The question of how rubbing alcohol differs from liquor starts with a fundamental chemical distinction. While both are members of the alcohol chemical family, the specific compound in liquor is ethanol (ethyl alcohol). Rubbing alcohol, conversely, is most commonly a solution of isopropyl alcohol (isopropanol). Though they share the hydroxyl (\(\text{OH}\)) functional group, classifying them as alcohols, their distinct molecular structures lead to profound variations in safety, metabolism, and use.
Chemical Identity and Molecular Structure
The chemical formula for ethanol, the alcohol in liquor, is \(\text{C}_2\text{H}_5\text{OH}\). It is characterized as a primary alcohol because its hydroxyl (\(\text{OH}\)) group is attached to a carbon atom connected to only one other carbon atom, positioned at the end of the chain. This structure is the compound produced by the fermentation of sugars in grains or fruits.
Isopropyl alcohol, found in rubbing alcohol, has the formula \(\text{C}_3\text{H}_7\text{OH}\). It is classified as a secondary alcohol, meaning its hydroxyl group is attached to a carbon atom bonded to two other carbon atoms, placing the \(\text{OH}\) group in the middle of the three-carbon chain. This structural placement distinguishes it chemically from ethanol.
Toxicity and How the Body Processes Each
The body processes ethanol and isopropyl alcohol using the same initial enzyme, alcohol dehydrogenase, but the metabolic results differ drastically. When ethanol is consumed, the liver first converts it into acetaldehyde. Although toxic and responsible for hangover effects, acetaldehyde is quickly converted by a second enzyme into harmless acetic acid.
Ingesting isopropyl alcohol is severely dangerous because of its metabolic byproduct. Isopropyl alcohol is metabolized into acetone, a ketone compound familiar as a component of nail polish remover. Acetone is a central nervous system depressant and is significantly more toxic than ethanol.
The body excretes acetone much more slowly than the harmless acetic acid produced from ethanol, leading to prolonged and severe poisoning. Ingesting even relatively small amounts of isopropyl alcohol can cause rapid and intense central nervous system depression, leading to dangerously low blood pressure, respiratory failure, organ damage, and death. Isopropyl alcohol is acutely toxic and unsafe for human consumption.
Denaturing and Regulatory Differences
The distinction between these substances is reinforced by regulatory measures, particularly concerning denatured alcohol. Denatured alcohol is ethanol intentionally made toxic and unpalatable by adding various chemicals, a process known as denaturing. Common denaturants include methanol, acetone, and bittering agents like denatonium benzoate. This process ensures the ethanol cannot be consumed recreationally.
The primary motivation for denaturing is financial and regulatory. Pure ethanol intended for beverages is subject to heavy government excise taxes. Denaturing the alcohol makes it unfit for human consumption, exempting it from these taxes and allowing it to be sold affordably for industrial uses, such as solvents or fuels.
This tax-exempt, denatured ethanol is separate from the isopropyl alcohol found in most rubbing alcohol, though isopropyl alcohol is sometimes used as a denaturant itself. The existence of denatured ethanol and the inherent toxicity of isopropyl alcohol establish clear regulatory boundaries. These boundaries ensure that industrial and cleaning products are not mistaken for or consumed as liquor.