Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Problem 47c

Chapter 4, Problem 47c

The following table of strain energies is associated with a variety of 1,2-gauche interactions. Use this table to answer the questions (i)–(iv).

For each of the bond rotations shown, (i) identify which you believe to be more stable, (ii) calculate ∆G°, (iii) calculate the equilibrium constant, and (iv) draw the transition state.
(c)

Verified step by step guidance

Step 1: Begin by analyzing the given table of strain energies for 1,2-gauche interactions. Identify the specific strain energy values associated with the bond rotations in question (c). These values will be critical for calculating the stability and thermodynamic parameters.

Step 2: To determine which bond rotation is more stable (i), compare the strain energies of the two conformations. The conformation with the lower strain energy will be more stable because it experiences less steric hindrance or torsional strain.

Step 3: Calculate the Gibbs free energy change (∆G°) (ii) using the formula:

Δ G ° = E

_strain(less−stable)−E_strain(more−stable). This equation subtracts the strain energy of the more stable conformation from the less stable conformation.

Step 4: Calculate the equilibrium constant (iii) using the relationship between ∆G° and the equilibrium constant:

K = e^{(- Δ G ° / RT)}

, where R is the gas constant (8.314 J/mol·K) and T is the temperature (assumed to be 298 K). Plug in the calculated ∆G° value to find the equilibrium constant.

Step 5: Draw the transition state (iv) for the bond rotation. The transition state represents the highest energy point during the rotation process. Use your knowledge of molecular geometry and steric interactions to depict the arrangement of atoms and bonds in the transition state. Highlight any significant steric clashes or torsional strain that contribute to the energy barrier.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Strain Energy

Strain energy refers to the energy stored in a molecule due to the distortion of its normal bond angles and lengths. In organic chemistry, this is particularly relevant in conformational analysis, where different spatial arrangements of atoms can lead to varying levels of stability. Understanding strain energy helps predict the stability of different conformers, such as those involved in 1,2-gauche interactions.

Gibbs Free Energy (∆G°)

Gibbs Free Energy (∆G°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It is crucial for determining the spontaneity of a reaction; a negative ∆G° indicates a spontaneous process. In the context of bond rotations, calculating ∆G° helps assess the stability of different conformations.

Equilibrium Constant (K)

The equilibrium constant (K) quantifies the ratio of the concentrations of products to reactants at equilibrium for a reversible reaction. It is directly related to the Gibbs Free Energy change (∆G°) through the equation ∆G° = -RT ln(K), where R is the gas constant and T is the temperature in Kelvin. Understanding K is essential for predicting the favorability of different conformations in the context of bond rotations.

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Related Practice

Textbook Question

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.

(c)

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Textbook Question

For each of the following acid–base reactions, (i) predict which side of the reaction you expect to be favored. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

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Textbook Question

The radical fluorination of 2-methyl propane resulted in a 14:86 ratio of products.

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Textbook Question

For each of the following acid–base reactions, (iv) draw the transition state, paying close attention to the degree of bond forming and breaking present in the transition state. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

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Textbook Question

For each of the following acid–base reactions, (ii) calculate K_eq. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

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Textbook Question

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.

(a)

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