What a learner can do afterwards
- Classifies an alcohol as primary, secondary or tertiary from its structure
- Predicts the oxidation product for each class and names the apparatus that stops the reaction at an aldehyde
- Writes the dehydration of an alcohol to an alkene with the catalyst and conditions
- Explains why alcohols boil far higher than alkanes of similar molecular mass
1 · Read
An open bottle of wine turns toward vinegar. That change is an oxidation of its alcohol, and how far it goes depends on the alcohol class. Count the carbons joined to the C-OH carbon: one means primary, two means secondary, three means tertiary. That single count decides what oxidation does to each.
Warm acidified dichromate sorts them out. A primary alcohol oxidises to an aldehyde, then to a carboxylic acid: distil off the aldehyde to stop there, reflux to push on to the acid. A secondary alcohol gives a ketone and stops. A tertiary alcohol resists, so the mixture stays orange. Orange chromium(VI) turning green chromium(III) shows oxidation happening.
Dehydration takes an alcohol back to an alkene when the next carbon holds a hydrogen to lose. Heat with concentrated sulfuric acid, or pass the vapour over hot aluminium oxide: water leaves and a double bond forms. For example, ethanol loses water this way to give ethene, losing a hydrogen with the OH group much as a hydrogen leaves with the halogen when an alkene forms from a haloalkane. Methanol cannot follow this path: with no next carbon, no double bond can form.
Alcohols boil far higher than alkanes of similar mass because OH groups hydrogen bond to each other, which takes extra energy to break. Alkanes only have weak London forces between molecules. Name the forces on each side.
Count the neighbours of the OH carbon and the oxidation result follows.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.