Aldehydes and Ketones: Reactions of the Carbonyl Group (9620 A2 Chemistry)

Welcome to the exciting world of aldehydes and ketones! These are two groups of organic compounds that share a very important feature: the carbonyl group. This chapter is vital because it introduces a crucial type of reaction mechanism—the nucleophilic addition—which explains how these compounds react. Understanding this structure and its polarity will unlock all their chemical properties!

1. Structure and Reactivity of the Carbonyl Group

Both aldehydes and ketones contain the carbonyl group, represented by \(C=O\).

Defining Aldehydes and Ketones
  • Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom (and typically an alkyl group, R, or another H). The functional group is usually written as \(-CHO\).
    Example: Ethanal (\(\text{CH}_3\text{CHO}\)).
  • Ketones: The carbonyl carbon is bonded to two alkyl groups (R and R'). The functional group is often written as \(-CO-\).
    Example: Propanone (\(\text{CH}_3\text{COCH}_3\)).
The Key to Reactivity: Polarity and Electrophilicity

The oxygen atom is significantly more electronegative than the carbon atom.

  • The electron pair in the \(C=O\) double bond is pulled strongly towards the oxygen atom.
  • This creates a strong dipole moment: the oxygen atom gains a partial negative charge (\(\delta-\)), and the carbon atom gains a partial positive charge (\(\delta+\)).
  • The carbonyl carbon is therefore electron deficient (an electrophile) and is highly susceptible to attack by nucleophiles (species that love positive charge).
Quick Review: Why Nucleophiles Attack

The \(C=O\) bond is highly polar. The positive carbon (\(C^{\delta+}\)) acts as an easy target for electron-rich nucleophiles, leading to the characteristic reaction type: Nucleophilic Addition.

2. Distinguishing Between Aldehydes and Ketones: Oxidation

The single most important chemical difference between aldehydes and ketones is their behaviour when faced with oxidising agents.

3.3.8.1 Oxidation Reactions
  • Aldehydes are readily oxidised to form carboxylic acids. They require only mild oxidising agents.
    Overall Equation (using \([\text{O}]\) for the oxidant):

    \(\text{RCHO} + [\text{O}] \rightarrow \text{RCOOH}\)

  • Ketones are resistant to oxidation. Because the carbonyl carbon is bonded to two other carbon atoms, breaking the molecule requires strong oxidising agents and harsh conditions, resulting in mixtures of products (this is outside the scope of typical lab tests).
Chemical Tests to Distinguish Them

Since aldehydes are easily oxidized and ketones are not, we use mild oxidising agents whose change is visible, acting as distinguishing tests.

Tollens' Reagent (The Silver Mirror Test)

Tollens' reagent is an aqueous solution of silver nitrate dissolved in ammonia, containing the complex ion \([\text{Ag}(\text{NH}_3)_2]^+\).

  • Result with Aldehydes: The aldehyde is oxidized to a carboxylic acid (or its salt), and the silver ions are reduced to metallic silver. This metallic silver forms a beautiful "silver mirror" on the inside of the test tube.

    \(\text{RCHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}(s) + 4\text{NH}_3 + 2\text{H}_2\text{O}\)

  • Result with Ketones: No visible reaction. The solution remains clear.

Memory Aid: Tollens' test tells you if you have an Aldehyde. Think A for Aldehyde and Ag (Silver).

Fehling's Solution (or Benedict's Solution)

Fehling's solution contains blue aqueous copper(II) ions (\(\text{Cu}^{2+}\)).

  • Result with Aldehydes: The aldehyde is oxidized to a carboxylic acid (or its salt), and the soluble blue copper(II) ions are reduced to insoluble, brick-red copper(I) oxide (\(\text{Cu}_2\text{O}\)).

    \(\text{RCHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \rightarrow \text{RCOO}^- + \text{Cu}_2\text{O}(s) + 3\text{H}_2\text{O}\)

  • Result with Ketones: No visible reaction. The solution remains blue.
Did You Know?
The sugar glucose, found in your blood, is an aldehyde (aldohexose). Fehling's and Benedict's tests were historically important (and still used in basic biology) to detect the presence of sugars like glucose in samples (like urine).

3. Reduction of Carbonyl Compounds

Reduction is the opposite of oxidation. When aldehydes and ketones are reduced, the \(C=O\) double bond is broken, and hydrogen is added, converting them into alcohols.

Reduction Products
  • Aldehydes are reduced to primary alcohols (\(\text{RCH}_2\text{OH}\)).
  • Ketones are reduced to secondary alcohols (\(\text{R}_2\text{CHOH}\)).
The Reducing Agent: Sodium Borohydride (\(\text{NaBH}_4\))

The most common and safest reducing agent used in the lab (as specified by the syllabus) is sodium borohydride (\(\text{NaBH}_4\)), used in aqueous solution.

Overall Reduction Equations

When writing overall equations for reduction, we represent the reducing agent as \([\text{H}]\).

Reduction of Aldehydes:

$$\text{RCHO} + 2[\text{H}] \rightarrow \text{RCH}_2\text{OH}$$

Reduction of Ketones:

$$\text{RCOR'} + 2[\text{H}] \rightarrow \text{RCH}(\text{OH})\text{R'}$$

Step-by-Step Mechanism: Nucleophilic Addition (Reduction)

Reduction using \(\text{NaBH}_4\) is a classic example of nucleophilic addition. The actual nucleophile is the hydride ion (\(\text{H}^-\)), which acts as an electron donor.

Step 1: Nucleophilic Attack

The hydride ion (\(\text{H}^-\)) attacks the electron-deficient carbon atom (\(C^{\delta+}\)) of the carbonyl group. Simultaneously, the pi bond (\(\pi\)) breaks, and the electron pair shifts onto the oxygen atom. This forms an unstable intermediate species called an alkoxide ion.

Step 2: Protonation (Addition of \(\text{H}^+\))

The alkoxide ion is rapidly protonated (it gains an \(\text{H}^+\) ion from the solvent, usually water or dilute acid, following the reaction). This step completes the alcohol functional group.

(Note: In your mechanism drawings, use curly arrows to show the movement of the electron pairs. Ensure the nucleophile for this reaction is specifically shown as \(\text{H}^-\) coming from \(\text{NaBH}_4\)).

Common Mistake Alert!
Do not confuse the reducing agent \(\text{NaBH}_4\) with a standard acid-base reaction. The hydride ion (\(\text{H}^-\)) is the nucleophile, not a proton (\(\text{H}^+\)).

4. Nucleophilic Addition with Hydrogen Cyanide (HCN)

Another important nucleophilic addition reaction involves the use of potassium cyanide (\(\text{KCN}\)) followed by dilute acid. This reaction lengthens the carbon chain by one carbon atom and produces compounds called hydroxynitriles (also known as cyanohydrins).

Formation of Hydroxynitriles

The product contains both a hydroxyl group (\(-\text{OH}\)) and a nitrile group (\(-\text{CN}\)) attached to the same carbon atom.

Overall Equation (using \(\text{HCN}\) for simplicity, though KCN/acid is used in practice):

$$\text{RCOR'} + \text{HCN} \rightarrow \text{R}(\text{OH})(\text{CN})\text{R'}$$

Step-by-Step Mechanism (KCN followed by dilute acid)

We use potassium cyanide because hydrogen cyanide gas (\(\text{HCN}\)) is highly toxic. Potassium cyanide reacts with the carbonyl compound via the cyanide ion (\(\text{CN}^-\)) which acts as the nucleophile.

Step 1: Nucleophilic Attack

The cyanide ion (\(\text{CN}^-\)) attacks the carbonyl carbon (\(C^{\delta+}\)). The \(\pi\) bond breaks, and electrons move onto the oxygen, forming an alkoxide intermediate.

Step 2: Protonation

The alkoxide ion immediately reacts with the acid (or water) present in the solution to gain a proton (\(\text{H}^+\)), forming the final hydroxynitrile product.

Stereochemistry: The Formation of Racemates

When the nucleophile (\(\text{CN}^-\)) attacks the planar \(C=O\) carbonyl group, it can approach the molecule from either the "top" face or the "bottom" face.

If the reaction creates a product that contains a chiral centre (an asymmetric carbon atom bonded to four different groups), then two different optical isomers (enantiomers) will be formed.

  • If the resulting hydroxynitrile has a chiral centre: The top and bottom attacks are equally likely. This results in an equal mixture of the two enantiomers (50% of one optical isomer and 50% of the mirror image).
  • This 50:50 mixture is called a racemic mixture (or racemate) and is optically inactive, as the rotation of plane-polarised light caused by one enantiomer is cancelled out by the other.

Example: Ethanal (\(\text{CH}_3\text{CHO}\)) is achiral, but its product, 2-hydroxypropanenitrile, has a chiral carbon, resulting in a racemate.

Crucially: This only happens if the ketone is unsymmetrical or if an aldehyde is used (unless it is methanal, \(\text{HCHO}\)).

Safety Note: Hazards of KCN

Potassium cyanide (\(\text{KCN}\)) is highly hazardous. When \(\text{KCN}\) is mixed with acid, it produces highly toxic, volatile hydrogen cyanide gas (\(\text{HCN}\)). Therefore, this reaction must be carried out in a well-ventilated fume cupboard, and strict safety procedures must be followed. Chemists must always consider the hazards of reagents when planning syntheses.

Key Takeaway for Aldehydes and Ketones
The fundamental reaction type is Nucleophilic Addition, driven by the polarity of the \(C=O\) bond.
1. Oxidation (Distinction): Aldehydes $\rightarrow$ Carboxylic acids (use Tollens' or Fehling's). Ketones resist.
2. Reduction (\(\text{H}^-\)): A $\rightarrow$ Primary Alcohol; K $\rightarrow$ Secondary Alcohol.
3. Cyanide Addition (\(\text{CN}^-\)): Forms hydroxynitriles; creates racemic mixtures if a new chiral centre is formed.