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Question 3.42: Consider 2-methylbutane (isopentane). Sighting along the C2–......

Consider 2-methylbutane (isopentane). Sighting along the C2–C3 bond:

(a) Draw a Newman projection of the most stable conformation.

(b) Draw a Newman projection of the least stable conformation.

(c) If a CH_3 ↔ CH_3 eclipsing interaction costs 11 kJ/mol (2.5 kcal/mol) and a CH_3 ↔ CH_3 gauche interaction costs 3.8 kJ/mol (0.9 kcal/mol), make a quantitative plot of energy versus rotation about the C2–C3 bond.

Question Data is a breakdown of the data given in the question above.

CH3 ↔ CH3 eclipsing interaction costs 11 kJ/mol (2.5 kcal/mol)
CH3 ↔ CH3 gauche interaction costs 3.8 kJ/mol (0.9 kcal/mol)

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Step 1:
the energy difference between the two conformations is calculated by subtracting the energy of the less stable conformation (3.8 kJ/mol) from the energy of the more stable conformation (1.0 + 6.0 + 4.0 kJ/mol). This gives a value of 17.2 kJ/mol.
Step 2:
the different conformations are obtained by rotating the back of the projection while keeping the front unchanged. The energy values for each conformation are given as follows: at 60°: 3.8 kJ/mol, at 120°: 18.0 kJ/mol, at 180°: 3.8 kJ/mol, at 240°: 21.0 kJ/mol, and at 300°: 7.6 kJ/mol.
Step 3:
the lowest energy conformation is considered as the energy minimum. The highest energy conformation is then determined to be 17.2 kJ/mol higher in energy than the lowest energy conformation.

Final Answer

(a), (b)

The energy difference between the two conformations is (1 1.0 + 6.0 + 4.0) kJ/mol – 3.8kJ/mol = 17.2kJ/mol.

(c) Consider the least stable conformation to be at zero degrees. Keeping the front of the projection unchanged, rotate the back by 60° to obtain each conformation.

\underline{at  60° }: energy = 3.8 kJ/mol \underline{at  120° }: energy = 18.0 kJ/mol \underline{at  180°} : energy = 3.8 kJ/mol

\underline{at  240°}: energy = 2 1 .0 kJ/mol \underline{at   300°} : energy = 7.6 kJ/mol

Use the lowest energy conformation as the energy minimum. The highest energy conformation is 17.2 kJ/mol higher in energy than the lowest energy conformation.

1915043-3.42.4
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