Aldehydes and ketones

Welcome to Aldehydes and Ketones!

In this chapter, we are diving into the world of carbonyl compounds. These molecules are everywhere—from the smell of fresh cinnamon to the chemicals our bodies use for energy. We will explore how their structures make them react, how to tell them apart in a lab, and the clever ways we can turn them into other useful substances like alcohols or plastics. Don't worry if the mechanisms look like a lot of arrows at first; we will break them down step-by-step!

Quick Review: Both aldehydes and ketones contain the carbonyl group, which is a carbon atom double-bonded to an oxygen atom \( (C=O) \). The difference is just where that group sits:
• Aldehydes: The \( C=O \) is at the end of the carbon chain (e.g., propanal).
• Ketones: The \( C=O \) is in the middle of the carbon chain (e.g., propanone).

1. The Carbonyl Group: Why does it react?

The \( C=O \) bond is very polar. Oxygen is much more electronegative than carbon, so it pulls the electrons toward itself. This makes the oxygen slightly negative \( (\delta-) \) and the carbon slightly positive \( (\delta+) \).

Because that carbon atom is "electron-poor," it is a prime target for nucleophiles (species that love positive charges). This is why the main reaction for these compounds is nucleophilic addition.

Analogy: Think of the carbonyl carbon like a person who has lost their wallet. They are "positive" and looking for someone with "extra" electrons (a nucleophile) to come and help them out!

2. Oxidation: Telling them apart

This is a classic exam topic. You need to know how aldehydes and ketones behave when you try to oxidise them.

Aldehydes vs. Ketones

• Aldehydes are easily oxidised to carboxylic acids. For example, ethanal turns into ethanoic acid.
Equation: \( CH_3CHO + [O] \rightarrow CH_3COOH \)
• Ketones are not easily oxidised because you would have to break a strong C-C bond to do it. Under normal lab conditions, they stay exactly as they are.

The Lab Tests

Since aldehydes react and ketones don't, we can use two special "oxidising agents" to tell them apart in a test tube:

1. Tollens’ Reagent (The Silver Mirror Test):
• Aldehyde: A beautiful silver mirror forms on the inside of the test tube as silver ions \( (Ag^+) \) are reduced to metallic silver \( (Ag) \).
• Ketone: No change (remains a clear solution).

2. Fehling’s Solution:
• Aldehyde: The blue solution turns into a brick-red precipitate of copper(I) oxide.
• Ketone: Stays blue.

Key Takeaway: Aldehydes say "Yes" to oxidation (Silver mirror/Red ppt), Ketones say "No."

3. Reduction: Turning them back into Alcohols

We can go backwards! By adding hydrogen, we can turn aldehydes and ketones back into alcohols. The reagent used is \( NaBH_4 \) (sodium tetrahydridoborate) in aqueous solution.

• Aldehydes are reduced to primary alcohols.
\( RCHO + 2[H] \rightarrow RCH_2OH \)
• Ketones are reduced to secondary alcohols.
\( RCOR' + 2[H] \rightarrow RCH(OH)R' \)

The Mechanism: Nucleophilic Addition

In this reaction, the nucleophile is the hydride ion \( (H^-) \) from the \( NaBH_4 \).
1. The \( H^- \) ion attacks the \( \delta+ \) carbon of the carbonyl group.
2. The \( C=O \) double bond breaks, and the electron pair moves onto the oxygen, making it \( O^- \).
3. The oxygen then picks up a \( H^+ \) (usually from water or the solvent) to form an \( -OH \) group.

Common Mistake to Avoid: When drawing the mechanism, make sure your arrow starts at the lone pair or the negative charge of the \( H^- \) and goes directly to the carbonyl carbon.

4. Reaction with KCN: Building the Chain

Aldehydes and ketones can react with potassium cyanide (KCN) followed by dilute acid to form hydroxynitriles. This is a very useful reaction because it adds one more carbon atom to the chain.

The Hazard: KCN is extremely toxic. In a lab, we often use a mixture of \( NaCN \) and \( H_2SO_4 \) to provide the \( HCN \) needed, but this must be done in a fume cupboard!

Step-by-Step Mechanism

1. The cyanide ion \( (:CN^-) \) attacks the \( \delta+ \) carbon.
2. The \( C=O \) bond breaks, shifting electrons to the oxygen.
3. The \( O^- \) then reacts with \( H^+ \) from the acid to form an \( -OH \) group.

Key takeaway: You end up with a molecule that has both an alcohol group (-OH) and a nitrile group (-CN) on the same carbon.

5. Optical Isomerism and Carbonyls

Something very interesting happens when you react an aldehyde (other than methanal) or an unsymmetrical ketone with KCN.

The carbonyl group \( (C=O) \) is planar (flat). This means the \( CN^- \) ion can attack from above or below with equal probability.
• If the product formed has a chiral centre (a carbon with 4 different groups), you will get two different optical isomers (enantiomers).
• Because the attack is 50/50 from either side, you get an equal mixture of both isomers. This is called a racemic mixture (or racemate).
• Quick Review: A racemic mixture is optically inactive because the two isomers rotate plane-polarised light in opposite directions, cancelling each other out.

Analogy: Imagine a flat piece of paper on a table. You can drop a marble on it from the top, or if it were hanging in the air, you could hit it from the bottom. Both are just as easy to do!

Summary Review Box

• Functional Group: \( C=O \) (Carbonyl).
• Polarity: Carbon is \( \delta+ \), Oxygen is \( \delta- \).
• Reagent for Reduction: \( NaBH_4 \) (makes alcohols).
• Reagent for Chain Extension: \( KCN/H^+ \) (makes hydroxynitriles).
• Aldehyde Test: Tollens' (Silver Mirror) or Fehling's (Red ppt).
• Mechanism: Nucleophilic Addition.

Don't worry if this seems tricky at first! Organic chemistry is all about patterns. Once you see that the \( \delta+ \) carbon is always the target, the mechanisms start to make perfect sense. Keep practicing those curly arrows!