D.A. Evans Conformational Analysis: Part -3 Chem 206 Conformational Analysis of Cyclic Systems http://www.courses.fasharvard.edu/-chem206/ Three Types of Strain Cher Prelog Strain: van der Waals interactions Baeyer Strain: bond angle distortion away from the ideal Advanced Organic Chemistry Pitzer Strain: torsional rotation about a sigma bond Lecture number 6 Baeyer Strain for selected ring sizes size of ring Ht of Combustion Total Strain Strain per CH2 "angle strain" ol(kcal mol) deviation from 109 28 4998 24°44 Conformational analysis-3 26.3 9°44 9448 5°16 Cyclopropane 11083 1.21 Conformational Analysis of C4 >C& Rings 14296 126 140 10 1586.8 12.4 1.24 1743.1 11.3 12 18934 051.9 5.2 14 15 Reading Assignment for week Eliel, E. L, Wilen, S. H. Stereochemistry of Organic Compounds Chapter 11, John Wiley Sons, 1994 A. Carey Sundberg: Part A; Chapter 3 a Baeyer"angle strain "is calculated from the deviation of the planar bond angles from the ideal tetrahedral bond angle de meijere, "Bonding Properties of Cyclopropane their Chemical Characteristics Angew Chem. Int Ed. 1979, 18, 809-826(handout) a Discrepancies between calculated strain/CH2 and the"angle strain" results from puckering to minimize van der Waals or Snyder, J. P JACS, 2000, 122, 544 eclipsing torsional strain between vicinal hydrogens Why is there an increase in strain for medium sized rings even Monda though they also can access puckered conformations free of Matthew d shair September 30, 2002 angle strain? The answer is transannular strain-van der Waals interactions between hydrogens across the ringD. A. Evans Chem 206 Matthew D. Shair Monday, September 30, 2002 http://www.courses.fas.harvard.edu/~chem206/ ■ Reading Assignment for week A. Carey & Sundberg: Part A; Chapter 3 Conformational Analysis: Part–3 Chemistry 206 Advanced Organic Chemistry Lecture Number 6 Conformational Analysis-3 ■ Cyclopropane ■ Conformational Analysis of C4 ® C8 Rings Three Types of Strain: Prelog Strain: van der Waals interactions Baeyer Strain: bond angle distortion away from the ideal Pitzer Strain: torsional rotation about a sigma bond Baeyer Strain for selected ring sizes size of ring Ht of Combustion (kcal/mol) Total Strain (kcal/mol) Strain per CH2 (kcal.mol) "angle strain" deviation from 109°28' 3 4 5 6 7 8 9 10 11 12 13 14 15 499.8 656.1 793.5 944.8 1108.3 1269.2 1429.6 1586.8 1743.1 1893.4 2051.9 2206.1 2363.5 27.5 26.3 6.2 0.1 6.2 9.7 12.6 12.4 11.3 4.1 5.2 1.9 1.9 9.17 6.58 1.24 0.02 0.89 1.21 1.40 1.24 1.02 0.34 0.40 0.14 0.13 24°44' 9°44' 0°44' -5°16' Eliel, E. L., Wilen, S. H. Stereochemistry of Organic Compounds Chapter 11, John Wiley & Sons, 1994. ■ Baeyer "angle strain" is calculated from the deviation of the planar bond angles from the ideal tetrahedral bond angle. ■ Discrepancies between calculated strain/CH2 and the "angle strain" results from puckering to minimize van der Waals or eclipsing torsional strain between vicinal hydrogens. ■ Why is there an increase in strain for medium sized rings even though they also can access puckered conformations free of angle strain? The answer is transannular strain- van der Waals interactions between hydrogens across the ring. Conformational Analysis of Cyclic Systems de Meijere, "Bonding Properties of Cyclopropane & their Chemical Characteristics" Angew Chem. Int. Ed. 1979, 18, 809-826 (handout) Snyder, J. P. JACS, 2000, 122, 544