Aldehydes, Ketones and Carboxylic Acids

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

investigate the structural formulae, properties and functional group including: – primary, secondary and tertiary alcohols – aldehydes and ketones (ACSCH127) – amines and amides – carboxylic acids
explain the properties within and between the homologous series of carboxylic acids, amines and amides with reference to the intermolecular and intramolecular bonding present
investigate the differences between an organic acid and organic base

Carbonyl Compounds: Structure, Properties & Reactions

This video introduces a new group of organic compounds – carbonyl compounds, including the structure, properties and reactions of

aldehydes
ketones and
carboxylic acids (organic acid).

Structure and Nomenclature

Aldehyde, ketone and carboxylic acids all contain a carbonyl carbon that is sp² This means both functional groups contain a C=O bond, of which one is a reactive π-bond, the other is an unreactive s-bond.

Functional group	Suffix	Prefix	Generic structure	Example
Aldehyde	-al	Formyl-
Ketone	-one	Oxo-
Carboxylic acid	-oic acid	Carboxyl-

Nomenclature priority

- In order of decreasing priority: carboxylic acid, aldehyde, ketone, alcohol, alkene, alkyne and alkanes.

- In the presence of carboxylic acid, aldehyde or ketone functional group, an alcohol will be referred to by its prefix ‘hydroxyl’

Properties of Aldehydes and Ketones

Boiling and Melting Points

Smaller aldehydes and ketones are polar molecules and there can form dipole-dipole forces on top of dispersion forces.
Aldehydes and ketones generally have stronger intermolecular forces than hydrocarbons of similar molecular mass. Thus, they have higher boiling and melting points
Compared to alcohols, aldehydes and ketones generally have weaker intermolecular forces because they cannot form hydrogen bonds, unlike alcohol molecules. Alcohol molecules contain hydroxyl (–OH) groups that can participate in hydrogen bonding as either a donor or acceptor.

In their own homologous series, boiling and melting points of aldehydes and ketones increase with molecular mass due to stronger dispersion forces.

Solubility in water

Aldehydes and ketones are polar compounds, primarily due to the presence of an electronegative oxygen atom in their functional groups. Smaller aldehyde and keton molecules are soluble in water.
Aldehydes and ketones cannot form hydrogen bonds with themselves (among molecules of only aldehyde and ketone), they can however form hydrogen bonds with water molecules.
This enables small aldehyde and ketone molecules to dissolve in water (molecules with low number of carbon atoms).
As the length of non-polar carbon chain increases, the polarity of aldehydes and ketones decreases. This reduces the solubility of aldehydes and ketones in water.

Table: melting and boiling points of aldehydes and ketones increase with molecular weight (size) while their solubilities decrease with molecular weight.

Properties of Carboxylic Acid

Acidity of Carboxylic Acids

Carboxylic acids are organic weak acids.
The deprotonation of hydrogen from a carboxylic acid forms a carboxylate ion.

The proton or hydrogen atom attached to oxygen is acidic because:
- O–H bond is polarised and weak due to oxygen’s high electronegativity.
- Resonance stabilisation of the conjugate base (carboxylate ion)

When carboxylic acids are halogenated, the O–H bond becomes more polarised. This means pK_a decreases and acidity increases.

As the carbon chain of carboxylic acids increases in length, acidity decreases and pK_a This is because alkyl groups have the opposite effect to that of halogens.

Boiling and Melting Points

Carboxylic acids are polar compounds. They are generally considered more polar than aldehydes and ketones due to the presence of an additional electronegative oxygen atom.

Carboxylic acids can form hydrogen bonds – hydrogen atom attached to oxygen acts as a bond donor while an electron lone pair acts as a bond acceptor.

The electron lone pair can either be from –OH or C=O.

Carboxylic acids have much higher boiling and melting points than hydrocarbons, alcohols, aldehydes and ketones of similar molecular weight.

The formation of two hydrogen bonds between two molecules of carboxylic acid forms a dimer configuration which further increases the strength of dispersion forces between the two molecules.

As a result of this dimer configuration, carboxylic acids have higher boiling points than alcohols of similar molar mass, despite both functional groups being able to form hydrogen bonds.

Hydrogen boding between molecules of carboxylic acids creates a dimer.

Hydrogen bonding between molecules of ethanol does not produce dimers.

Table: compounds that can form hydrogen bonds have, in general, stronger intermolecular force and higher boiling and melting points than those that do not.

Compound	Functional group	Molar mass (g mol^–1)	Type of intermolecular force	Boiling point (ºC)
Butane	Alkane	58	Dispersion	–1
Butanal	Aldehyde	72	Strong dipole	49
Butanone	Ketone	72	Stronger dipole	56
Butanol	Alcohol	74	Hydrogen bonding	97
Butanoic acid	Carboxylic acid	88	Hydrogen bonding	118

Solubility in Water

Similar to alcohols, aldehydes and ketones, small carboxylic acids are soluble in water.

- Carboxylic acids are more soluble in water than alcohol, aldehydes and ketones of similar molecular weight because they can form more hydrogen bonds.

Carboxylic acids’ solubilities in water decrease with molecular weight (number of carbons in its chain). The extension of the carbon chain decreases the overall polarity of the molecule.

Reactions of Aldehydes, Ketones and Carboxylic acids

Oxidation

Oxidation of alcohols produces aldehyde, ketone and carboxylic according to the following table

Reactant	Reagent/catalyst/condition	Product
Primary alcohol	Mild oxidising agent pyridinium chlorochromate (PCC)	Aldehyde
Primary alcohol	Strong oxidising agent Acidified potassium permanganate (H⁺/KMnO₄) Acidified sodium dichromate (H⁺/NaCr₂O₇) Jones Reagent (CrO₃/H⁺) Tollens’ Reagent (Ag(NH₃)₂⁺) (silver mirror test)	Carboxylic acid
Secondary alcohol	Any oxidising agent Acidified potassium permanganate (H⁺/KMnO₄) Acidified sodium dichromate (H⁺/NaCr₂O₇) Jones Reagent (CrO₃/H⁺) Tollens’ Reagent (Ag(NH₃)₂⁺) (silver mirror test)	Ketone

Oxidation of an aldehyde produces a carboxylic acid

Carboxylic Acid and Base Reactions

Carboxylic acids are weak acids that react with Arrhenius and Brønsted-Lowry bases
Each carboxylic acid functional group is monoprotic i.e. donates one proton
Carboxylic acid + metal hydroxide  salt + water

Example: reaction between acetic acid (C₂H₄O₂) and sodium hydroxide to produce sodium acetate and water

C₂H₄O₂(aq) + NaOH " NaC₂H₃O₂(aq) + H₂O(l)

Carboxylic acid + metal carbonate  salt + carbon dioxide and water

Example: reaction between acetic acid (C₂H₄O₂) and sodium carbonate to produce sodium acetate, carbon dioxide and water

2 C₂H₄O₂(aq) + Na₂CO₃(aq) " 2 NaC₂H₃O₂(aq) + CO₂(g) + H₂O(l)

Carboxylic acid + metal hydrogen carbonate  salt + carbon dioxide and water

Example: reaction between acetic acid (C₂H₄O₂) and sodium hydrogen carbonate to produce sodium acetate, carbon dioxide and water

C₂H₄O₂(aq) + NaHCO₃(aq) " NaC₂H₃O₂(aq) + CO₂(g) + H₂O(l)

Chemical Tests for Aldehydes, Ketones and Carboxylic acids

Dichromate (Cr₂O₇^2–) oxidation test

This test is used to identify ketones from aldehydes (or vice versa).
Dichromate is a strong oxidising agent that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Cr⁶⁺ ions (in dichromate) are reduced to form Cr³⁺, the solution turns from orange to green.
Acidified dichromate solutions will turn green in the presence of an aldehyde.
Acidified dichromate solutions will remain orange in the presence of a ketone.

Cr⁶⁺(orange) reduces to form Cr³⁺(green)

Tollens reagent (silver mirror test)

This test is used to identify ketones from aldehydes (or vice versa).
Tollens reagent contains Ag⁺ ions which are strong oxidising agents that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Ag⁺ ions are reduced to form Ag, a layer of silver will coat the test tube. This creates a mirror-like appearance.
Tollens reagent will create a mirror-like appearance in the presence of an aldehyde.
Tollens reagent will remain colourless in the presence of a ketone.

Tollen’s reagent

Permanganate (MnO₄^–) oxidation test

This test is used to identify ketones from aldehydes (or vice versa).
Acidified KMnO₄ solution contains Mn⁷⁺ ions which are strong oxidising agents that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Mn⁷⁺ions (purple) are reduced to form Mn²⁺, the solution changes from purple to colourless.
Acidified permanganate solutions will decolourise (from purple) in the presence of an aldehyde.
Acidified permanganate solutions will remain purple in the presence of a ketone.

Mn⁷⁺ (purple) reduces to form Mn²⁺ (colourless)

Identifying Organic Acids

This test is used to identify a carboxylic acid from aldehydes and ketones.
Carboxylic acids undergo acid-base reaction with carbonate and hydrogen carbonates to produce salt, water and carbon dioxide. No acid and base reactions occur between an aldehyde/ketone and carbonates.

When Na₂CO₃ or NaHCO₃ is added to a test tube, formation of bubbles indicates the production of carbon dioxide, which in turn indicates the presence of a carboxylic acids.
Production of CO₂ can be further confirmed by using limewater test.

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Using pH indicators

A solution containing carboxylic acid will have a pH < 7 at 25ºC. This means a blue litmus paper will turn red, bromothymol blue will be yellow.
Ketones and aldehydes do not contain acidic hydrogens so their solutions will be neutral.

Limewater test of carbon dioxide