In chemistry, an alcohol is any organic compound that contains a hydroxyl group (an oxygen atom bonded to a hydrogen atom, written as -OH) attached to a carbon atom. Ethanol, the alcohol in beverages, is just one member of a large family. Methanol, isopropanol, and glycerol are all alcohols too, each with different properties and uses. What unites them is that -OH group, which defines how they behave in reactions, how they dissolve in water, and why they boil at higher temperatures than similar-sized molecules without it.
The Hydroxyl Group: What Makes an Alcohol
The defining feature of every alcohol is the hydroxyl group, -OH, bonded to a carbon atom. This group contains two reactive bonds: the bond between carbon and oxygen (C-O) and the bond between oxygen and hydrogen (O-H). Oxygen is far more electronegative than either carbon or hydrogen, meaning it pulls electron density toward itself. This makes the oxygen end of the molecule slightly negative and the carbon and hydrogen ends slightly positive, creating a polar molecule.
That polarity is the reason alcohols behave so differently from hydrocarbons like gasoline or wax. It allows alcohol molecules to form hydrogen bonds with each other and with water, which dramatically affects their boiling points and solubility.
Primary, Secondary, and Tertiary Alcohols
Chemists classify alcohols based on how many other carbon atoms are attached to the carbon holding the -OH group. This distinction matters because it determines how the alcohol reacts in chemical processes like oxidation.
- Primary (1°) alcohol: The carbon bearing the -OH group is connected to only one other carbon. Ethanol (CH₃CH₂OH) and 1-propanol (CH₃CH₂CH₂OH) are common examples. Methanol (CH₃OH) is also classified as primary even though its -OH carbon has no other carbon attached at all.
- Secondary (2°) alcohol: The -OH carbon is connected to two other carbons. Isopropyl alcohol (2-propanol), the rubbing alcohol you buy at a pharmacy, is a secondary alcohol.
- Tertiary (3°) alcohol: The -OH carbon is connected to three other carbons. Tert-butyl alcohol (2-methyl-2-propanol) is a typical example.
How Alcohols Are Named
The formal naming system (IUPAC nomenclature) replaces the “-e” ending of the parent hydrocarbon with “-ol.” So a one-carbon alcohol is methanol, a two-carbon alcohol is ethanol, and a three-carbon alcohol is propanol. When the -OH group could be on different positions along the chain, a number indicates its location: 1-propanol has the -OH on the end carbon, while 2-propanol has it on the middle carbon.
You’ll also encounter older common names. Methanol is sometimes called methyl alcohol or wood alcohol. Ethanol goes by ethyl alcohol or grain alcohol. Isopropyl alcohol is the common name for 2-propanol. These informal names are still widely used in everyday contexts, while IUPAC names dominate in scientific writing.
Why Alcohols Have High Boiling Points
Alcohols boil at much higher temperatures than hydrocarbons of similar size. This is entirely because of hydrogen bonding. In an alcohol, the slightly positive hydrogen on one molecule’s -OH group is attracted to the lone pair of electrons on the oxygen of a neighboring molecule. These hydrogen bonds are significantly stronger than the weak attractions (van der Waals forces) that hold hydrocarbon molecules together, so it takes more heat energy to pull alcohol molecules apart and turn them into vapor.
As alcohol molecules get larger, their boiling points climb even higher. The hydrogen bonding stays roughly the same, but the larger hydrocarbon portion of the molecule adds more van der Waals attractions on top of it. Both forces must be overcome before the liquid boils.
Hydrogen bonding also explains why small alcohols dissolve easily in water. Methanol and ethanol are completely miscible with water because their -OH groups hydrogen-bond with water molecules. As the hydrocarbon chain grows longer, though, the nonpolar portion of the molecule begins to dominate, and water solubility drops. By the time you reach alcohols with five or six carbons, they barely dissolve in water at all, and their physical properties start to resemble those of hydrocarbons.
Alcohols With More Than One -OH Group
Some alcohols carry two or more hydroxyl groups on the same molecule. A diol has two, and a triol has three. Ethylene glycol, the main ingredient in automotive antifreeze, is a diol with -OH groups on each of its two carbon atoms. Glycerol (also called glycerin) is a triol: a three-carbon chain with an -OH on every carbon, giving it the formula C₃H₈O₃.
These polyhydric alcohols are exceptionally good at hydrogen bonding because of their multiple -OH groups. Glycerol, for instance, has three hydrogen bond donors and three hydrogen bond acceptors. This is why glycerol is thick, syrupy, and hygroscopic (it pulls moisture from the air). It’s completely soluble in water and is used in everything from food products to skin care.
How Alcohols React: Oxidation
One of the most important reactions alcohols undergo is oxidation, where the carbon bonded to the -OH group loses hydrogen atoms and gains bonds to oxygen. The products depend on the alcohol’s classification.
A primary alcohol first oxidizes to an aldehyde, a compound with a carbon double-bonded to oxygen at the end of a chain. Under stronger conditions, that aldehyde can oxidize further to a carboxylic acid (the type of compound that gives vinegar its sourness). A secondary alcohol oxidizes to a ketone, where the carbon-oxygen double bond sits in the middle of the chain rather than at the end. Tertiary alcohols resist oxidation under normal conditions because there’s no hydrogen on the -OH carbon left to remove.
This stepwise oxidation is exactly what happens in your liver when you drink ethanol. Enzymes convert ethanol first to acetaldehyde (an aldehyde), then to acetic acid (a carboxylic acid), which the body can eventually break down into carbon dioxide and water.
Acidity Compared to Water
Alcohols are very weak acids. The -OH group can lose its hydrogen as a proton, but it does so far less readily than a strong acid would. Methanol has a pKa around 15.5, and most simple alcohols fall in the range of 14 to 16. For comparison, water has a conventional pKa of 14.0, making it roughly 35 times more acidic than methanol. This means water gives up a proton more easily than most alcohols do, though both are extremely weak acids in practical terms.
Methanol vs. Ethanol: A Critical Difference
Methanol (CH₃OH) and ethanol (CH₃CH₂OH) differ by just one carbon atom, but the difference in toxicity is enormous. Both are processed in the liver by the same enzyme, alcohol dehydrogenase, but the breakdown products are completely different. Ethanol is converted to acetaldehyde and then to acetic acid, which the body handles without major trouble. Methanol, however, is converted to formaldehyde and then to formic acid.
Formic acid is the dangerous part. It accumulates in the body, causes severe metabolic acidosis (a dangerous drop in blood pH), and directly damages the retina and brain. Even small amounts of methanol can cause permanent blindness or death. One reason ethanol is actually used as a treatment for methanol poisoning is that ethanol has a higher affinity for the same liver enzyme, essentially blocking methanol from being converted into its toxic byproducts and buying time for the body to clear it.
How Ethanol Is Produced
Ethanol is produced at industrial scale through two main routes. The older method is fermentation, where yeast breaks down sugars in the absence of oxygen. The core reaction converts one molecule of glucose into two molecules of ethanol and two molecules of carbon dioxide (C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂). This is the same process behind brewing and winemaking, and it’s also how most fuel ethanol is made, increasingly from cellulose-rich plant waste rather than food crops.
The second method is catalytic hydration of ethylene, a petrochemical process where water is added across the double bond of ethylene gas in the presence of a catalyst. This produces high-purity ethanol efficiently and is the primary method for making industrial-grade ethanol used in solvents, disinfectants, and chemical manufacturing.