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.

Common traps

Writing a positive Tollens' for a ketone. Ketones do not give a silver mirror. Only aldehydes do.

Forgetting the cyclic dimer. When explaining the high boiling point of carboxylic acids, mention the dimer explicitly; it is what marks the answer.

Confusing the test for "carbonyl group" with the test for "aldehyde". The carbonyl is in both aldehydes and ketones. Tollens and Fehling distinguish aldehyde from ketone, not carbonyl from non-carbonyl.

Oxidising a carboxylic acid further. Under HSC conditions, carboxylic acids are the terminus of the oxidation pathway. They do not go further.

Naming a propanone "propan-2-one" only. Acetone and propan-2-one are both accepted but the IUPAC name is propan-2-one (or simply propanone, since the locant is unambiguous on a 3-carbon chain).

In one sentence

A primary alcohol oxidises to an aldehyde and then to a carboxylic acid, a secondary alcohol oxidises only to a ketone, you tell aldehyde from ketone with the Tollens' silver mirror or Fehling's brick-red $Cu_2O$ tests, and you identify a carboxylic acid by its effervescence with carbonate or its reaction with metals.