Alcohols: The Hydroxyl Functional Group

What makes an alcohol

Alcohols are organic compounds in which a hydroxyl group (–OH) is bonded to a saturated (sp3) carbon atom. They follow the general formula CnH2n+1OH — the alkane formula with one hydrogen replaced by –OH.

Do not confuse the –OH group with the hydroxide ion. In NaOH the OH− is a separate ion making the compound basic; in ethanol the –OH is covalently bonded to carbon and the molecule is neutral.

The –OH group transforms the molecule's behavior: it enables hydrogen bonding, raises boiling points dramatically, allows mixing with water, and opens up oxidation and substitution chemistry that alkanes never show.

Primary, secondary, and tertiary

Alcohols are classified by the carbon bearing the –OH group:

• Primary: the carbon is attached to one other carbon. Example: ethanol (CH3CH2OH).
• Secondary: the carbon is attached to two other carbons. Example: propan-2-ol (CH3CH(OH)CH3).
• Tertiary: the carbon is attached to three other carbons. Example: 2-methylpropan-2-ol ((CH3)3COH).

The classification is not bookkeeping — it predicts chemistry. Primary alcohols oxidize in two stages, secondary alcohols in one, and tertiary alcohols resist oxidation entirely, as the next sections show.

Learn to classify at a glance: find the –OH, count the carbon's carbon neighbors, and you know the class.

Hydrogen bonding and physical properties

The –OH group hydrogen-bonds: the partially positive hydrogen of one molecule attracts the partially negative oxygen of another. These attractions are far stronger than the dispersion forces between alkanes.

The effect on boiling point is dramatic. Ethanol (molar mass 46) boils at 78°C, while ethane (molar mass 30) boils at −89°C — a difference of over 160 degrees from a single –OH group.

Small alcohols mix freely with water because their –OH groups hydrogen-bond to water molecules. But as the carbon chain lengthens, the nonpolar hydrocarbon portion dominates: butanol is only slightly soluble, and octanol is essentially insoluble. Every alcohol is a tug-of-war between its water-loving head and its water-fearing tail.

Combustion

Alcohols burn cleanly and completely in oxygen:

C2H5OH + 3O2 → 2CO2 + 2H2O

Ethanol's clean combustion and renewable production make it a major biofuel, blended into petrol in many countries to reduce fossil-fuel use. 'E10' petrol, for example, contains 10% ethanol.

Compared with alkanes, alcohols release slightly less energy per gram because they are already partially oxidized — the oxygen in the –OH group means less oxygen needs to be added during burning. But their renewability and cleaner burn keep them central to fuel policy.

Oxidation

Oxidation of alcohols removes hydrogen from the carbon bearing the –OH group, and the outcome depends on the class:

• Primary alcohol → aldehyde → carboxylic acid (two stages: ethanol → ethanal → ethanoic acid).
• Secondary alcohol → ketone (one stage: propan-2-ol → propanone).
• Tertiary alcohol → no reaction under normal oxidizing conditions — there is no hydrogen on the bearing carbon to remove.

This pattern is both a synthetic tool and an analytical test: the different outcomes distinguish the three classes. It also explains why the body metabolizes ethanol stepwise, with ethanal as the intermediate responsible for many of alcohol's toxic effects.

Ethanol and fermentation

Yeast converts sugars into ethanol and carbon dioxide in the absence of oxygen:

C6H12O6 → 2C2H5OH + 2CO2

This fermentation is among humanity's oldest biotechnologies — brewing and winemaking are fermentation harnessed. The CO2 produced is what makes bread rise and beer fizzy; the ethanol is the product in beverages.

Industrially, fermentation of plant sugars and starches produces fuel ethanol at enormous scale, while the same chemistry in sanitizer manufacturing supplies the ethanol in hand disinfectants. One biological process thus serves food, fuel, and hygiene — a remarkable return on a simple reaction.

Alcohols to handle with care

Not all alcohols are benign, and the differences matter:

• Methanol (CH3OH) is acutely toxic: even small swallowed amounts can cause blindness or death, because the body oxidizes it to formaldehyde and formic acid. Never consume anything containing methanol.
• Propan-2-ol (isopropyl alcohol, rubbing alcohol) is for external disinfection only — it is poisonous if swallowed.
• Ethanol itself is toxic in excess and addictive; beverage use carries well-documented health risks.

The lesson generalizes: a functional group tells you the reaction family, but each member's safety profile is its own. Always check before handling any chemical.

Common mistakes

Real-world applications

• Ethanol fuels vehicles as biofuel and blends into petrol in many countries.

• Alcohol-based sanitizers use ethanol or propan-2-ol to denature microbial proteins.

• Methanol and ethanol serve as industrial solvents and feedstocks for formaldehyde and other chemicals.

• Glycerol (propane-1,2,3-triol), a triple alcohol, is used in foods, cosmetics, and antifreeze.