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A primary alcohol has its OH-bearing carbon bonded to one other carbon, a secondary alcohol to two and a tertiary alcohol to three; methanol is treated with primary oxidation behaviour. Acidified dichromate oxidises primary alcohols to aldehydes and then carboxylic acids, while secondary alcohols form ketones. Tertiary alcohols do not undergo the same simple oxidation because the OH-bearing carbon lacks a hydrogen. A colour change is an observation; identifying the product requires structure and conditions.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Predict alcohol oxidation products
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does a secondary alcohol oxidise to?
A ketone. The carbonyl forms at the existing OH-bearing carbon.
After watching 2: What is the purpose of reflux?
Return condensed vapour to the reaction mixture. It permits heating without losing volatile material.
What you'll be able to do
A primary alcohol has its OH-bearing carbon bonded to one other carbon, a secondary alcohol to two and a tertiary alcohol to three; methanol is treated with primary oxidation behaviour. Acidified dichromate oxidises primary alcohols to aldehydes and then carboxylic acids, while secondary alcohols form ketones. Tertiary alcohols do not undergo the same simple oxidation because the OH-bearing carbon lacks a hydrogen. A colour change is an observation; identifying the product requires structure and conditions.
To obtain an aldehyde from a primary alcohol, use controlled oxidation and distil it away as it forms. Reflux with excess oxidant retains volatile reactants and products in the apparatus, favouring complete oxidation to the carboxylic acid. A condenser returns vapour in reflux, while a distillation arrangement collects a separated volatile fraction. Never seal a heated apparatus. Removing a product can change reaction outcome, so apparatus choice is part of the chemistry rather than an optional drawing detail.
Fermentation converts sugars to ethanol using enzymes under controlled warm, anaerobic conditions, giving a dilute product requiring separation. Hydration of ethene uses steam and an acid catalyst and can operate continuously, but uses an alkene feedstock and demanding conditions. Dehydration of an alcohol gives an alkene using an acid catalyst and heat. Alcohols can also react with carboxylic acids to form esters. Evaluate yield, purity, energy, feedstock renewability and rate instead of assuming one route is always environmentally superior.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: How can ethanol be oxidised mainly to ethanal rather than ethanoic acid?
Prevent prolonged contact with oxidant by removing the aldehyde.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.