson Group Equivalents

 

            C10H10  - Even though [4n + 2], generally non-aromatic.  Why?

 

            Non-planar, too much strain to maintain planarity (build a model to see this)

           

            However, a somewhat planar conformer displays some aromatic character (Schleyer/Schaefer III)

 

            STRAIN           “Unusual effets observed when compared to a strain-free model”

 

What is a strain-free model?  For the purposes of hydrocarbon rings, we can assume a strain-free model of acyclic hydrocarbons (they may be strained, however they are the LEAST strained systems)

 

Benson Group Equivalents can be used to assess strain:

 

e.g. CH3CH2CH2CH3              (2)(-10.2 for CH3) + (2)(-4.93 for –CH2-) = -30.3 kcal/mol

                                                c.f. DHf° = -30.4 kcal/mol, virtually no strain

 

Cyclopropane, C3H6                 (3)(-4.93 for –CH2-) = -14.79 kcal/mol

                                                c.f. DHf° = 12.74 kcal/mol, therefore (12.74 - -14.79) kcal/mol

                                                27.5 kcal/mol of RING STRAIN ENERGY

 

“Strain” is DHf of system vs. strain-free model (non-corrected Benson estimate)

 

            Limitations of Benson Group Equivalents

 

            Cannot account for certain isomers

(for example, it would give the same prediction for cis/trans-butadiene)

 

            Assumes non-interacting groups (unless otherwise parameterized)

 

            Doesn’t seem to work well with halonitriles, e.g. X-CH2-CN, X = F, Cl, Br, I

 

            In general, group additivity can give DHf within ± 1-2 kcal/mol

 

 

PAPERS MENTIONED

 

Sulzbach, H.M., Schleyer, P.v.R., Jiao, H., Xie, Y., and Schaefer III, H.F.  A [10]Annulene Isomer May Be Aromatic, After All!  J. Am. Chem. Soc. (1995) 117, 1369-1373.

Benson, S. Thermochemical Kinetics, (1973) (monograph).

 

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