Investigate the structural formulae, properties and reactions of aldehydes, ketones and carboxylic acids, including their formation by oxidation of alcohols and chemical tests that distinguish them

What this dot point is asking

NESA wants you to identify the structural feature of each carbonyl class, predict whether a given alcohol oxidises to an aldehyde, a ketone or a carboxylic acid, describe the chemical tests that distinguish aldehyde from ketone (Tollens, Fehling, Benedict), and explain why carboxylic acids are weak acids that react with metals, carbonates and bases.

The answer

The three functional groups

All three contain a carbonyl group $C=O$. They differ in what else is attached to the carbonyl carbon.

An aldehyde has at least one $H$ on the carbonyl carbon; a ketone has two carbons on it; a carboxylic acid has an $-OH$ directly on the carbonyl carbon.

Formation: oxidation of alcohols

• Primary alcohol ($R-CH_2OH$) $\xrightarrow{[O]}$ aldehyde ($R-CHO$) $\xrightarrow{[O]}$ carboxylic acid ($R-COOH$).
• Secondary alcohol ($R-CH(OH)-R'$) $\xrightarrow{[O]}$ ketone ($R-CO-R'$). Stops there.
• Tertiary alcohol: no oxidation.

The oxidant is acidified $K_2Cr_2O_7$ (orange to green) or acidified $KMnO_4$ (purple to colourless). To stop a primary alcohol at the aldehyde, distil the aldehyde off as it forms (aldehydes have lower boiling points than alcohols). To go to the acid, reflux with excess oxidant.

Carboxylic acids cannot be made from ketones without breaking $C-C$ bonds, which does not happen under HSC conditions.

Physical properties

Boiling point trend (same carbon number): alkane < aldehyde/ketone < alcohol < carboxylic acid.

• Aldehydes and ketones have $C=O$ dipole-dipole forces but no hydrogen bonding, so they boil above alkanes but below alcohols.
• Alcohols hydrogen-bond and boil higher.
• Carboxylic acids form cyclic dimers in the liquid phase, with two hydrogen bonds per pair, so they boil highest of all.

Solubility in water decreases with chain length. Short-chain carbonyls (acetone, propanal, ethanoic acid) are fully miscible because the polar functional group hydrogen bonds with water. Beyond about C5, the alkyl chain dominates and solubility falls.

Tests that distinguish aldehyde from ketone

Tollens' reagent (silver mirror test). $[Ag(NH_3)_2]^+$ in alkaline solution. Warm gently in a clean glass test tube.

• Aldehyde: silver metal deposits on the glass as a silver mirror. $RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag + 4NH_3 + 2H_2O$.
• Ketone: no reaction.

Fehling's solution or Benedict's solution. $Cu^{2+}$ ions in alkaline tartrate or citrate complex, blue.

• Aldehyde: brick-red precipitate of $Cu_2O$ forms on warming.
• Ketone: no reaction.

Both tests work because aldehydes are easily oxidised to carboxylates; ketones are not. Only aliphatic aldehydes give a positive Fehling's; aromatic aldehydes are negative. Tollens works for both.

A third option is to oxidise with acidified dichromate. Both aldehydes and primary alcohols decolourise (orange to green); ketones do not. If you suspect aldehyde, the silver mirror confirms it.

Reactions of carboxylic acids

Carboxylic acids are weak acids ($pK_a$ about 4 to 5). They ionise partially in water:

They undergo all the standard acid reactions.

With reactive metals (Mg, Zn, Fe) to give salt plus hydrogen:

With carbonates and hydrogencarbonates to give salt plus water plus carbon dioxide (this is the diagnostic test, since aldehydes, ketones and alcohols do not react):

With bases (neutralisation) to give salt plus water:

With alcohols (esterification, $H_2SO_4$ catalyst, reflux) to give an ester plus water. See the esters dot point.

Distinguishing all four classes

A flowchart that handles alcohol, aldehyde, ketone, carboxylic acid:

1. Sodium carbonate or blue litmus. Effervescence/red colour identifies the carboxylic acid. Remove it from consideration.
2. Tollens' reagent on the remaining three. Silver mirror identifies the aldehyde.
3. Acidified dichromate on the last two. Orange to green identifies the alcohol; orange remains for the ketone.

Examples in context

Example 1. Industrial production of ethanoic acid at Penrice Soda. Although Penrice's main plant in South Australia made carbonate, the NSW import pipeline relied on truck-tankered ethanoic acid produced by catalytic oxidation of ethanol. Air-oxidised ethanol over a copper catalyst gives ethanal as a removable middle product, then ethanoic acid: $CH_3CH_2OH \rightarrow CH_3CHO \rightarrow CH_3COOH$. Commercial vinegar packed at the Cornwell vinegar plant near Lidcombe is the dilute aqueous version, around 4 to 8 percent acid. The HSC oxidation tree predicts every transformation in the plant flowsheet. Quality control labs verify the product with sodium hydrogen carbonate effervescence and with phenolphthalein titration against NaOH.

Example 2. Distinguishing propanal from propanone in a Stage 6 lab. A NSW HSC depth study common across schools gives students unlabelled propanal and propanone and asks them to identify each chemically. A few drops of Tollens' reagent (silver-ammonia) added and warmed in a water bath produces a silver mirror with propanal (the aldehyde) but no change with propanone (the ketone). Repeating with Fehling's solution gives brick-red $Cu_2O$ with propanal and a clear blue solution with propanone. Students must write the half-equations and explain why the aldehyde, but not the ketone, can lose another H to become a carboxylic acid. This is exactly the test NESA examines in Section II tasks.

Try this

Q1. Write structural formulae for butanal, butan-2-one and butanoic acid, and identify which can be distinguished by Tollens' reagent. [3 marks]

• Cue. Butanal is an aldehyde (gives silver mirror); butan-2-one is a ketone (negative); butanoic acid is an acid (negative with Tollens', positive with $Na_2CO_3$).

Q2. A 1.50 g sample of butanoic acid is titrated with 0.250 mol L$^{-1}$ NaOH. Calculate the volume of NaOH required for complete neutralisation. [3 marks]

• Cue. $n(\text{acid}) = 1.50 / 88.11 = 0.0170$ mol; 1:1 stoichiometry; $V = 0.0170 / 0.250 = 0.0681$ L $= 68.1$ mL.

Q3. A student is given three unknowns A, B and C in unlabelled bottles. A gives a silver mirror with Tollens', B effervesces with sodium hydrogen carbonate, C gives neither reaction. (a) Classify each compound. (b) Write a balanced ionic equation for the carbonate reaction. (c) Predict the oxidation product of A. [2+2+1 marks]

• Cue. (a) A aldehyde, B carboxylic acid, C ketone or alcohol. (b) $2RCOOH + CO_3^{2-} \rightarrow 2RCOO^- + H_2O + CO_2$. (c) Carboxylic acid.