Alcohols

Q: Why do alcohols have significantly higher boiling points than alkanes of similar relative molecular mass?

Alcohols can form strong intermolecular hydrogen bonds between their polar \(\text{O}-\text{H}\) groups, which require much more thermal energy to break than the weak London dispersion forces found in alkanes.

Q: How do secondary and tertiary alcohols differ in their behavior towards acidified potassium dichromate(VI)?

Secondary alcohols are oxidised under reflux to ketones (orange turns green), whereas tertiary alcohols cannot be oxidised and the solution remains orange.

Chemistry 1110 · CCEA A Level · 7 min read · Updated: Aug 31, 2026

Welcome to Alcohols!

Welcome to one of the most exciting and central chapters in organic chemistry! You come across alcohols every day — from the ethanol in hand sanitisers and drinks, to the isopropyl alcohol used to clean electronic screens. In this chapter, we will explore what makes alcohols special, how they behave, and the chemical reactions that allow chemists to convert them into many other useful organic molecules.

Don't worry if organic chemistry has felt overwhelming before. We will break every concept down into clear, bite-sized steps with memory aids and practical examples to make your revision as smooth as possible.

1. What is an Alcohol? Structure and Classification

Alcohols are organic compounds that contain the hydroxyl functional group, written as \(-\text{OH}\). When bonded to an aliphatic carbon chain (an alkyl group), their general formula is \(C_n H_{2n+1}\text{OH}\).

Classification of Alcohols

Just like halogenoalkanes, alcohols are classified into three types depending on what is attached to the carbon atom directly bonded to the \(-\text{OH}\) group (we call this the carbinol carbon):

1. Primary (\(1^\circ\)) Alcohols: The carbon with the \(-\text{OH}\) group is attached to one other carbon atom (or zero, in the case of methanol, \(CH_3\text{OH}\)).
Example: Ethanol, \(CH_3CH_2\text{OH}\), or Propan-1-ol, \(CH_3CH_2CH_2\text{OH}\).

2. Secondary (\(2^\circ\)) Alcohols: The carbon with the \(-\text{OH}\) group is attached to two other carbon atoms.
Example: Propan-2-ol, \(CH_3CH(\text{OH})CH_3\).

3. Tertiary (\(3^\circ\)) Alcohols: The carbon with the \(-\text{OH}\) group is attached to three other carbon atoms.
Example: 2-Methylpropan-2-ol, \((CH_3)_3C\text{OH}\).

Memory Trick: Count the hydrogen atoms on the carbon bonded to \(-\text{OH}\)!
• \(1^\circ\) alcohol: 2 (or 3) hydrogens on that carbon (\(-\text{CH}_2\text{OH}\))
• \(2^\circ\) alcohol: 1 hydrogen on that carbon (\(-\text{CH}(\text{OH})-\))
• \(3^\circ\) alcohol: 0 hydrogens on that carbon (\(-\text{C}(\text{OH})-\))

Key Takeaway

Alcohols contain an \(-\text{OH}\) group. Identifying whether an alcohol is \(1^\circ\), \(2^\circ\), or \(3^\circ\) is essential because their chemical reactivity (especially during oxidation) depends entirely on this classification!

2. Physical Properties of Alcohols

Boiling Points: Why are Alcohols Less Volatile than Alkanes?

If you compare an alcohol to an alkane of similar relative molecular mass (\(M_r\)), the alcohol has a significantly higher boiling point and is far less volatile.

Why?
• Alkanes only possess weak London dispersion forces (induced dipole-dipole interactions) between their non-polar molecules.
• Alcohols contain an oxygen atom bonded directly to a hydrogen atom (\(\text{O}-\text{H}\)). Oxygen is much more electronegative than hydrogen, creating strong permanent dipoles: \(O^{\delta-}-H^{\delta+}\).
• This allows alcohol molecules to form hydrogen bonds with each other. Hydrogen bonds are the strongest type of intermolecular force and require substantial thermal energy to overcome.

Solubility in Water

• Small alcohols (such as methanol, ethanol, and propan-1-ol) are completely miscible in water. This is because their \(-\text{OH}\) group can readily form hydrogen bonds with polar water (\(H_2O\)) molecules.
• Larger alcohols become progressively less soluble. As the non-polar hydrocarbon chain (the "tail") gets longer, it disrupts the hydrogen bonding network of water without contributing favorable interactions. The non-polar hydrophobic effect dominates over the polar \(-\text{OH}\) group.

Key Takeaway

Hydrogen bonding explains why alcohols have high boiling points and why small alcohols dissolve easily in water. As chain length increases, boiling point increases (stronger London forces) but water solubility decreases.

3. Chemical Reactions of Alcohols

A. Combustion

Alcohols burn cleanly in a plentiful supply of oxygen to produce carbon dioxide and water:

\(C_2H_5\text{OH}_{(l)} + 3O_{2(g)} \rightarrow 2CO_{2(g)} + 3H_2O_{(l)}\)

Because they burn with a clean, blue flame and release a large amount of energy, alcohols like ethanol are widely used as renewable fuels and additives in petrol.

B. Reaction with Sodium Metal

When a small piece of clean sodium metal (\(Na\)) is added to an alcohol, it reacts steadily (less vigorously than with water):

Equation: \(2CH_3CH_2\text{OH} + 2Na \rightarrow 2CH_3CH_2O^-Na^+ + H_2\)

Observations:
• Effervescence / bubbling (due to hydrogen gas, \(H_2\), being evolved).
• The sodium metal dissolves/sinks and disappears.
• A white solid (sodium alkoxide, e.g., sodium ethoxide) is formed.
• The mixture gets warm (exothermic reaction).

C. Substitution Reactions (Halogenation)

Halogenation replaces the hydroxyl group (\(-\text{OH}\)) with a halogen atom (\(-Cl\), \(-Br\), or \(-I\)), producing a halogenoalkane.

1. Formation of Chloroalkanes (Test for \(-\text{OH}\) group)

• Reagent: Solid phosphorus(V) chloride, \(PCl_5\).
• Conditions: Room temperature, dry/anhydrous conditions.
• Equation: \(CH_3CH_2\text{OH} + PCl_5 \rightarrow CH_3CH_2Cl + POCl_3 + HCl_{(g)}\)
• Observation: Steamy white fumes of hydrogen chloride gas (\(HCl\)) are produced, which turn damp blue litmus paper red.

2. Formation of Bromoalkanes

• Reagents: Potassium bromide (\(KBr\)) and \(50\%\) concentrated sulfuric acid (\(H_2SO_4\)).
• How it works: The reagents react in situ to produce hydrogen bromide (\(HBr\)):
\(KBr + H_2SO_4 \rightarrow KHSO_4 + HBr\)
Then the \(HBr\) reacts with the alcohol under reflux:
\(CH_3CH_2\text{OH} + HBr \rightarrow CH_3CH_2Br + H_2O\)

3. Formation of Iodoalkanes

• Reagents: Red phosphorus (\(P\)) and iodine (\(I_2\)), heated under reflux.
• How it works: Phosphorus(III) iodide (\(PI_3\)) is generated in situ:
\(2P + 3I_2 \rightarrow 2PI_3\)
\(3CH_3CH_2\text{OH} + PI_3 \rightarrow 3CH_3CH_2I + H_3PO_3\)

D. Oxidation of Alcohols

This is one of the most frequently examined topics in AS Chemistry! Make sure you know the reagents, conditions, and color changes thoroughly.

The Oxidising Agent: Acidified potassium dichromate(VI), written as \(K_2Cr_2O_7 / H_2SO_4\) or \(Cr_2O_7^{2-} / H^+\).
Color Change: The orange dichromate(VI) ion (\(Cr_2O_7^{2-}\)) is reduced to the green chromium(III) ion (\(Cr^{3+}\)).

1. Oxidation of Primary (\(1^\circ\)) Alcohols

Primary alcohols can be oxidised in two distinct stages:

Stage 1: Partial Oxidation to an Aldehyde
• Condition: Excess alcohol, warm gently, and distil off the product immediately as it forms.
• Why distil immediately? Aldehydes have lower boiling points than alcohols (no hydrogen bonding between aldehyde molecules) and will vaporise easily. Removing them prevents further oxidation.
• Equation: \(CH_3CH_2\text{OH} + [O] \rightarrow CH_3CHO + H_2O\)
(Ethanol \(\rightarrow\) Ethanal + Water)

Stage 2: Complete Oxidation to a Carboxylic Acid
• Condition: Excess oxidising agent (\(K_2Cr_2O_7 / H^+\)), heated under reflux.
• What is reflux? Continuous boiling and condensing of the reaction mixture to ensure volatile vapours do not escape before full oxidation occurs.
• Equation: \(CH_3CH_2\text{OH} + 2[O] \rightarrow CH_3COOH + H_2O\)
(Ethanol \(\rightarrow\) Ethanoic acid + Water)

2. Oxidation of Secondary (\(2^\circ\)) Alcohols

• Secondary alcohols are oxidised to ketones.
• Condition: Heated under reflux with acidified potassium dichromate(VI).
• Ketones cannot be oxidised further without breaking the carbon skeleton.
• Equation: \(CH_3CH(\text{OH})CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O\)
(Propan-2-ol \(\rightarrow\) Propanone + Water)

3. Oxidation of Tertiary (\(3^\circ\)) Alcohols

• Tertiary alcohols cannot be oxidised by acidified potassium dichromate(VI).
• Observation: The solution remains orange (no reaction).
• Why? There is no hydrogen atom attached to the carbon bonded to the \(-\text{OH}\) group to be removed during oxidation.

E. Dehydration (Elimination Reaction)

Dehydration is an elimination reaction in which an alcohol loses a molecule of water to form an alkene.

Conditions:
• Method 1: Heated with concentrated sulfuric acid (\(conc.\ H_2SO_4\)) or concentrated phosphoric acid (\(conc.\ H_3PO_4\)) at around \(170^\circ\text{C}\).
• Method 2: Passing alcohol vapours over a hot aluminium oxide (\(Al_2O_3\)) catalyst at \(300^\circ\text{C}\).

Equation:
\(CH_3CH_2\text{OH} \rightarrow CH_2=CH_2 + H_2O\)
(Ethanol \(\rightarrow\) Ethene + Water)

Be Careful with Asymmetric Alcohols!
When dehydrating an asymmetric alcohol like butan-2-ol, the double bond can form in different positions, yielding a mixture of structural and geometric isomers (e.g., but-1-ene, cis-but-2-ene, and trans-but-2-ene).

Key Takeaway

• \(1^\circ\) alcohol \(\xrightarrow{\text{distil}}\) Aldehyde \(\xrightarrow{\text{reflux}}\) Carboxylic Acid (Orange \(\rightarrow\) Green)
• \(2^\circ\) alcohol \(\xrightarrow{\text{reflux}}\) Ketone (Orange \(\rightarrow\) Green)
• \(3^\circ\) alcohol \(\xrightarrow{\text{reflux}}\) No Reaction (Remains Orange)
• Dehydration of alcohols eliminates \(H_2O\) to form an alkene.

4. Summary of Qualitative Diagnostic Tests

Here is a quick reference guide to identify and differentiate alcohols in laboratory questions:

1. Test for the presence of an \(-\text{OH}\) group:
• Add solid \(PCl_5\) at room temperature.
• Positive result: Steamy white fumes of \(HCl\) gas that turn damp blue litmus paper red.

2. Test for \(1^\circ\) and \(2^\circ\) vs \(3^\circ\) alcohols:
• Warm with acidified potassium dichromate(VI), \(K_2Cr_2O_7 / H^+\).
• \(1^\circ\) or \(2^\circ\) alcohol: Orange solution turns green.
• \(3^\circ\) alcohol: Solution remains orange.

3. Distinguishing an Aldehyde from a Ketone (Products of Oxidation):
• Tollens' Reagent: Aldehydes form a silver mirror; Ketones show no change.
• Fehling's Solution: Aldehydes turn the blue solution into a brick-red precipitate (\(Cu_2O\)); Ketones show no change.

5. Common Mistakes to Avoid

• Forgetting the acid: Always write acidified potassium dichromate(VI) or include \(H^+\) (e.g., \(K_2Cr_2O_7 / H_2SO_4\)). Dichromate will not act as an oxidising agent in neutral conditions.
• Confusing Distillation and Reflux: Remember, distillation is used to separate the aldehyde immediately so it is not oxidised further. Reflux keeps all reactants in the flask to force full oxidation to a carboxylic acid.
• Missing the byproduct: In oxidation equations using \([O]\), always remember to balance the equation by including \(H_2O\) on the product side!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the functional group of an alcohol and how are alcohols classified into primary, secondary, and tertiary types?

Alcohols contain the hydroxyl group (\(-\text{OH}\)). They are classified as primary (\(1^\circ\)), secondary (\(2^\circ\)), or tertiary (\(3^\circ\)) based on whether the carbon bonded to the \(-\text{OH}\) group is attached to one (or zero), two, or three other carbon atoms.

Why do alcohols have significantly higher boiling points than alkanes of similar relative molecular mass?

Alcohols can form strong intermolecular hydrogen bonds between their polar \(\text{O}-\text{H}\) groups, which require much more thermal energy to break than the weak London dispersion forces found in alkanes.

How and why does the water solubility of alcohols change as their carbon chain length increases?

Small alcohols are completely miscible in water due to hydrogen bonding with water molecules, but solubility decreases as chain length increases because the non-polar hydrophobic hydrocarbon chain disrupts water's hydrogen bond network.

What are the products and observations when an alcohol reacts with sodium metal?

The reaction produces a sodium alkoxide (e.g., sodium ethoxide) and hydrogen gas (\(H_2\)), observed as effervescence, the sodium dissolving, and heat being released.

How can phosphorus(V) chloride (\(PCl_5\)) be used to test for the presence of an \(-\text{OH}\) group?

Adding solid \(PCl_5\) to an alcohol at room temperature produces steamy white fumes of hydrogen chloride gas (\(HCl\)), which turn damp blue litmus paper red.

What reagents and conditions are used to convert alcohols into bromoalkanes and iodoalkanes?

Bromoalkanes are prepared using \(KBr\) and \(50\%\) concentrated \(H_2SO_4\) under reflux, while iodoalkanes are prepared by heating under reflux with red phosphorus (\(P\)) and iodine (\(I_2\)).

What are the oxidation products of a primary alcohol under distillation versus reflux conditions?

Partial oxidation with gentle heating and immediate distillation yields an aldehyde, whereas complete oxidation heated under reflux with excess oxidising agent yields a carboxylic acid.

How do secondary and tertiary alcohols differ in their behavior towards acidified potassium dichromate(VI)?

Secondary alcohols are oxidised under reflux to ketones (orange turns green), whereas tertiary alcohols cannot be oxidised and the solution remains orange.

What is the dehydration of an alcohol and what reaction conditions are required?

Dehydration is an elimination reaction where an alcohol loses a water molecule to form an alkene, using concentrated \(H_2SO_4\) / \(H_3PO_4\) at \(170^\circ\text{C}\) or hot \(Al_2O_3\) at \(300^\circ\text{C}\).

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