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SAChemistrySyllabus dot point

How does the oxidation product of an alcohol depend on its class?

Describe the oxidation of primary, secondary and tertiary alcohols and the products formed.

How primary alcohols oxidise to aldehydes then carboxylic acids, secondary alcohols to ketones, and why tertiary alcohols resist oxidation, using oxidants such as acidified dichromate.

Generated by Claude Opus 4.77 min answer

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  1. What this dot point is asking
  2. Classifying alcohols
  3. The oxidising agent
  4. Primary alcohols
  5. Secondary alcohols
  6. Tertiary alcohols

What this dot point is asking

You must classify an alcohol as primary, secondary or tertiary and state its oxidation product.

Classifying alcohols

For example, ethanol (-OH on an end carbon) is primary, propan-2-ol is secondary, and 2-methylpropan-2-ol is tertiary.

The oxidising agent

Acidified potassium dichromate (K2Cr2O7/H+\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}^+) is the standard oxidant. As it oxidises the alcohol it is itself reduced, changing colour:

Cr2O72− (orange)→Cr3+ (green)\text{Cr}_2\text{O}_7^{2-}\ (\text{orange}) \rightarrow \text{Cr}^{3+}\ (\text{green})

The orange-to-green colour change confirms that oxidation occurred - so it distinguishes oxidisable (primary/secondary) from non-oxidisable (tertiary) alcohols.

Primary alcohols

Oxidation proceeds in two steps. With limited oxidant and distillation, the reaction stops at the aldehyde; with excess oxidant under reflux, it continues to the carboxylic acid:

CH3CH2OH→[O]CH3CHO→[O]CH3COOH\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{[O]} \text{CH}_3\text{CHO} \xrightarrow{[O]} \text{CH}_3\text{COOH}

(ethanol → ethanal → ethanoic acid). [O][O] represents oxygen supplied by the oxidant.

Secondary alcohols

A secondary alcohol oxidises once, to a ketone, and no further (there is no H on the carbonyl carbon to remove):

CH3CH(OH)CH3→[O]CH3COCH3\text{CH}_3\text{CH(OH)CH}_3 \xrightarrow{[O]} \text{CH}_3\text{COCH}_3

(propan-2-ol → propanone).

Tertiary alcohols

A tertiary alcohol has no hydrogen on the carbon bearing the -OH, so it cannot be oxidised to a carbonyl compound under these conditions. The dichromate stays orange.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2022 SACE Stage 23 marksA sample of enzyme solution was added to sucrose. Explain which one of Tollens reagent or acidified potassium dichromate solution should be used to identify whether the sucrose was converted into its monosaccharides.
Show worked answer →

The monosaccharides glucose and fructose are reducing sugars; sucrose is not.

  1. Acidified potassium dichromate is not suitable because it can oxidise both the reducing sugars and is not selective enough to confirm a positive identification distinctly here.

  2. Tollens reagent should be used: it specifically detects an aldehyde group. The monosaccharides produced (in particular glucose, an aldose) contain an aldehyde group that reduces Tollens reagent.

  3. A positive result is the formation of a silver mirror (or grey precipitate), confirming the monosaccharides are present. One mark for choosing Tollens, one for the reason (it tests for the aldehyde of the reducing sugar), one for the silver-mirror observation.

2024 SACE Stage 22 marksSorbitol can be oxidised to form an aldehyde. State one laboratory test and the resulting observation that would confirm the presence of the new aldehyde group.
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Use a mild oxidising test that distinguishes an aldehyde.

Test 1: Add Tollens reagent and warm; a positive result is the formation of a silver mirror on the inside of the test tube.

OR Test 2: Add acidified potassium dichromate solution and warm; a positive result is a colour change from orange to green (the dichromate is reduced as the aldehyde is oxidised to a carboxylic acid).

One mark for a valid test reagent, one mark for the correct observation.

2023 SACE Stage 21 marksMolecules A, B, and C are commonly found in fermentation mixtures (an alkane, an alcohol, and an aldehyde). Identify which of the molecules A, B, or C would react with acidified dichromate solution.
Show worked answer →

Acidified dichromate is an oxidising agent that oxidises primary and secondary alcohols and aldehydes, but not alkanes.

The molecules that react are the alcohol and the aldehyde (the ones containing an oxidisable -OH on carbon or a -CHO group); the alkane does not react. One mark for correctly identifying the oxidisable molecule(s) (the alcohol and/or aldehyde) rather than the unreactive alkane.

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