C given the values below determine the δ h c of e 13

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c. Given the values below, determine the Δ H c ° of ( E )-1,3-pentadiene. H f ° (1,4-pentadiene) = 25 kcal mol -1 H c ° (1,4-pentadiene) = -769 kcal mol -1 H f ° (( E )-1,3-pentadiene) = 18 kcal mol -1 H c ° (( E )-1,3-pentadiene) = e. Estimate H c ° of ( Z )-1,3-pentadiene relative to ( E )-1,3-pentadiene? Briefly explain. 8. Given the Δ H f ° values indicated, rank the compounds below from most heat released upon combustion (most negative Δ H c °) to least heat released. Briefly explain your reasoning. Δ H f ° (kcal mol -1 ) 12.7 4.9 -0.1 6.4 d. Suggest a reason for the enhanced stability of ( E )-1,3-pentadiene isomer over 1,4-pentadiene. 5. Consider the structure of 1,4-pentadiene. a. write out a balanced chemical reaction for the heat of formation of 1,4 pentadiene. Chem 33, Kanan & Stack, 2012, PS 3; pg 5
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9. Conformational analysis of 2-methylbutane a. With Newman projections, draw out the lowest and highest energy conformer of 2-methylbutane viewing down the C 2 -C 3 bond. c. Estimate the relative amounts of the three most stable conformers at 300 K. DG° = –RT(lnK eq ) where R = 0.002 kcal mol -1 K -1 ; K eq = [conformation A]/[conformation B] b. With an H-C 2 -C 3 -H dihedral angle as a reference, create a relative energy versus dihedral angle graph for 2- methylbutane in increments of 60°, from -180° to +180°, assuming the following energies for butane conformations: Me H H Me H H Me H H H Me H Me H H Me H H Me H H H Me H 0 (kcal mol -1 ) 0.9 4.5 3.8 anti eclipsed gauche eclipsed Chem 33, Kanan & Stack, 2012, PS 3; pg 6
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