Last updated: 23 August 2026
The –OH group may look like a small addition to a carbon chain, but it changes the chemistry of an organic molecule dramatically. It allows an alcohol to react with sodium, undergo substitution to form a halogenoalkane, lose water to form an alkene, and — depending on its structure — undergo oxidation.
The important idea is therefore not to memorise a disconnected list of reactions. First identify the carbon bonded directly to –OH. Once you know whether that carbon is primary, secondary or tertiary, many of the products can be predicted logically.
Try these five questions before reading the chapter. Look carefully at the carbon bearing the –OH group and use the reaction conditions to decide what transformation is taking place.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
What is an alcohol?
An alcohol is an organic compound containing a hydroxy group, –OH, bonded to a saturated carbon atom. Ethanol, CH₃CH₂OH, is the simplest alcohol commonly used to illustrate the chemistry of this functional group.
The carbon bonded directly to –OH is especially important because its surrounding carbon groups determine whether the alcohol is primary, secondary or tertiary.
To classify an alcohol, look only at the carbon bonded directly to –OH and count how many other carbon atoms are attached to that carbon.
Primary, secondary and tertiary alcohols
Primary alcohol: the carbon bearing –OH is bonded directly to one other carbon atom.
Secondary alcohol: the carbon bearing –OH is bonded directly to two other carbon atoms.
Tertiary alcohol: the carbon bearing –OH is bonded directly to three other carbon atoms.
For propan-2-ol, CH₃CH(OH)CH₃, the carbon carrying –OH is bonded to two CH₃ groups. It is therefore a secondary alcohol. Do not classify the alcohol by counting the total number of carbon atoms in the molecule.
How acidic are alcohols?
The O–H bond of an alcohol can break heterolytically so that the alcohol loses H⁺ and forms an alkoxide ion.
However, ethanol is only a very weak acid. A useful comparison at this level is:
Why is ethanol less acidic than water?
When ethanol loses H⁺, it forms the ethoxide ion, CH₃CH₂O⁻. The ethyl group has an electron-donating effect and pushes electron density towards the negatively charged oxygen atom.
This makes the negative charge on the ethoxide ion less stable than the corresponding charge in hydroxide, OH⁻. A less stable conjugate base means the parent substance is less willing to lose H⁺.
Why is phenol more acidic than water and ethanol?
Phenol forms the phenoxide ion when it loses H⁺. In the phenoxide ion, electron density can be delocalised into the benzene ring. This delocalisation stabilises the conjugate base, making loss of H⁺ from phenol more favourable.
When comparing acidity, compare the stability of the conjugate bases: the more stable the conjugate base, the stronger the corresponding acid.
Reaction of alcohols with sodium
Alcohols react with reactive metals such as sodium because the hydrogen attached to oxygen can be replaced by the metal.
Ethanol therefore forms sodium ethoxide and hydrogen gas. The reaction provides evidence that an alcohol can behave as a weak acid.
What would you observe?
Effervescence is observed as hydrogen gas is evolved, and the sodium gradually disappears as it reacts. The reaction is less vigorous than the corresponding reaction of sodium with water.
Water: reacts more vigorously with sodium because water is more acidic than ethanol.
Ethanol: also produces hydrogen, but the reaction is slower.
Converting alcohols into halogenoalkanes
The –OH group of an alcohol can be replaced by a halogen atom. Overall, this is a substitution reaction.
Reaction with phosphorus(V) chloride
Alcohols react with phosphorus(V) chloride, PCl₅, replacing –OH by Cl. With ethanol:
Steamy white fumes are observed because hydrogen chloride gas is produced and reacts with moisture in the air.
PCl₅ can provide evidence for an –OH-containing functional group, but the test is not unique to alcohols because other compounds such as carboxylic acids can also react with PCl₅.
Reaction with hydrogen halides
Alcohols can also be converted into halogenoalkanes using hydrogen halides, HX. For example, ethanol can react with HBr to form bromoethane.
Hydrogen bromide may be generated in situ from a bromide salt such as NaBr together with an acid.
Oxidation of alcohols
Oxidation is one of the most important parts of alcohol chemistry because the product depends on whether the starting alcohol is primary, secondary or tertiary.
A commonly used oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇ / H⁺. During a successful oxidation, the solution changes from orange to green.
| Type of alcohol | Oxidation product | Typical observation |
|---|---|---|
| Primary | Aldehyde, then carboxylic acid | Orange → green |
| Secondary | Ketone | Orange → green |
| Tertiary | No reaction under normal oxidation conditions | Remains orange |
Oxidation of primary alcohols
A primary alcohol can be oxidised in two stages. Ethanol first forms ethanal and can then be oxidised further to ethanoic acid.
Why is distillation used to obtain an aldehyde?
The aldehyde is itself readily oxidised. If ethanal remains in contact with the oxidising mixture, it can be oxidised further to ethanoic acid.
To obtain ethanal, it is therefore distilled from the reaction mixture as it forms. Removing it reduces its opportunity to undergo further oxidation.
Oxidising a primary alcohol to a carboxylic acid
To produce the carboxylic acid, the primary alcohol is heated under reflux with excess oxidising agent. Reflux allows volatile substances to condense and return to the flask so that oxidation can continue.
Distillation: remove the aldehyde as it forms to minimise further oxidation.
Reflux: keep the reactants and intermediates in the reaction vessel so oxidation can continue to the carboxylic acid.
Oxidation of secondary alcohols
Secondary alcohols are oxidised to ketones.
The ketone does not undergo the easy further oxidation seen with aldehydes under these conditions. There is therefore no need to remove it immediately simply to prevent further oxidation.
Why are tertiary alcohols not readily oxidised?
For the usual oxidation of an alcohol to a carbonyl compound, the carbon bearing –OH must also have a hydrogen atom attached to it.
In a tertiary alcohol, that carbon is bonded to three carbon groups and therefore has no hydrogen atom attached to it. The usual oxidation cannot occur without breaking a C–C bond, so there is no reaction with acidified potassium dichromate(VI) under the normal conditions used here.
Primary → aldehyde → carboxylic acid
Secondary → ketone
Tertiary → no reaction under the usual conditions
Dehydration of alcohols
An alcohol can lose H₂O to form an alkene. This is an elimination reaction.
Ethanol can be heated with concentrated sulfuric acid or phosphoric acid to form ethene.
What is removed during dehydration?
The –OH group is removed from one carbon and a hydrogen atom is removed from an adjacent carbon. A C=C double bond then forms between the two carbon atoms.
The word dehydration tells you what is lost: H₂O. When working backwards in synthesis questions, an alkene can therefore suggest that it may have been obtained from an alcohol by elimination.
The tri-iodomethane test for alcohols
The tri-iodomethane test, also commonly called the iodoform test, detects particular structural features rather than the alcohol functional group in general.
For alcohols, a positive result is obtained when the molecule contains:
Ethanol, CH₃CH₂OH, is an important special case and also gives a positive result.
Reagents and observation
Warm the compound with I₂(aq) and NaOH(aq).
A positive result is the formation of a yellow precipitate of tri-iodomethane, CHI₃.
Ethanol is the only primary alcohol that gives a positive tri-iodomethane test. Other alcohols giving a positive result contain the CH₃CH(OH)– structural unit.
How to distinguish primary, secondary and tertiary alcohols
Chemical tests are most useful when several observations are combined. Acidified potassium dichromate(VI) can first separate a tertiary alcohol from oxidisable alcohols.
Step 1: Use acidified potassium dichromate(VI)
Warm each alcohol with acidified K₂Cr₂O₇.
Orange remains orange: the alcohol is tertiary under these test conditions.
Orange changes to green: the alcohol may be primary or secondary.
Step 2: Distinguish primary from secondary
Oxidise the primary and secondary candidates under conditions that allow the carbonyl product to be collected by distillation.
A primary alcohol forms an aldehyde, while a secondary alcohol forms a ketone.
Test the collected products using Tollens' reagent.
| Original alcohol | Oxidation product | Tollens' reagent |
|---|---|---|
| Primary | Aldehyde | Silver mirror |
| Secondary | Ketone | No reaction |
| Tertiary | No oxidation product in the first test | Not required |
Do not expect one reagent to distinguish every alcohol immediately. Use the oxidation behaviour first, then identify the oxidation product.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Classifying an alcohol by the total number of carbon atoms in the molecule. | Primary, secondary and tertiary refer specifically to the carbon bearing –OH. | Locate the C–OH carbon and count how many carbon atoms are bonded directly to it. |
| Assuming a primary alcohol always gives a carboxylic acid during oxidation. | Controlled oxidation with distillation can stop at the aldehyde. | Look at the conditions: distillation favours aldehyde collection; reflux with excess oxidant gives the carboxylic acid. |
| Saying tertiary alcohols do not oxidise because they have no hydrogen on the oxygen atom. | Tertiary alcohols do contain an O–H hydrogen. | State that the carbon bearing –OH has no hydrogen atom attached to it. |
| Assuming every alcohol gives a positive tri-iodomethane test. | The test requires a particular structural arrangement. | Look for CH₃CH(OH)–, remembering that ethanol is the special primary-alcohol case. |
| Saying PCl₅ is a test that uniquely identifies an alcohol. | Other –OH-containing compounds can also react with PCl₅. | Treat steamy HCl fumes as evidence of a reactive –OH-containing group, then use other information to identify the compound. |
What you should be able to do
Classify alcohols
Identify primary, secondary and tertiary alcohols from displayed, condensed or skeletal structures.
Compare acidity
Explain the order phenol > water > ethanol using the stability of the conjugate bases.
Write sodium reactions
Predict the alkoxide and hydrogen formed when an alcohol reacts with sodium.
Predict substitution products
Convert alcohol structures into the corresponding halogenoalkanes using reagents such as PCl₅ or HX.
Predict oxidation products
Use both alcohol type and reaction conditions to predict aldehydes, ketones and carboxylic acids.
Apply alcohol tests
Use dichromate oxidation, Tollens' reagent and the tri-iodomethane test to distinguish structures.
These ten questions test whether you can apply the chapter to structures, reagents, observations and reaction conditions.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions Students Often Ask About Alcohols
Alcohol chemistry becomes much easier once you connect each reaction to the structure of the carbon bearing the –OH group. These final questions clarify some of the distinctions that often cause confusion.
What is the difference between a primary, secondary and tertiary alcohol?
Count how many carbon atoms are bonded directly to the carbon carrying –OH. One gives a primary alcohol, two gives a secondary alcohol and three gives a tertiary alcohol.
Why do we distil when oxidising ethanol to ethanal?
Ethanal can itself be oxidised to ethanoic acid. Distilling the ethanal from the reaction mixture as it forms reduces its contact with the oxidising agent and helps prevent further oxidation.
Why is reflux used when making ethanoic acid from ethanol?
Reflux allows the mixture to be heated for an extended time without losing volatile reactants or intermediates. With excess oxidising agent present, oxidation can therefore proceed fully to ethanoic acid.
Does every secondary alcohol give a positive tri-iodomethane test?
No. For an alcohol to give the positive test, it must contain the CH₃CH(OH)– structural unit. Some secondary alcohols contain this arrangement, but others do not.
Why does ethanol give the tri-iodomethane test even though it is a primary alcohol?
Ethanol is the exceptional primary alcohol. Under the test conditions it can be oxidised to ethanal, CH₃CHO, which has the structural arrangement required for the subsequent reactions that produce yellow CHI₃.
The central idea in alcohol chemistry is structural: identify the carbon bearing –OH first, then let its surroundings and the reaction conditions guide you to the product. Once primary, secondary and tertiary alcohols are recognised confidently, oxidation, substitution, dehydration and distinguishing tests become a connected set of reactions rather than a list to memorise.