Video Transcript
The structures of two alkylbenzene sulfonates are shown below. Why might structure B be preferred over structure A as a commercial detergent? (A) Structure B is more thermally stable. (B) Structure A has a higher melting point. (C) Structure B is more biodegradable. (D) Structure B has a more aromatic smell. Or (E) structure A is more expensive.
Both of the alkylbenzene sulfonates shown can function as a detergent. A detergent is a class of substances with cleaning properties. The cleaning properties of a detergent are the results of its unique structure. A detergent contains both a hydrophilic head region which can interact with water molecules and a hydrophobic tail region which can interact with nonpolar fats and oils.
In an alkylbenzene sulfonate, the negatively charged sulfonate group is the head, and the alkylbenzene portion is the tail. The sodium shown in each of these structures is a positively charged counterion that accompanies the detergent so that the species is electrically neutral. We can see that both of structure A and B have the same head region and counterion. And while the tail region of each detergent is similar, the tail of structure A has a number of branching substituents.
Branched alkylbenzene sulfonates, like this one, were some of the first anionic detergents introduced in the 1930s. However, these substances do not easily biodegrade. Their use caused foaming in sewage systems and waterways, which contributed to the contamination of the drinking water supply. Detergents, like structure B, which are more linear or have fewer branches biodegrade more easily and therefore pose less of a threat to environmental systems. So, structure B is preferred over structure A as a commercial detergent because structure B is more biodegradable, or answer choice (C).