rotate H o = Approximate Gauche Energy (kcal/mole) isopropyl = i-pr

Transcription

1 alculating confmational energies in open chain compounds. 0 o 0 o 0 o 0 o 0 o (0 o ) (0 o ) (80 o ) (0 o ) (00 o ) 0 o (0 o ) repeat o = o = o = o = o = o = pproximate Eclipsing Energy Values pproximate Gauche Energy (/) Values (/) Me Et i-pr t-u Ph Me Et i-pr t-u Ph G K eq =0 --.RT Me Et i-pr Me Et i-pr t-u t-u Ph Ph bbreviations f substituent groups in Newman projections. hydrogen methyl = Me ethyl = Et isopropyl = i-pr t-butyl = t-u phenyl = Ph Draw all of the Newman projections f rotations around each of the indicated bonds in the structures below and calculate the confmational energies f each confmation using the tables above. tart with the confmation. You can use the above abbreviations in your structures. alculate the ratios of the two lowest confmational energies and the two farthest apart energies.,-dimethylhexane (use the bond) Youhavetodrawthis. 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 80 o 0 o 00 o y:\files\classes\\ pecial andouts\ confmational energies.doc

2 Possible answer: onfmational energies of,-dimethylhexane front dot = =, Me, Et back circle = =,, i-pr o 0 o 0 o 0 o 0 o total =. total =. total =. total =.8 total =.7 total = o 0 o 0 o 80 o 0 o 00 o,,-triimethyl--phenylhexane (use the bond) Youhavetodrawthis. 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 80 o 0 o 00 o y:\files\classes\\ pecial andouts\ confmational energies.doc

3 -methyl--phenylpentane (use the bond) You have to draw this. 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 80 o 0 o 00 o -phenyl--methylpentane (use the bond) R R R R Ph Ph Ph Ph D enantiomers enantiomers diastereomers = (,) (,D) (,) (,D) 0 o 0 o 0 o 0 o 0 o 0 o 0 o 0 o 80 o 0 o 00 o y:\files\classes\\ pecial andouts\ confmational energies.doc

4 alculating confmational energies in substituted cyclohexane s. equatial Use Trends to compare. = substituent group (table of values below) axial Use PE equatial G o = axial - - O r F. -( ). -O. -( ) >.0 -O 0.9 -F 0. -O 0. -l r I =.7 -O. -==. -O e.0 -N 0. -Te (phenyl).9 -N.( ),.7( O) - (benzyl).7 -N( ). ( ),.( O) -O 0. -NO. -O.0 -gr -=O 0.7 -gl -0. -Mgr ( ). -( ) >.0 -=.7 -==. - (phenyl).9 - (benzyl) (benzyl) e.0 -Te 0.9 ubstituent G o ( value) -N.( ), -N.7( O) -O 0. -O.0 -F 0. -l 0. -r 0. -I 0. -=.7 -=O N 0. y:\files\classes\\ pecial andouts\ confmational energies.doc

5 Disubstituted yclohexanes awth Projections - s are drawn flat and substituents are written in with straight lines on the top bottom. R R - -- cis-- -- trans-- -- cis-- -- trans-- -- cis-- -- trans-- -- There are two chair confmations f each cyclohexane example above. One axial group increases the by an amount listed in the table f that substituent. If two axial substituents are on the same side (cis), add an additional / to the sum of their axial energies (arbitrary value f our course). If two substituents are, gauche increase the by an additional 0.8 / (arbitrary value f our course). In some cases the two confmations are equivalent in if =, (,, cis-,, trans-, and cis-,), but in the other cases one confmation is preferred because it has lower. cis trans cis trans cis trans same if = same if = same if = same if = One axial group increases the by an amount listed in the table f that substituent, If two axial substituents are on the same side (cis), add an additional / to the sum of their axial energies (arbitrary value f our course), If two substituents are, gauche increase the by an additional 0.8 / (arbitrary value f our course). Or, you can use the table of open chain 'gauche' values. We need a systematic method of analysis we will quickly become hopelessly lost in the wilderness of flipflopping cyclohexane s. I recommend the following strategy f every cyclohexane analysis. Possible systematic approach to nalyze yclohexane onfmations. Draw the cyclohexane framewk as a bond-line fmula (chair).. dd both axial and equatial positions. xial positions point straight up down in alternating fashion (the points to the axial positions). Equatials are off to the side (use the axial positions to guide you as to top and bottom positions). oth alternate on top bottom of the face of the as you move around the. Use parallel bonds in the to guide you where to draw the equatial bonds.. dd in the necessary substituents accding to the name of the structure (fill in the blank). It is generally easier to visualize substituents drawn on the extreme left extreme right carbon atoms of the because those bonds will be in the plane of the paper, so these are good places to draw your first substituent.. Draw the other confmation by flipping one side up and flip the other side down. ll of the axial and equatial positions will interchange, but the top will still be the top and the bottom will still be the bottom.. If required, draw Newman projections of any specified compounds accding to the indicated perspective.. In addition to,-diaxial interactions look f an extra gauche interactions when substituents are substituted, (vicinal substitution). We will use 0.8 / f all gauche interactions of side groups, which is sometimes increct ( you can use the table of open chain gauche values.. y:\files\classes\\ pecial andouts\ confmational energies.doc

6 Evaluate which is the preferred confmation with the available values. Use the following equation to estimate the ratio between the two confmations.,-disubstituted (If > then is the preferred confmation.) Use Use Use same if = cis-,-disubstituted (If > then is the preferred confmation.) trans-,-disubstituted ( is always the preferred confmation.) cis-,-disubstituted ( is always the preferred confmation.) trans-,-disubstituted (If > then is the preferred confmation.) cis-,-disubstituted (If > then is the preferred confmation.) Use same if = Use Use y:\files\classes\\ pecial andouts\ confmational energies.doc

7 trans-,-disubstituted ( is always the preferred confmation.) Use Use Use Example: cis--isopropyl--methylcyclohexane Use the gauche table f open chain confmations to calculate relative costs from each perspective and add them to find the total cost f each chair confmation. me stable confmation = perspectives f each 's substituents (from all possible views) total strain 0. view. view 0.8 view total =. view view view perspectives f each 's substituents (from all possible views) less stable confmation = total strain. view. view view total =. view view view from above total = Full Newman projection of me stable confmation... Full Newman projection of me less stable confmation. from above.. total =. K = 0 - G.RT.RT = 80 cal/mol when R = cal / (mol-k) and T = 00 K. y:\files\classes\\ pecial andouts\ confmational energies.doc