Antibacterial potencies of primary alcohols increase with molecular weight up to how many carbons only?

• A. C3
• B. C4
• C. C6
• D. C8

Answer: (D)

The antibacterial activity of primary alcohols is driven by their ability to interact with and disrupt bacterial membranes, which improves as the molecule becomes more lipophilic. Adding carbon atoms increases hydrophobicity and helps the alcohol partition into lipid membranes, boosting potency—up to a practical limit. That limit is eight carbons: up to C8 the potency increases, but beyond eight carbons the alcohol becomes too poorly soluble in water to reach and diffuse to the bacteria effectively, so the potency no longer rises. So, eight carbons is the point where the increasing trend stops.