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Unformatted text preview: Chem 057 CENTER FOR LEARNING AND TEACHING Handout #7 J. Walcott LEARNING STRATEGIES CENTER Fall 2007 Review Guide #4 Lectures: 9/10/07 – 9/14/07 and Chapter 3 I. Cycloalkanes Rings can be subject to two sources of strain – angle strain and torsional strain. Angle strain results from sp 3 hybridized carbons assuming angles that vary from the optimal 109.5 ° . Torsional strain results from eclipsing interactions. A. Cyclopropane has a ring strain of ¡ 30 kcal/mol that results from three small angles (60 ° interior angles) and six pairs of eclipsing interactions. The angle strain (24kcal/mol) + torsional strain (6 kcals/mol) = ring strain (30 kcals/mol) A. Cyclobutane also has a ring strain of ¡ 30 kcal/mol that results from four small angles (90 ° interior angles) and eight pairs of eclipsing interactions when it is flat. The angle strain (22 kcal/mol) + torsional strain (8 kcals/mol) = ring strain (30 kcals/mol). It puckers a bit to relieve torsional strain, which perhaps increases angle strain. B. Cyclopentane has minimal ring strain. The interior bond angle of 108 ° is close to 109.5 ° but it has ten pairs of eclipsing interactions when it is flat. The angle strain (~2 kcal/mol) + torsional strain (10 kcals/mol) = ring strain (12 kcals/mol). It has a ¡ rotating envelope flap” effect to relieve torsional strain giving the overall ring strain of ~5 kcals/mol. C. Cyclohexane also has no ring strain. The expected interior bond angle of 120 ° is actually 109.5 ° due to formation of chair conformations. The chair conformations also avoid eclipsing interactions since the hydrogens on adjacent carbons are staggered ....
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This note was uploaded on 09/12/2008 for the course CHEM 057 taught by Professor Walcott, j during the Fall '07 term at Cornell University (Engineering School).
- Fall '07
- WALCOTT, J