Alcohols: The Hydroxyl Family
What Exactly is an Alcohol?
In everyday language, "alcohol" often refers to the specific compound found in alcoholic beverages, ethanol. However, in chemistry, the term has a much broader meaning. An alcohol is any organic compound where a hydroxyl group (-OH) is bonded to a carbon atom. The key here is that the carbon atom must be saturated, meaning it is connected to other atoms by single bonds. This distinguishes alcohols from other compounds containing -OH groups, like carboxylic acids or phenols[1].
The simplest alcohol is methanol ($CH_3OH$), which has one carbon atom. The next is ethanol ($C_2H_5OH$), with two carbon atoms. As you add more carbon atoms, you get a series of related compounds. This is known as a homologous series[2]—a family of compounds with the same functional group and general formula, where each member differs from the next by a $CH_2$ group. For alcohols, the general formula is $C_nH_{2n+1}OH$.
Classifying Alcohols: Primary, Secondary, and Tertiary
Not all alcohols are the same. They are classified based on the number of carbon atoms attached to the carbon that holds the hydroxyl group (the carbinol carbon). This classification significantly impacts their chemical reactivity.
- Primary (1°) Alcohols: The carbinol carbon is attached to only one other carbon atom. Example: Ethanol ($CH_3-CH_2-OH$).
- Secondary (2°) Alcohols: The carbinol carbon is attached to two other carbon atoms. Example: Propan-2-ol ($CH_3-CH(OH)-CH_3$).
- Tertiary (3°) Alcohols: The carbinol carbon is attached to three other carbon atoms. Example: 2-methylpropan-2-ol ($(CH_3)_3C-OH$).
This structural difference means that primary alcohols are generally more easily oxidized to aldehydes and then carboxylic acids, while tertiary alcohols are resistant to oxidation.
Naming Alcohols: The IUPAC System
To avoid confusion, scientists use a standardized system for naming chemical compounds called IUPAC[3] nomenclature. Here is a step-by-step guide to naming a simple alcohol:
- Identify the longest continuous carbon chain that contains the -OH group. This forms the base name (e.g., meth-, eth-, prop-, but-).
- Number the carbon atoms in this chain so that the carbon bearing the -OH group gets the lowest possible number.
- Drop the final "e" from the base name of the alkane and add the suffix "-ol".
- Use the number from step 2 to indicate the position of the -OH group.
- Name and number any side branches (alkyl groups) and place them at the front of the name.
Example: $CH_3-CH_2-CH_2-OH$ is called propan-1-ol. The chain is three carbons long (prop-), the -OH is on the first carbon, so we add "-1-ol".
| IUPAC Name | Common Name | Molecular Formula | Class | Common Use |
|---|---|---|---|---|
| Methanol | Wood Alcohol | $CH_3OH$ | Primary | Antifreeze, solvent |
| Ethanol | Grain Alcohol | $C_2H_5OH$ | Primary | Beverages, fuel, disinfectant |
| Propan-2-ol | Rubbing Alcohol | $C_3H_7OH$ | Secondary | Disinfectant, solvent |
| Butan-1-ol | Butyl Alcohol | $C_4H_9OH$ | Primary | Solvent for paints, varnishes |
Physical Properties and Trends in the Homologous Series
The physical properties of alcohols change in a predictable way as we move up the homologous series, from methanol to longer-chain alcohols like octanol.
Boiling Points: Alcohols have significantly higher boiling points than alkanes[4] of similar molecular mass. This is due to hydrogen bonding. The -OH group is highly polar, and the hydrogen atom attached to the oxygen can form a strong attraction to the oxygen atom of another alcohol molecule. This intermolecular force requires more energy (heat) to break than the weaker London forces found in alkanes. For example, ethanol ($C_2H_5OH$) boils at 78.37 °C, while propane ($C_3H_8$), which has a similar mass, boils at -42 °C. Within the series, boiling point increases with chain length.
Solubility in Water: Smaller alcohols like methanol, ethanol, and propanol are completely miscible with water because their -OH groups can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, the non-polar part of the molecule starts to dominate, and the solubility in water decreases. Larger alcohols, like pentanol and above, are much less soluble.
Alcohols in Action: From Labs to Daily Life
Alcohols are not just theoretical concepts; they are workhorse molecules in both the laboratory and our everyday lives.
As Solvents: Ethanol and propan-2-ol are excellent solvents. They can dissolve a wide range of polar and non-polar substances, making them ideal for use in paints, inks, perfumes, and medicinal tinctures. The hand sanitizer you use relies on the solvent action of alcohol to disrupt the lipid membranes of microbes.
As Fuels: Ethanol is a major biofuel. It is produced by the fermentation of sugars from crops like corn and sugarcane. When burned, it releases energy and produces carbon dioxide and water. Many countries mix ethanol with gasoline (e.g., E10, which is 10% ethanol) to create a cleaner-burning fuel.
As Disinfectants: As mentioned, alcohols like ethanol (60-90%) and isopropanol are effective disinfectants. They kill microorganisms by denaturing their proteins and dissolving their cell membranes.
As Antifreeze: Methanol and ethylene glycol (a diol, an alcohol with two -OH groups) are used in antifreeze for car radiators. Their ability to form hydrogen bonds with water lowers the freezing point of the mixture, preventing the engine coolant from freezing in winter.
Important Questions
Why is methanol poisonous while ethanol is safe to drink in small quantities?
The human body metabolizes alcohols using enzymes. The liver enzyme alcohol dehydrogenase first oxidizes methanol to formaldehyde, which is highly toxic and can cause blindness or death. Ethanol, on the other hand, is oxidized to acetaldehyde and then acetic acid (vinegar), which is much less toxic and can be processed by the body. In cases of methanol poisoning, ethanol is sometimes administered as an antidote because the enzyme has a higher affinity for ethanol, slowing down the production of formaldehyde.
What is the difference between denatured alcohol and pure ethanol?
Pure ethanol (100%) is subject to heavy taxation in many countries because it is used in alcoholic beverages. To make it unfit for drinking and thus tax-free for industrial use, it is "denatured" by adding small amounts of unpleasant or toxic substances like methanol, benzene, or denatonium benzoate (the most bitter compound known). Denatured alcohol is perfectly fine for use as a solvent or fuel but is extremely dangerous to consume.
How can you test for the presence of an alcohol?
A common test for primary and secondary alcohols is the oxidation test with acidified potassium dichromate($K_2Cr_2O_7$). This orange solution will turn green in the presence of an alcohol that can be oxidized (primary and secondary, but not tertiary). This color change is a clear visual indicator of a reaction. Breathalyzer tests used by police are based on a similar oxidation reaction of ethanol.
Footnote
[1] Phenols: A class of organic compounds where the -OH group is directly attached to a carbon atom that is part of an aromatic benzene ring. They have different chemical properties from alcohols.
[2] Homologous Series: A series of organic compounds with the same functional group, where each successive member differs by a $CH_2$ unit. Members show a gradual change in physical properties and similar chemical properties.
[3] IUPAC: International Union of Pure and Applied Chemistry. This body establishes the standardized rules for chemical nomenclature.
[4] Alkanes: A homologous series of saturated hydrocarbons with the general formula $C_nH_{2n+2}$. They contain only carbon-carbon and carbon-hydrogen single bonds.