Answer to Question #305147 in General Chemistry for Kel

Question

Why is ethanol denser than isopropanol?

We are asked to compare its specific gravity using hydrometer. My readings are 0.820 SG for ethanol and 0.795 SG for isopropanol.

Answer 1

Fuel ethanol is an agriculturally-derived product. It’s use is mandated under the Energy Policy Act and subsequent legislation. According to the law, the material used to comply must be derived from biomass. So ethanol is mandated by law while isopropyl alcohol isn’t. Ethanol can be made similarly to isopropyl alcohol but even that doesn’t comply. It has to come from biomass.

Energy density is nice but irrelevant. The point of fuel ethanol is not to get an energy dense fuel but to use up corn and support crop prices. The excuses are energy supply security and renewability. If energy density is the goal, gasoline is cheaper than either ethanol or isopropyl alcohol and has considerably higher energy density.

Isopropyl alcohol is manufactured from propylene a common petrochemical precursor. Propylene is reacted with water to form isopropyl alcohol, a hydration reaction. Ethylene also can be hydrated to make ethanol but that doesn’t comply with the law, as mentioned above.

There’s simply no point in making a fuel from propylene by hydration. A simpler process is to react the same propylene molecule with isobutane in an alkylation reaction. Many refineries have that equipment and it’s a widely used process. An alternative use for propylene is making polypropylene and other polymers, a higher value application than fuel for many refineries. Some propylene can be used as fuel directly in commercial grade LPG.

Isopropyl alcohol is a dandy solvent but that’s about it.