Chapter 5: Rings QuickTime and a TIFF (Uncompressed)
Chapter 5: Rings QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Ring Structures Remember that sp3 carbon wants to be tetrahderal with 109.5 bond angles: 109.5 R R R R When confined to a ring, bond angles are forced to change: cyclopropane planar structrue; C-C bond angles are defined as 60
Bond angles that deviate from the ideal (acyclic) angles increase the energy of the system through angle strain . 60 bond angles 90 (if planar) severe angle strain 120 (if planar) small angle strain 150 (if planar) no angle strain Torsional Strain H H H H H H H
HH H H H cyclopropane planar structrue; C-C bond angles are defined as 60 Cyclopropane is planar because it has to be! All other cycloalkanes would have severe torsional strain if they remained planar (in addition to angle strain): H H H H H H G = 3 Kcal/mol HH H H H H Each H-H eclipsing interaction is 1 Kcal/mol
If planar, each of the following cycloalkanes would have a lot of torsional strain: 6 Kcal/mol 8 Kcal/mol 10 Kcal/mol 12 Kcal/mol Torsional Strain To relieve torsional strain (and sometimes angle strain too!), rings larger than 3 atoms will pucker (bend out of planarity): H H H H pull "tip" up or down Both 4 and 5 membered rings are "fluxional" - the bulge moves rapidly around the ring. Ring Structures Cyclohexane commonly puckers into a conformer called a chair. This conformation has 109.5 bond angles and a staggered arrangement for all atoms Chair Conformation of Cyclohexane H H
H H H H H H If you squint enough, this roughly resembles a beach chair: Ring Structures Substituents on cyclohexane have 2 different environments: axial positions equatorial positions However, chair-chair interconversion will equilibrate the positions: G = 0 Substitution of a methyl makes the 2 chair forms non-equivalent: CH3 G not 0 CH3 Ring Structures Recall the butane conformational isomers: CH3
H H H H H CH3 H CH3 H CH3 H anti-butane gauche-butane (+0.6 kcal/mol) This relates to cycohexane equilibria: CH3 G not 0 CH3 H CH3
H H H H CH3 H H H H equatorial substituents are in antibutane conforamation (lowest energy) H CH3 H H H H axial substituent has 2 gauchebutane interactions (+1.8 kcal/mol) CH3
Ring Structures The third kind of ring strain is steric strain ("bumping"). G = ? R R G (kcal/mol) equatorial:axial ratio (25 C) Me 1.74 19:1 Et 1.78 21:1 Pr 2.21 43:1 iPr 2.61
86:1 tBu 5.5 11,900:1 R Ring Structures Ring size Total Strain Strain per CH2 (kcal/mol) (kcal/mol) 3 4 5 6 7 8 9 10 11 12 13 14 15 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 Total strain is the sum of: angle strain, eclipsing interactions (torsional strain), van der Waals (steric strain) interactions Multicyclic Ring Structures Adamantium! Why its so cool: One of the strongest substances in the universe Why its not so cool: Doesnt exist
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