Alcohols — common mistakes

Remember the key colour changes for redox reactions involving alcohols. With acidified potassium dichromate(VI), a positive test for a primary or secondary alcohol is a colour change from orange ($Cr_2O_7^{2-}$) to green ($Cr^{3+}$). With acidified potassium manganate(VII), the colour change is from purple ($MnO_4^−$) to colourless ($Mn^{2+}$). These are essential observations for practical-based questions.

Oxidation of an alcohol (with agents like dichromate) involves removing a hydrogen atom from the -OH group and another hydrogen from the carbon atom that is bonded to the -OH group. Tertiary alcohols do not have a hydrogen atom on this specific carbon, so this type of oxidation cannot occur without breaking a much stronger C-C bond, which requires very harsh conditions.

Distillation separates substances by boiling point. When oxidising a primary alcohol, the aldehyde product has a lower boiling point than the alcohol and is distilled off as it forms, preventing its further oxidation. Reflux involves continuous boiling and condensation, returning the mixture to the reaction flask. This allows for prolonged heating at a constant temperature, ensuring the reaction goes to completion, which is necessary to fully oxidise a primary alcohol to a carboxylic acid.

Yes. Dehydration of an unsymmetrical secondary alcohol like butan-2-ol can lead to a mixture of products by removing a hydrogen from an adjacent carbon. Elimination of the -OH group and a hydrogen from the adjacent methyl ($CH_3$) group gives but-1-ene. Elimination of the -OH group and a hydrogen from the adjacent methylene ($CH_2$) group gives but-2-ene. According to Saytzeff's (Zaitsev's) rule, the more substituted alkene (but-2-ene) is the major product. But-2-ene also exists as E/Z stereoisomers.