D. A. Evans Acyclic Conformational Analysis: Pentane Chem 206 n-Pentane Rotation about both the C2-C3 and C3-C4 bonds in either direction(+or-:: The double-gauche pentane conformation The new high-energy conformation:(9g) Estimate of 1, 3-Dimethyl Eclipsing Interaction AG°=X+2 Y where 1(t 2(g X=1,3(Me> Me)&Y=1, 3(Me+H) Ant(2.3)-Ant(34) Gauche(23)-An“(34) 1,3(Me+H)=Skew-butane=0.88 kcal mol -1 13MeMe)=△G°-2Y=55-1,76=+37 kcal mol 1, 3(Me+- Me)=+3.7 kcal mol-1 Estimates of In-Plane 1, 2&1, 3-Dimethyl Eclipsing Interactions 4(g’g) 3(g’g Gauche e(2, 3)-Gauche'(3, 4)+ Gauche (2, 3)-Gauche(3, 4) double gauche pentane 3.7 3.9 76 It may be concluded that in-plane 1, 3(Me+Me)interactions are Ca +4 cal/mol while 1, 2(Me>Me)interactions are destablizing by Ca 2.2 kcal/mol~ 2.2 It may be concluded that in-plane 1,3(Me«Me) interactions are Ca +4 kcal/mol while 1,2(Me«Me) interactions are destabliizing by Ca 2.2 kcal/mol. ~ 3.7 ~3.9 ~ 7.6 Estimates of In-Plane 1,2 &1,3-Dimethyl Eclipsing Interactions 1,3(Me«Me) = + 3.7 kcal mol -1 D G° = +5.5 kcal mol -1 D G° = X + 2Y where: X = 1,3(Me«Me) & Y = 1,3(Me«H) Estimate of 1,3-Dimethyl Eclipsing Interaction 1,3(Me«Me) = D G° – 2Y = 5.5 –1.76 = + 3.7 kcal mol -1 1,3(Me«H) = Skew-butane = 0.88 kcal mol -1 The double-gauche pentane conformation n-Pentane D. A. Evans Acyclic Conformational Analysis: Pentane Chem 206 Me Me Me Me Me Me Me Me Me Me H H H H Me H H H H Me Me H H Me H H Me Me Me Me Me Me Me Me Me Me Me Me Rotation about both the C2 -C3 and C3 -C4 bonds in either direction (+ or -): tg+ g-g+ g-t g-g￾tg￾g+g￾g+t g+g+ t,t From prior discussion, you should be able to estimate energies of 2 & 3 (relative to 1). On the other hand, the least stable conformer 4 requires additional data before is relative energy can be evaluated. Anti(2,3)-Anti(3,4) 1 1 1 1 3 3 3 3 5 5 5 5 Gauche(2,3)-Anti(3,4) Gauche(2,3)-Gauche'(3,4) Gauche(2,3)-Gauche(3,4) double gauche pentane 1 (t,t) 4 (g+g – ) 3 (g+g + ) 2 (g+ t) The new high-energy conformation: (g+g – ) X Y