Oxidation of Alcohols and Related Concepts
Quick Notes
- Alcohols can be oxidised using an oxidising agent:
- Primary alcohols → aldehydes → carboxylic acids (two-step oxidation)
- Secondary alcohols → ketones (one-step oxidation)
- Tertiary alcohols can’t be oxidised under standard conditions
- Distillation collects aldehydes and reflux allows full oxidation to acids
- Oxidation changes the functional group, affecting boiling point and physical properties
- Combustion = complete oxidation to CO2 and H2O
- Colour change occurs with transition metal oxidizing agents
(e.g. orange Cr2O72− → green Cr3+)
Full Notes
What Is Oxidation in Organic Chemistry?
Organic oxidation involves a carbon atom forming a bond to a more electronegative element than itself and losing a bond to a less electronegative element.
Usually this means:
- Addition of a carbon–oxygen bond
- Removal of a carbon–hydrogen bond
There is an increase in the oxidation state of a carbon atom in the compound.
Organic functional groups such as alcohols can be oxidised in two main ways:
- Combustion – reacting directly with oxygen.
- Chemical oxidation – using an oxidising agent.
Combustion of Alcohols
Alcohols (especially short-chain ones) burn cleanly in oxygen to produce carbon dioxide and water.
This reaction is exothermic, as the products (CO2 and H2O) are lower in energy than the alcohol.
Short-chain alcohols are useful fuels for this reason.
Example Combustion of Ethanol:
CH3CH2OH + 3O2 → 2CO2 + 3H2O
Oxidation of Alcohols – Oxidising Agents
Primary and secondary alcohols can be oxidised using agents such as acidified potassium dichromate(VI) or acidified potassium manganate(VII).
In equations, we represent oxygen from the oxidising agent as [O].
In organic chemistry, oxidation means the carbon bonded to the OH group gains a bond to oxygen (or loses hydrogen) – see top of the page.
For primary alcohols:
Primary alcohols oxidise first to aldehydes, then to carboxylic acids using heat and acidified potassium dichromate (VI) as an oxidising agent.
Observation: Orange Cr2O72− (dichromate) turns green (Cr3+ formed).
1. Formation of Aldehyde (Distillation):
Distillation is needed to obtain the aldehyde because the aldehyde has a low boiling point and will easily evaporate once formed, leaving the reaction mixture as a vapour.
Example Ethanol → Ethanal
CH3CH2OH + [O] → CH3CHO + H2O
2. Formation of Carboxylic Acid (Reflux):
Reflux is needed to obtain the carboxylic acid because the aldehyde must be continually condensed and forced to re-enter the reaction mixture, enabling it to be further oxidised.
Example Ethanal → Ethanoic Acid
CH3CHO + [O] → CH3COOH
For secondary alcohols:
Secondary alcohols oxidise to ketones when heated with acidified potassium dichromate(VI)
Observation: Orange Cr2O72− (dichromate) turns green (Cr3+ formed).
Example Propan-2-ol → Propanone
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O
Tertiary Alcohols
Tertiary alcohols have no hydrogen atom attached to the carbon that holds the –OH group. Since oxidation involves removing this hydrogen (to form a C=O bond), tertiary alcohols cannot be oxidised using common oxidising agents like acidified potassium dichromate.
Summary
- Primary alcohols oxidise to aldehydes and then carboxylic acids
- Secondary alcohols oxidise to ketones
- Tertiary alcohols do not oxidise under standard conditions
- Distillation collects aldehydes while reflux allows full oxidation
- Combustion is complete oxidation producing CO2 and H2O
- Oxidising agents show colour changes due to transition metal reduction
Linked Course Questions
How does the nature of the functional group in a molecule affect its physical properties, such as boiling point?
Functional groups affect boiling point by influencing the strength of intermolecular forces:
- Hydrogen bonding (e.g. in alcohols, carboxylic acids) leads to high boiling points.
- Polar groups (e.g. in aldehydes, ketones) cause dipole–dipole attractions, raising boiling points moderately.
- Non-polar groups (e.g. alkanes) only have London forces, so they have low boiling points.
- Carboxylic acids often have the highest boiling points due to strong hydrogen bonding and dimer formation.
Boiling point increases with stronger intermolecular forces and larger molecular size.
What is the difference between combustion and oxidation of an alcohol?
Combustion: The complete reaction of an alcohol with excess oxygen, producing carbon dioxide and water. It is highly exothermic and used for energy (e.g. ethanol as a fuel).
Example: CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O
Oxidation (organic chemistry): Involves reacting an alcohol with an oxidising agent (e.g. acidified K₂Cr₂O₇) to form aldehydes, ketones, or carboxylic acids, depending on the alcohol type.
Why is there a colour change when an alcohol is oxidized by a transition element compound?
The colour change occurs because the transition metal in the oxidising agent changes oxidation state during the redox reaction. For example:
- In acidified potassium dichromate (VI), the Cr⁶⁺ ion (orange) is reduced to Cr³⁺ (green) as the alcohol is oxidised.
This visible change is typical of transition metals, which can exist in multiple oxidation states – each with a distinct colour. The colour change acts as a clear indicator that oxidation has occurred.