Oxidation & Distinguishing Tests

CHEReactions of Aldehydes & Ketones

Aldehydes are readily oxidised but ketones are not, which underlies the classic distinguishing tests. This subtopic tests Tollens', Fehling's, the iodoform test, and how to tell aldehydes, ketones, alcohols and aromatic carbonyls apart by a single observation.

Tollens' test (silver mirror)

RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O

Ag(I)

reduced to metallic

Ag

; aldehyde oxidised to carboxylate

Fehling's test (red precipitate)

RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O\downarrow + 3H_2O

red-brown

Cu_2O

; aromatic aldehydes give NO reaction

Iodoform test

CH_3COR + 3I_2 + 4NaOH \rightarrow CHI_3\downarrow + RCOONa + 3NaI + 3H_2O

yellow

CHI_3

from a

CH_3CO\text{-}

CH_3CH(OH)\text{-}

group;

R = H

for ethanal

Tollens' reagent (ammoniacal $AgNO_3$ ): aldehydes give a silver mirror; ketones do not. It distinguishes an aldehyde from a ketone.
Fehling's solution (alkaline $Cu^{2+}$ with sodium potassium tartrate): aliphatic aldehydes give a red-brown $Cu_2O$ precipitate; ketones and aromatic aldehydes (e.g. benzaldehyde) do not respond - so Fehling's separates ethanal from benzaldehyde.
Iodoform test ( $I_2/NaOH$ ): yellow $CHI_3$ precipitate from compounds with a $CH_3CO\text{-}$ group or a $CH_3CH(OH)\text{-}$ group oxidisable to it - positive for ethanal, propanone, ethanol, propan-2-ol; negative for propanal, pentan-3-one, methanol.
Formic acid is the exception: $HCOOH$ has a $\text{-}CHO$ -like unit and reduces Tollens' reagent (gives a silver mirror), unlike other carboxylic acids.
2,4-DNP gives an orange/yellow precipitate with all aldehydes and ketones (confirms a carbonyl) but cannot itself distinguish aldehyde from ketone - pair it with Tollens'/Fehling's for that.
Reasoning trap: a compound giving 2,4-DNP (orange ppt) but no Tollens' mirror is a ketone; follow up with the iodoform test to check for a methyl ketone.
Distinguishing alcohols from carbonyls: an aldehyde vs a $1^\circ$ alcohol is told apart by Tollens' (only the aldehyde reacts); ethanol and propan-2-ol are both iodoform-positive so iodoform cannot separate them - distinguish instead by oxidising and testing the product.
Why ketones resist oxidation: oxidising a ketone requires breaking a strong $\text{C-C}$ bond, so it happens only under drastic conditions and gives a mixture of smaller acids (cleaved on either side of the $\text{C=O}$ ).
Both Tollens' and Fehling's are MILD oxidants used in alkaline medium; the aldehyde is oxidised to the carboxylate (salt), not the free acid, because the medium is basic.
Tollens' is the more general aldehyde test: it works for both aliphatic and aromatic aldehydes (benzaldehyde DOES give a silver mirror), whereas Fehling's fails with aromatic aldehydes.
Schiff's reagent (decolourised fuchsin) is restored to pink by aldehydes but not ketones - another quick aldehyde-vs-ketone test you may be asked to name.
Putting the tests together to identify an unknown: 2,4-DNP first (carbonyl yes/no), then Tollens'/Fehling's (aldehyde vs ketone), then iodoform (methyl-ketone/methyl-carbinol pattern).

Where the marks go

Claiming benzaldehyde gives a positive Fehling's test - aromatic aldehydes do NOT reduce Fehling's, even though they reduce Tollens'.
Saying propanal or pentan-3-one is iodoform-positive - they lack the $CH_3CO\text{-}$ / $CH_3CH(OH)\text{-}$ group; only methyl ketones and methyl carbinols (plus ethanal/ethanol) respond.
Forgetting that formic acid ( $HCOOH$ ) and formaldehyde reduce Tollens'/Fehling's - a common "trick" exception to the rule that carboxylic acids are unreactive.
Treating 2,4-DNP as able to distinguish aldehyde from ketone - it only confirms the presence of a carbonyl.
Writing the oxidation product as the free acid in Tollens'/Fehling's - in the alkaline medium it is the carboxylate salt ( $RCOO^-$ ).

How the board asks it

Distinguishsingle-observation tollens', fehling's and iodoform tests
How will you distinguish between the following pairs by a single chemical test, giving the observation in each case: (i) ethanal and propanone; (ii) ethanal and benzaldehyde; (iii) propanal and propan-2-ol?
Identify / classify2,4-DNP then tollens' then iodoform sequence
An organic compound $A$ ( $C_3H_6O$ ) gives an orange precipitate with 2,4-DNP and a yellow precipitate with $I_2/NaOH$ , but does not give a silver mirror with Tollens' reagent. Identify $A$ and explain each observation.
Give reasonsketones resist mild oxidation; formic acid exception
Account for the following: (i) ketones do not respond to Fehling's or Tollens' test whereas aldehydes do; (ii) formic acid reduces Tollens' reagent though other carboxylic acids do not.
Conversionoxidation to carboxylate in alkaline medium
Write the equation for the reaction of ethanal with Tollens' reagent (ammoniacal $AgNO_3$ ), clearly showing the oxidation product formed in the alkaline medium.
Structure / namingreagent and observation for a methyl ketone
Name one reagent you would use to confirm that a given liquid is a methyl ketone, and state the observation obtained when the result is positive.

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.