Chapter 5 Rings Ring Structures 109 5 R Remember that sp3 carbon wants to be tetrahderal with 109 5 bond angles 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 H H 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 R G 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 it s so cool One of the strongest substances in the universe Why it s not so cool Doesn t exist