- Define and explain Alcohols and Oxidation in your own words
- Use key terms such as functional group accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Alcohols and Oxidation: classify alcohols and follow their oxidation to carbonyls and acids.
Classify alcohols and follow their oxidation to carbonyls and acids.
Key ideas
Alcohols oxidise in stages
Primary alcohols oxidise first to aldehydes and then to carboxylic acids: ethanol → ethanal → ethanoic acid, using acidified potassium dichromate. Gentle distillation collects the aldehyde; reflux drives oxidation all the way to the acid. Secondary alcohols stop at ketones, and tertiary alcohols resist oxidation — a difference used to classify unknown alcohols.
Synthesis is planned backwards
Planning a synthesis means working backwards from the target molecule, choosing reactions that build each functional group in turn. Each step must use conditions that do not destroy groups already present — for instance, oxidise an alcohol before introducing an alkene that the oxidiser would attack. Good routes are short, high-yielding and use cheap starting materials.
Key term — functional group: The atom or group of atoms responsible for a compound's characteristic reactions, e.g. –OH in alcohols.
Hydrogen bromide adds to propene (CH₃CH=CH₂). Describe the mechanism and name the major product.
The C=C pi bond's electrons attack the H of H–Br, breaking the pi bond. H⁺ adds to the end carbon (C1), forming the more stable secondary carbocation on C2. Br⁻ then attacks the carbocation on C2. Overall: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (Markownikoff addition).
Answer: Major product: 2-bromopropane, formed via the secondary carbocation (Markownikoff's rule).
- Oxidising a tertiary alcohol Tertiary alcohols have no hydrogen on the carbon bearing the –OH group, so they cannot be oxidised without breaking carbon–carbon bonds. Acidified dichromate stays orange with a tertiary alcohol — a classic test result.
- Drawing curly arrows starting at atoms Curly arrows show electron pairs moving: they start at the electron source (a bond or lone pair) and point to where the electrons go. An arrow starting at an atom rather than the bond is meaningless.
Practice
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
The orange bromine water decolourises; the product is 1,2-dibromoethane.
H⁺ adds to C1, giving a secondary carbocation on C2, which is more stable than the primary alternative; Br⁻ then attacks C2 (Markownikoff's rule).
Ethanoic acid; the orange dichromate turns green as it is reduced.
Quick check
Which of these best defines "functional group"?
Plan a two-step synthesis of ethanoic acid from ethene.
Name the structural isomers of C₄H₁₀.
- Alcohols and Oxidation: classify alcohols and follow their oxidation to carbonyls and acids.
- Alcohols oxidise in stages: Primary alcohols oxidise first to aldehydes and then to carboxylic acids: ethanol → ethanal → ethanoic acid, using acidified potassium dichromate.
- electrophile: An electron-pair acceptor, attracted to electron-rich areas such as C=C double bonds.
- Watch out for: oxidising a tertiary alcohol