Exploring Carbonyl Compounds - Chapter 26.1

Making Carbonyl Compounds from Alcohols

You can create carbonyl compounds by oxidising different types of alcohols using acidified dichromate. This reaction is straightforward but depends entirely on what type of alcohol you start with.

Primary alcohols oxidise to form aldehydes (if you're careful) or carboxylic acids (if you use excess oxidising agent). Secondary alcohols always produce ketones when oxidised. Tertiary alcohols are stubborn - they won't react at all with acidified dichromate.

Both aldehydes and ketones contain the carbonyl functional group $C=O$, but they differ in structure. Aldehydes have their carbonyl at the end of the chain (suffix -al), whilst ketones have theirs in the middle, bonded to two carbon atoms (suffix -one).

Quick Check: Remember that aldehydes can be oxidised further, but ketones cannot - this is key for identifying them!

Oxidising Aldehydes vs Ketones

Here's a brilliant way to tell aldehydes and ketones apart: try oxidising them! Aldehydes readily oxidise to carboxylic acids when you reflux them with acidified dichromate.

During this reaction, the orange dichromate ions (Cr₂O₇²⁻) get reduced to green Cr³⁺ ions. You'll see a clear colour change from orange to green, which makes this an excellent test.

Ketones stubbornly refuse to oxidise under these conditions. The solution stays orange because the dichromate isn't being used up. This difference in behaviour is your key to identifying which type of carbonyl compound you're dealing with.

Test Tip: If it goes green, it's an aldehyde. If it stays orange, it's a ketone!

Understanding Nucleophilic Addition

The C=O bond consists of both sigma (σ) and pi (π) bonds. The sigma bond forms from direct orbital overlap, whilst the pi bond comes from sideways overlap of p-orbitals.

Because oxygen is more electronegative than carbon, the C=O bond is polar. This creates a partially positive carbon $δ+$ and partially negative oxygen $δ-$, making the carbon attractive to nucleophiles.

Nucleophiles are electron pair donors that attack the electron-deficient carbon atom. This leads to heterolytic fission where the pi bond breaks unevenly, and both electrons move to the oxygen atom.

Remember: Nucleophiles love positive charges - they're drawn to that δ+ carbon like a magnet!

Reduction with Sodium Borohydride

Sodium borohydride (NaBH₄) is your go-to reducing agent for carbonyl compounds. It works brilliantly in water and converts both aldehydes and ketones back into alcohols.

When you reduce an aldehyde, you get a primary alcohol. The reaction essentially reverses the oxidation process you learned about earlier.

Ketones reduce to form secondary alcohols. This makes perfect sense when you think about it - you're just adding hydrogen back to the carbonyl group.

This reduction reaction is super useful in organic synthesis because it allows you to control the oxidation state of your molecules precisely.

Synthesis Tip: NaBH₄ is much safer and easier to handle than other reducing agents like LiAlH₄!

Nucleophilic Addition Mechanisms

The nucleophilic addition mechanism with NaBH₄ follows a clear pattern. The hydride ion (H⁻) attacks the partially positive carbon, forming a new C-H bond whilst the pi electrons move to oxygen.

Hydrogen cyanide (HCN) also undergoes nucleophilic addition with carbonyl compounds. You generate HCN safely in situ by mixing NaCN with H₂SO₄ - never handle pure HCN as it's extremely poisonous.

The product of HCN addition is a hydroxynitrile (like 2-hydroxybutanenitrile). This reaction is particularly valuable because it extends the carbon chain length by one carbon atom.

Safety Warning: Always generate HCN in a fume cupboard - it's one of the most dangerous chemicals you'll encounter in A-level chemistry!

The HCN Addition Mechanism

The cyanide ion (CN⁻) acts as the nucleophile in this reaction, attacking the electron-deficient carbon of the carbonyl group. Meanwhile, a hydrogen ion (H⁺) from the acid adds to the oxygen.

This mechanism creates a hydroxynitrile product with both an -OH group and a -CN group attached to the same carbon atom. The reaction works with both aldehydes and ketones.

The beauty of this reaction lies in its ability to build longer carbon chains, making it invaluable for organic synthesis. You're essentially adding a carbon atom that can be modified further in subsequent reactions.

Mechanism Tip: Draw the curly arrows carefully - CN⁻ attacks carbon, whilst H⁺ goes to the oxygen!