Organic Chemistry: Carbonyls

Revise Organic Chemistry: Carbonyls for Chemistry 4 (IAL) WCH14 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Chemistry — Organic Chemistry

Carbonyls: Aldehydes & Ketones

The big idea: Carbonyls (aldehydes and ketones) all share the same reactive C=O group — but a tiny structural difference (an H atom vs. two alkyl groups) changes everything about how they behave, and five classic tests let you tell them apart.

Summary — What This Chapter Covers

Aldehydes have the carbonyl group at the end of the chain (bonded to at least one H); ketones have it in the middle (bonded to two alkyl groups).
Both have a polarised C=O bond (δ+ carbon, δ− oxygen) — this drives almost everything they do.
Carbonyls can't hydrogen bond with each other (no O–H or N–H), so their boiling points sit below the equivalent alcohol — but small ones dissolve in water via hydrogen bonding with water molecules.
Aldehydes are easily oxidised to carboxylic acids; ketones resist oxidation. This difference is the basis of three classic distinguishing tests: acidified K₂Cr₂O₇, Tollens' reagent, and Fehling's solution.
Both are reduced by LiAlH₄: aldehydes → primary alcohols, ketones → secondary alcohols.
Both undergo nucleophilic addition with HCN, extending the carbon chain by one and forming hydroxynitriles.
2,4-DNPH detects any carbonyl (orange precipitate); the iodoform test specifically detects methyl ketones (and ethanal) with a yellow precipitate.

1. What Are Carbonyls?

Structure

The C=O group — and why position matters

Aldehydes and ketones are both built around the carbonyl functional group, C=O. That's the shared feature. The difference between them comes down to where that carbonyl sits on the carbon chain and what it's attached to.

Think of it like this: the carbonyl carbon needs two more bonds besides the double bond to oxygen. In an aldehyde, one of those two remaining bonds is always to a hydrogen atom (R–CHO). Because of this, the carbonyl carbon can never be "buried" in the middle of a chain — it's always forced to be at the very end, which is why it's always carbon number 1 and you never need to write the "1" in the name (e.g. it's "propanal," not "propan-1-al").

In a ketone, both of the remaining bonds go to alkyl (R) groups — there's no carbonyl-hydrogen. That means the carbonyl carbon needs a carbon chain on both sides, so it's physically impossible for it to sit at the end. It's always somewhere in the middle.

Aldehyde R–CHO (H always attached to the carbonyl carbon — simplest example: methanal, HCHO)

Ketone R–CO–R′ (two alkyl groups either side — simplest example: propan-2-one / acetone, CH₃COCH₃)

Quick reference table

Structural formula	Name	Molecular formula	Type
HCHO	Methanal (formaldehyde)	CH₂O	Aldehyde
CH₃CHO	Ethanal	C₂H₄O	Aldehyde
CH₃CH₂CHO	Propanal	C₃H₆O	Aldehyde
CH₃COCH₃	Propanone (acetone)	C₃H₆O	Ketone
CH₃COCH₂CH₂CH₃	Pentan-2-one	C₅H₁₀O	Ketone

Notice

Propanal and propanone have the same molecular formula (C₃H₆O)! This is exactly why the distinguishing tests later in this chapter matter so much — formula alone won't tell you which one you've got.

Practice Question 1

Explain, in terms of bonding, why the carbonyl group in a ketone can never be at the end of a carbon chain.

Practice Question 2

2. Polarity, Boiling Points & Solubility

Intermolecular Forces