Oxidation of Alcohols & Distinguishing Tests

CHEAlcohols: Preparation & Reactions

Oxidation behaviour separates the three classes of alcohol and underpins lab identification — acidified $K_2Cr_2O_7$ / $KMnO_4$ , the Victor Meyer test, and tests to tell methanol from ethanol. The pattern is that $1^\circ$ , $2^\circ$ and $3^\circ$ alcohols give characteristically different oxidation products.

Oxidation of a primary alcohol

RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH

Oxidation of a secondary alcohol

R_2CHOH \xrightarrow{[O]} R_2C{=}O \ (\text{ketone})

Acidity order of alcohols

1^\circ > 2^\circ > 3^\circ \qquad pK_a \approx 16\text{-}18

Acidified $K_2Cr_2O_7$ (orange $\rightarrow$ green): $1^\circ$ alcohol $\rightarrow$ aldehyde $\rightarrow$ carboxylic acid; $2^\circ$ alcohol $\rightarrow$ ketone; $3^\circ$ alcohol resists oxidation (no H on the C bearing $\text{-}OH$ ).
$3^\circ$ alcohols give no oxidation under mild conditions, and only break the chain (to smaller acids/ketones) under drastic conditions — a key distinguishing point.
Victor Meyer test (final nitrolic-acid colour): $1^\circ$ alcohol gives blood-red, $2^\circ$ gives blue, $3^\circ$ gives no colour (colourless).
Distinguish methanol from ethanol by the iodoform test (only ethanol gives yellow $CHI_3$ ).
A second methanol/ethanol distinction: warm with $I_2$ /NaOH — ethanol gives iodoform; methanol does not.
Acidity of alcohols: $1^\circ > 2^\circ > 3^\circ$ , because more alkyl groups push electron density onto oxygen (positive inductive effect), destabilising the alkoxide $RO^-$ .
Alcohols are weaker acids than water ( $pK_a \approx 16\text{-}18$ ); the $+I$ effect of $R$ makes $RO^-$ less stable than $OH^-$ .
To stop a $1^\circ$ alcohol at the aldehyde stage use a mild/controlled oxidant such as PCC (pyridinium chlorochromate) or Cu at $573\,\text{K}$ (catalytic dehydrogenation), which avoids over-oxidation to the acid.
Catalytic dehydrogenation over hot Cu ( $573\,\text{K}$ ): $1^\circ \rightarrow$ aldehyde, $2^\circ \rightarrow$ ketone, and $3^\circ$ undergoes dehydration to an alkene (no H on the carbinol carbon to remove).
Acidified $KMnO_4$ (purple $\rightarrow$ colourless) oxidises $1^\circ$ alcohols all the way to carboxylic acids and $2^\circ$ to ketones; the decolourisation of the purple $MnO_4^-$ (to $Mn^{2+}$ ) is itself a distinguishing observation.
Victor Meyer steps: alcohol $\rightarrow$ alkyl iodide ( $P/I_2$ ) $\rightarrow$ nitroalkane ( $AgNO_2$ ) $\rightarrow$ react with $HNO_2$ then add NaOH to develop the colour ( $1^\circ$ red, $2^\circ$ blue, $3^\circ$ none).
Methanol gives a negative iodoform test and is also distinguished from ethanol by the fact that ethanol oxidises to acetaldehyde/acetic acid while methanol gives formaldehyde/formic acid (which still reduces Tollens' reagent).

Where the marks go

Forgetting that a $3^\circ$ alcohol is NOT oxidised by acidified $K_2Cr_2O_7$ under mild conditions — there is no H on the carbinol carbon, so the orange colour does not turn green.
Swapping the Victor Meyer colours — $1^\circ$ is blood-red, $2^\circ$ is blue, $3^\circ$ is colourless; reversing red and blue is a common slip.
Stopping a $1^\circ$ alcohol at the aldehyde with $K_2Cr_2O_7$ / $KMnO_4$ — these strong oxidants drive on to the carboxylic acid; use PCC or distil off the aldehyde for the partial product.
Writing the acidity order $3^\circ > 2^\circ > 1^\circ$ — it is the reverse: $1^\circ > 2^\circ > 3^\circ$ because alkyl $+I$ destabilises the alkoxide.
Claiming alcohols are more acidic than water — they are weaker acids than water, so an alkoxide cannot be made simply by adding NaOH in water (use Na metal).

How the board asks it

Distinguishacidified $K_2Cr_2O_7$ oxidation and the iodoform test
How will you distinguish between propan- $1$ -ol and propan- $2$ -ol by a chemical test? State the reagent and the observation in each case.
Predict the product $1^\circ/2^\circ/3^\circ$ oxidation products with acidified $K_2Cr_2O_7$ or $KMnO_4$
Predict the organic product formed when (i) ethanol and (ii) propan- $2$ -ol are oxidised with acidified $K_2Cr_2O_7$ , and write the equation for (i).
Distinguishiodoform test for the $CH_3CH(OH)\text{-}$ group
How will you distinguish between methanol and ethanol using a chemical test? Give the reagent, the observation, and name the test.
Give reasonsno hydrogen on the carbinol carbon of a $3^\circ$ alcohol
Give reasons: a tertiary alcohol is not oxidised by acidified $K_2Cr_2O_7$ under mild conditions, whereas a primary alcohol is readily oxidised.
Conversioncontrolled oxidation with $PCC$ or catalytic dehydrogenation over $Cu$ at $573\,\text{K}$
How will you convert ethanol into ethanal without it being oxidised further to ethanoic acid? Give the reagent or conditions and the equation.
Identify / classifythe Victor Meyer test sequence and its final nitrolic-acid colours
An alcohol $A$ is treated with $P/I_2$ , then $AgNO_2$ , and the product on reaction with $HNO_2$ followed by $NaOH$ gives a blood-red colour. State the class of $A$ and name the test.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.