Question 1

Given the equation $$\Delta G^\circ_{AB} = -RT \ln \frac{[A]}{[B]}$$, what is the equilibrium ratio $$\frac{[B]}{[A]} if$$ $$\Delta G^\circ_{AB} = -2.8$$ kcal/mol at 298 K? (Assume $$R = 0.001987$$ kcal/(mol·K))

Accepted Answer

$$\frac{[B]}{[A]} = 100$$

Question 2

A reaction has a $$\Delta G^\circ of$$ $$+4.2$$ kcal/mol. Using the rule that $$1.4$$ kcal/mol corresponds to a factor of $$10 in $$equilibrium constant, what is the approximate ratio of starting material to product at equilibrium?

Accepted Answer

1000:1

Question 3

If the transition state for product X is $$1.8$$ kcal/mol lower than for product Y, what is the expected product ratio $$\frac{[X]}{[Y]}$$ under kinetic control?

Accepted Answer

95:5

Question 4

Which of the following best explains why macrocyclic ring formation is difficult in organic synthesis?

Accepted Answer

The entropic cost is high due to many possible conformers.

Question 5

What is the entropic cost (in kcal/mol) of pairing two molecules in a 1 M solution, and how does this affect the stability of the complex?

Accepted Answer

$$+14$$ kcal/mol; makes the complex $$10^{10}$$ times less stable

Question 6

Why is the formation of a 5-membered ring generally faster than a 3-membered or 8-membered ring in cyclization reactions?

Accepted Answer

5-membered rings have minimal ring strain and favorable entropy.

Question 7

If an ideal hydrogen bond between two water molecules is $$-4.6$$ kcal/mol, but only $$1$$ out of $$16$$ water pairs forms this bond at equilibrium, what is the main reason for this observation?

Accepted Answer

The entropy term ($$-T\Delta S) $$favors many other arrangements.

Question 8

According to the molecular orbital (MO) diagram trends, which bond is most nucleophilic?

Accepted Answer

A longer C–C bond

Question 9

Which of the following statements about nucleophilicity and electrophilicity parameters is correct based on Prof. Mayr’s database?

Accepted Answer

Enolates are orders of magnitude more nucleophilic than alkenes.

Question 10

If the methyl cation affinity of a species correlates with its nucleophilicity on a log scale, which of the following is true?

Accepted Answer

A higher methyl cation affinity means higher nucleophilicity.

Question 11

A reaction has a $$\Delta G^\circ of$$ $$-1.4$$ kcal/mol. What is the equilibrium constant $$K_{eq}$$ for this reaction?

Accepted Answer

$$K_{eq} = 10$$

Question 12

Which of the following best describes why unimolecular (intramolecular) reactions are generally more favorable than bimolecular reactions?

Accepted Answer

They avoid the large entropic cost of bringing two molecules together.

Question 13

If the equilibrium ratio of two conformers is $$20:1$$, what is the approximate free energy difference between them?

Accepted Answer

$$1.8$$ kcal/mol

Question 14

Which of the following is a correct application of the rule: $$1.4$$ kcal/mol corresponds to a factor of $$10 in$$ $$K_{eq} or$$ $$k$$?

Accepted Answer

A reaction with $$\Delta G^\circ = 2.8$$ kcal/mol will have $$K_{eq} \approx 0.01$$

Question 15

Which factor most strongly disfavors the formation of an 8-membered ring in a cyclization reaction?

Accepted Answer

High entropic cost due to many possible conformations

Question 16

If the transition state energy for the endo product is $$1.8$$ kcal/mol lower than for the exo product, what is the expected kinetic product ratio?

Accepted Answer

95:5

Question 17

Which of the following statements about the effect of temperature on entropy in chemical reactions is correct?

Accepted Answer

Higher temperature increases the entropic penalty for unfavorable orientations.

Thermodynamics, Kinetics, and Reactivity in Organic Chemistry

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