3. Consider the following coupled reaction

Acetaldehyde + 2H⁺+2e^– à Ethanol   ΔE⁰= 0.16 V
NAD+ 2H⁺+2e^– à NADH + H⁺ ΔE⁰ = -032 V
Under standard conditions the transfer of electron is from
(1) NAD to acetaldehyde
(2) NADH to ethanol
(3) NADH to acetaldehyde
(4) Ethanol to NADH

In biochemical systems, coupled redox reactions are essential for energy transfer and metabolism. One such important reaction involves the reduction of acetaldehyde to ethanol, coupled with the oxidation of NADH to NAD+. Understanding which molecule donates electrons in this process relies on comparing their standard redox potentials (E0).

The Coupled Reaction and Redox Potentials

Consider the following half-reactions with their standard redox potentials:

• Acetaldehyde + 2H^+ + 2e^- → Ethanol  ΔE0 = +0.16 V

• NAD^+ + 2H^+ + 2e^- → NADH + H^+    ΔE0 = -0.32 V

Note: The NAD^+/NADH couple is often represented with E0 ≈ -0.32 V, indicating its strong reducing power when in the NADH form.

Determining Electron Flow Direction

Electrons flow spontaneously from the half-reaction with lower redox potential to the one with higher redox potential. Here:

• NADH/NAD^+ couple has a redox potential of -0.32 V (more negative)

• Acetaldehyde/ethanol couple has a redox potential of +0.16 V (more positive)

This means NADH donates electrons to acetaldehyde, reducing it to ethanol.

Evaluating the Options

1. NAD to acetaldehyde: NAD^+ is the oxidized form and cannot donate electrons; this is incorrect.

2. NADH to ethanol: NADH donates electrons to acetaldehyde, not to ethanol (the product); this is incorrect.

3. NADH to acetaldehyde: Correct. NADH donates electrons to acetaldehyde, reducing it to ethanol.

4. Ethanol to NADH: Ethanol would have to donate electrons to NAD^+, which is not spontaneous under standard conditions; this is incorrect.

Conclusion

The correct electron transfer under standard conditions is from NADH to acetaldehyde, facilitating the reduction of acetaldehyde to ethanol.

Answer: (3) NADH to acetaldehyde

Additional Context

This reaction is catalyzed by alcohol dehydrogenase in biological systems, where NADH serves as the electron donor, and acetaldehyde is the electron acceptor. The favorable redox potential difference drives the reaction forward, allowing cells to regenerate NAD^+ for glycolysis and produce ethanol, especially under anaerobic conditions.