Problem 39 - Basic Principles of Organic Chemistry

Question

Explain how resonance can be used to account for the fact that the \(\Delta H^{0}\) for reduction of \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) to \(\mathrm{CH}_{3} \mathrm{CHO}\) is about \(18 \mathrm{kcal} \mathrm{mol}^{-1}\) more positive than calculated from bond energies, whereas \(\Delta H^{0}\) for the corresponding reduction of \(\mathrm{CH}_{3} \mathrm{COCl}\) to \(\mathrm{CH}_{3} \mathrm{CHO}\) is about as expected from bond energies. Would you expect \(\Delta H^{0}\) for reduction of \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\) to be as expected from the pertinent bond energies? Why?

Step-by-Step Solution

Answer

The reduction of \(\mathrm{CH}_3 \mathrm{CONH}_2\) may have a more positive \(\Delta H^0\) due to resonance in the amide group, unlike \(\mathrm{CH}_3 \mathrm{COCl}\).

Step 1: Understanding Resonance Energy

Resonance energy is the difference between the bond energy of a real molecule and the bond energy calculated from a single resonance structure. It accounts for the stability added by having multiple contributing structures.

Step 2: Analyze Reduction of CH3CO2H

The reduction of acetic acid (\(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\)) to acetaldehyde (\(\mathrm{CH}_3 \mathrm{CHO}\)) involves breaking and forming bonds, where the calculated enthalpy change (\(\Delta H^0\)) does not match the experimental value, being \(18 \mathrm{kcal} \mathrm{mol}^{-1}\) more positive due to extra stability from resonance.

Step 3: Compare with Reduction of CH3COCl

In the reduction of acetyl chloride (\(\mathrm{CH}_3 \mathrm{COCl}\)) to acetaldehyde, resonance is not as significant, so the calculated \(\Delta H^0\) using bond energies aligns with the experimental value, indicating no additional stabilization from resonance.

Step 4: Predicting for CH3CONH2

The reduction of acetamide (\(\mathrm{CH}_3 \mathrm{CONH}_2\)) to acetaldehyde would likely be influenced by resonance, as amides exhibit resonance stabilization. Hence, \(\Delta H^0\) might be more positive than expected from bond energies because of this additional resonance stabilization.

Key Concepts

Resonance Energy

Resonance energy is a crucial concept in understanding the stability of certain molecules in organic chemistry. It is the energy difference between the actual structure of a molecule and the structure predicted by considering only a single resonance form. Multiple resonance structures suggest increased stabilization because the electrons are more delocalized, spreading out any charge more evenly across the molecule. This delocalization leads to lower energy and thus greater stability.

• Real molecules often exist as a hybrid of their resonance forms.
• The more resonance structures, the greater the stability due to electron delocalization.
• Resonance energy can be understood by comparing calculated bond energies with experimental values.

In the case of reducing acetic acid to acetaldehyde, resonance within the original structure of acetic acid accounts for an additional stabilization, making the enthalpy change more positive than what is calculated from simple bond energies.

Reduction Reactions

Reduction reactions involve the formal gain of electrons or the loss of oxygen atoms in organic chemistry. When reducing a compound such as acetic acid to acetaldehyde, it's important to consider the changes in bonding during the reaction. The calculated enthalpy change (\(\Delta H^0\)) may not always match the observed values due to resonance effects.

• For acetic acid, the reduction unexpectedly resulted in a more positive enthalpy change by 18 kcal/mol due to resonance stabilization.
• Acetyl chloride, lacking such resonance, has a reduction reaction with enthalpy more in line with expected bond energies.
• The comparison suggests that the predicted results of reduction reactions can significantly deviate due to the presence or absence of resonance structures.

Reduction reactions vary greatly depending on the nature of the reactants, particularly their resonance stabilization. Therefore, resonance must always be considered when predicting enthalpy changes for these reactions.

Enthalpy Change

Enthalpy change (\(\Delta H^0\)) is the heat change at constant pressure during a chemical reaction. It helps determine the favorability of a reaction. Experiments often compare this value to one computed purely from bond energies. The enthalpy change for the reduction of certain compounds sometimes deviates from expectations due to resonance.

• For molecules like acetic acid, resonance causes a higher positive enthalpy change than predicted.
• This implies that resonance contributes additional stability, affecting the observed enthalpy.
• Amides, such as acetamide, are also expected to exhibit these deviations due to their resonance characteristics, predicting a more positive \(\Delta H^0\) than simple calculation would suggest.

In summary, enthalpy change is a valuable tool for understanding reaction energetics, but must be interpreted with consideration of resonance, as it provides the structural stability impacting this thermodynamic parameter.