7.2.1 Aldehydes & Ketones

The carbonyl group is the defining feature of two closely related families of organic compounds: aldehydes and ketones.

This functional group is highly prevalent in biological molecules and plays a crucial role in many biochemical processes. Carbonyl groups are found in a variety of essential organic compounds, including carbohydrates, fats, proteins, nucleic acids, hormones, and vitamins, all of which are fundamental to the structure and function of living organisms.

Both aldehydes and ketones are classified as carbonyl compounds, as they feature a carbon-oxygen double bond, referred to as the carbonyl group $C=O$.

The difference between aldehydes and ketones is the groups bonded to the carbon of the carbonyl group

Aldehydes

In the presence of acidified potassium dichromate, aldehydes can undergo further oxidation to form carboxylic acids.

For example:

Ethanol can be oxidised to ethanal, which further oxidises to ethanoic acid in the reaction with an oxidising agent. The balanced chemical equation for this process shows the conversion of an aldehyde to a carboxylic acid:

$${CH}_3{CH}_2{OH} + [O] {CH}_3{CHO} + [O] → {CH}_3{COOH}$$

Aldehydes and Their Reactions

Aldehydes can be tested using Tollen's reagent or Fehling's solution, as they produce a positive result with both reagents if present. These tests are useful in distinguishing aldehydes from ketones, since only aldehydes will react.

Ketones

Ketones are organic compounds characterized by the carbonyl group ($C=O$) and can be identified by this functional group. They are typically formed through the oxidation of secondary alcohols using acidified potassium dichromate.

An example of this process is the oxidation of a secondary alcohol to a ketone:

• For instance, the oxidation of isopropanol produces propanone and hydrogen gas. It is important to note that ketones do not undergo further oxidation, unlike aldehydes. Therefore, they show no visible reaction when tested with either Tollen's reagent or Fehling's solution, making these reagents useful for distinguishing ketones from aldehydes in organic analysis.

Further Oxidation of Alcohols and Aldehydes

When primary alcohols are oxidised to aldehydes, the apparatus must be arranged for distillation to remove the aldehyde as soon as it forms. This prevents further oxidation from occurring at that stage. However, if the reaction continues, the aldehyde can undergo further oxidation to form a carboxylic acid.

To directly oxidise a primary alcohol to a carboxylic acid, the reaction mixture is heated under reflux. In this setup, the aldehyde is still produced, but it evaporates, condenses, and returns to the mixture, allowing further oxidation to the carboxylic acid.

In all these oxidation reactions, the oxidising agent is acidified potassium dichromate ($K₂Cr₂O₇$), which is used in the presence of sulfuric acid ($H₂SO₄$).

Oxidation Resistance of Ketones

Ketones, on the other hand, are highly resistant to oxidation. No further oxidation occurs with secondary alcohols, as ketones lack a readily available hydrogen atom on the carbonyl carbon, unlike aldehydes or alcohols.

For a ketone to be oxidised, an extremely strong oxidising agent would be required. However, this oxidation is typically destructive, as it involves breaking a $C-C$ bond, rather than a simple oxidation process like that of aldehydes. This makes the further oxidation of ketones rare and chemically difficult.

Distinguishing Between Aldehydes and Ketones

Weak oxidising agents can be used to differentiate between aldehydes and ketones. Aldehydes are readily oxidised to carboxylic acids, whereas ketones do not undergo oxidation under the same conditions.

There are several methods for distinguishing between these two compounds, but the key ones you need to know for A-level Chemistry include:

• Tollens' reagent (the most common method)
• Fehling's solution
• Acidified potassium dichromate

Tollens' Reagent – The Silver Mirror Test

Tollens' reagent contains the silver(I) complex ion $[Ag(NH₃)₂]⁺$, which is formed by adding aqueous ammonia to a solution of silver nitrate. This reagent is also known as ammoniacal silver nitrate.

• When gently heated with an aldehyde, the aldehyde is oxidised to a carboxylic acid, and the $[Ag(NH₃)₂]⁺$ ions are reduced to form solid metallic silver ($Ag$), resulting in the formation of a silver mirror on the test tube.

Positive Test Result:

• The formation of a silver mirror when Tollens' reagent is heated with an aldehyde indicates a positive result, confirming the presence of an aldehyde.

Negative Test Result:

• When Tollens' reagent is heated with a ketone, no reaction occurs, and no silver mirror is formed. This indicates a negative result, as ketones do not undergo oxidation with Tollens' reagent.

Using Fehling's Solution

Fehling's solution is an alkaline solution containing copper(II) ions ($Cu²⁺$), which act as the oxidising agent. The solution is blue due to the presence of the copper(II) complex.

• When heated with an aldehyde, the aldehyde is oxidised to a carboxylic acid. During this process, the blue $Cu²⁺$ions are reduced to Cu⁺ ions, forming a brick-red precipitate of copper(I) oxide ($Cu₂O$).

Positive Test Result:

• A brick-red precipitate forms when Fehling's solution is heated with an aldehyde, indicating a positive result.

Negative Test Result:

• When heated with a ketone, no reaction occurs, and the solution remains blue, indicating a negative result since ketones do not react with Fehling's solution.

Using Acidified Potassium Dichromate

Acidified potassium dichromate can also be used to distinguish between aldehydes and ketones.

• When an aldehyde is heated with acidified potassium dichromate, the aldehyde is oxidised, and the solution changes from orange (due to the dichromate ions, $Cr₂O₇²⁻$) to green (due to the formation of $Cr³⁺$ions).
• When a ketone is heated with acidified potassium dichromate, no reaction occurs, and the solution remains orange. This simple colour change can easily distinguish between aldehydes and ketones.

Oxidation reactions table:

Oxidising Agent	1° Alcohol	2° Alcohol	Aldehyde	Ketone
Acidified Potassium Dichromate ($K₂Cr₂O₇$/$H₂SO₄$)	✓ Orange to green colour change • Forms aldehyde (distillation) • Forms carboxylic acid (reflux)	✓ Orange to green colour change • Forms ketone	✓ Orange to green colour change • Forms carboxylic acid	✗ No reaction
Tollens' Reagent	✗ No reaction	✗ No reaction	✓ Silver mirror seen • Forms carboxylic acid	✗ No reaction
Fehling's Solution	✗ No reaction	✗ No reaction	✓ Blue to brick-red precipitate • Forms carboxylic acid	✗ No reaction

Boiling Points

Van der Waals' forces and permanent dipole-dipole forces exist between aldehydes and other molecules - that's true for ketones too.

• This means that the b.p. of aldehydes and ketones are higher than those of alkanes with a similar $M_r$.
• As the length of the carbon chain increases, the b.p. of the aldehydes/ketones increases too. This is due to a greater number of electrons, and therefore stronger van der Waals' forces.
• As branching increases, which is common in aldehydes and ketones, the boiling point of the aldehydes/ketones decreases. This is due to less effective permanent dipole-dipole forces.

This table compares the molecular mass ($Mᵣ$) and boiling points of propane, ethanal, and ethanol across different homologous series.

Homologous Series	Molecule	$Mᵣ$	Boiling Point (°C)
Alkane	Propane	44	-42
Aldehyde	Ethanal	44	+21
Alcohol	Ethanol	46	+79

All 3 of these molecules have similar van der Waals' forces between their molecules (similar $M_r$, so a similar number of electrons.) The boiling point of the aldehyde is much higher than the b.p. of the alkane due to permanent dipole-dipole forces between the carbonyl groups on neighbouring aldehyde molecules. The boiling point of the alcohol is much higher than the boiling point of the aldehyde due to H-bonds between the hydroxyl groups on neighbouring alcohol molecules.

Solubility in water

All short chain aldehydes and ketones are soluble in water because the polar carbonyl groups are able to form H-bonds with water molecules.

• As the chain length increases, the aldehydes and ketones become less soluble. This is because the hydrocarbon chain that doesn't have the carbonyl group on it is non-polar.
• The longer this chain is, the less polar the molecule is so therefore less soluble.