Question

Strain Energy Increments Strain Energy for Alkanes Interaction/Compound kJ/mol kcal/ 4.0 H: Heclipsing 1.0 H: CH3 eclipsing 5.8 1.4 CH3 : CH3 eclipsing 11.0 2.6 gauche butane 3.8 0.9 cyclopropane 115 27.5 cyclobutane 110 26.3 cyclopentane 26.0 6.2 6.3 cycloheptane 26.2 cyclooctane 40.5 9.7 Axial Strain Energies for Monosubstituted Cyclohexanesab This table gives the sum of the values for the 1,3 diaxial interactions of the substituent with two hydrogen atoms. kJ/mol kcal/mol Substituent(solvent) -CH3 1.7 7.3 CH2CH3 7.5 1.8 -CH(CH3)2 9.2 2.2 -C(CH3)3 20 4.8 phenyl 11.7 2.8 cyclohexyl 9.2 2,2 CH-CH2 6.2 1.5 -CCH, ethyny 2.1 0.5 0.7 -сно 3.0 -соснз 5.1 1,2 -со 2н 5.9 1.4 CO2CH2CH3 5,0 1.2 -Cl 2,5 0.6 2,5 Br 0.6 -CN, cyano 0.8 0.2 -OH (cyclohexane) 2.5 0.6 -он (isopropanol) 4.0 1.0 -OCH3 2,5 0.6 -NH2 (toluene) 5.2 1,2 -NH2 (CH3OCH>CH20Н /н-0) 1.7 7,1 NO2 4.8 1.1 access important values if needed for this question Use the References a. Use strain energy increments in the OWL Table Reference (see References button, Strain Energy Increments) to calculate the energy difference between the two chair conformations of the compound below. b. Specify substituent positions (axial or equatorial) in the more stable chair c. Estimate the percent of the more stable chair at equilibrium at 25°C. (To determine the percent of the more stable chair at equilibrium, first calculate Keg, and then use this value to find the percentage.) H C-CH2 CH3 Answers kJ/mol a. The energy difference is b. In the more stable chair o The vinyl(ethenyl) group is in the o The methyl group is in the position. position. c. At 25°C the equilibrium percent of the more stable chair conformation is approximately

Answer 1

Chair Conformations of Cyclohexane position cis trans (a,a) or (e,e) 1,2 а,е (a,a) or (e,e) 1,3 а,е (a,a) or (e,e) а,е 1,4 cis-1-methyl-2-vinylcyclohexane CH CH2 1,2 trans conformation is represented in two forms 'CH3 -CHз CH=CH2 СH-CH Conformer Conformer lI Axial strain values for -CH=CH, is 6.2 kJ/mol - CH2 is 7.3 kJ/mol In Conformer I, -CH-CH, is in equitorial and methyl is at axial So energy for conformer 1 is 7.3 kJ/mol In Conformer II, -CH CH, is at axial and methyl is at equitorial So energy for conformer 2 is 6.2 kJ/mol a) Energy difference between these two conformers 7.3 - 6.2 = 1.1 kJ/mol b) In the most stable chair form the vinyl group is present at Axial position and the methyl group present at Equitorial position

Chair Conformations of Cyclohexane position cis trans (a,a) or (e,e) 1,2 а,е (a,a) or (e,e) 1,3 а,е (a,a) or (e,e) а,е 1,4 cis-1-methyl-2-vinylcyclohexane CH CH2 1,2 trans conformation is represented in two forms 'CH3 -CHз CH=CH2 СH-CH Conformer Conformer lI Axial strain values for -CH=CH, is 6.2 kJ/mol - CH2 is 7.3 kJ/mol In Conformer I, -CH-CH, is in equitorial and methyl is at axial So energy for conformer 1 is 7.3 kJ/mol In Conformer II, -CH CH, is at axial and methyl is at equitorial So energy for conformer 2 is 6.2 kJ/mol a) Energy difference between these two conformers 7.3 - 6.2 = 1.1 kJ/mol b) In the most stable chair form the vinyl group is present at Axial position and the methyl group present at Equitorial position

(c)

Energy 1.1 kJ.mol = 1100 J.mol1 We know that AG° = -RT ln K =-8.314J.K molx298 Kx ln K >1100 Jmol > In K = -0.444 >K = 0.6415 Less Stable 0.6415 More Stable 1 Percent of more stable conformer = x100 = -x 100 60.92% 10.6415 1.6415