Quick Summary
- Carbonyl group is C=O (carbon double-bonded to oxygen)
- Two main types: aldehydes (alkanals) and ketones (alkanones)
- Aldehydes have -CHO group at the end of carbon chain
- Ketones have C=O group in the middle of carbon chain
- Different chemical properties allow us to distinguish between them
What is the Carbonyl Group?
The carbonyl group is one of the most important functional groups in organic chemistry. It consists of a carbon atom that shares a double bond with an oxygen atom. This double bond (C=O) gives carbonyl compounds their special properties.
Think of the carbonyl group like the engine of a car. Just as the engine determines how a car behaves, the carbonyl group determines how these organic compounds react with other substances.
Types of Carbonyl Compounds
There are two main types of carbonyl compounds you need to know for your WAEC and NECO exams:
1. Aldehydes (Alkanals)
In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom. The functional group is written as -CHO. This group is always found at the end of a carbon chain. You cannot have an aldehyde group in the middle of a chain.
Common examples of aldehydes include:
- Methanal (Formaldehyde): HCHO – used to preserve biological specimens in your school lab
- Ethanal (Acetaldehyde): CH₃CHO – found in ripe fruits
- Propanal: CH₃CH₂CHO – has a pungent smell
2. Ketones (Alkanones)
In ketones, the carbonyl carbon is bonded to two other carbon atoms. The carbonyl group is written as C=O and appears in the middle of the carbon chain, never at the end.
Common examples of ketones include:
- Propanone (Acetone): CH₃COCH₃ – used as nail polish remover
- Butanone: CH₃COC₂H₅ – used as a solvent in industries
- Pentan-2-one: CH₃COCH₂CH₂CH₃ – found in some fruits
Comparing Aldehydes and Ketones
| Property | Aldehydes (Alkanals) | Ketones (Alkanones) |
|---|---|---|
| Functional Group | -CHO (at the end of chain) | C=O (in the middle of chain) |
| Structure | R-CHO (R = hydrogen or alkyl group) | R-CO-R’ (R and R’ = alkyl groups) |
| Naming | Replace -e with -al (e.g., methane → methanal) | Replace -e with -one (e.g., propane → propanone) |
| Oxidation | Easily oxidized to carboxylic acids | Resistant to oxidation |
| Reducing Agent Test | Reduces Fehling’s solution (positive test) | Does not reduce Fehling’s solution (negative test) |
| Tollen’s Test | Produces silver mirror (positive test) | No reaction (negative test) |
| Example | Ethanal (CH₃CHO) | Propanone (CH₃COCH₃) |
Physical Properties of Carbonyl Compounds
Boiling Points
Aldehydes and ketones have higher boiling points than alkanes of similar molecular mass. This is because the C=O bond is polar. The oxygen atom pulls electrons towards itself, creating a partial negative charge on oxygen and a partial positive charge on carbon.
However, their boiling points are lower than alcohols of similar mass. Why? Alcohols can form hydrogen bonds with each other, but aldehydes and ketones cannot form hydrogen bonds among themselves.
Solubility
Small carbonyl compounds (like methanal, ethanal, and propanone) dissolve well in water. This is because the oxygen atom can form hydrogen bonds with water molecules. As the carbon chain gets longer, the compounds become less soluble in water.
Smell
Aldehydes typically have pungent, sharp smells. Ketones usually have pleasant, sweet smells. This is why propanone (acetone) is used in nail polish remover – it has a less offensive smell than many aldehydes.
Chemical Properties and Tests
Oxidation
This is the most important difference between aldehydes and ketones for your exams:
Aldehydes are easily oxidized. When you add an oxidizing agent like acidified potassium dichromate (K₂Cr₂O₇), aldehydes are oxidized to carboxylic acids. The orange dichromate solution turns green.
Example: Ethanal → Ethanoic acid
CH₃CHO + [O] → CH₃COOH
Ketones resist oxidation. Under normal laboratory conditions, ketones do not react with oxidizing agents. This is because the carbonyl carbon in a ketone is bonded to two carbon atoms, making it more stable.
Fehling’s Test
This test helps you distinguish aldehydes from ketones in the laboratory.
Procedure: Add Fehling’s solution (a blue solution containing copper ions) to the carbonyl compound and heat gently.
Result:
- Aldehydes: The blue solution turns into a red-brown precipitate of copper(I) oxide (Cu₂O). This is a positive test.
- Ketones: No change occurs. The solution remains blue. This is a negative test.
Example: If you test propanal (an aldehyde), you will see the red-brown precipitate. If you test propanone (a ketone), nothing happens.
Tollen’s Test (Silver Mirror Test)
This is another test for aldehydes that often appears in WAEC practical exams.
Procedure: Add Tollen’s reagent (a colourless solution containing silver ions) to the carbonyl compound and warm gently.
Result:
- Aldehydes: A shiny silver mirror forms on the inside of the test tube. The aldehyde reduces silver ions (Ag⁺) to metallic silver (Ag).
- Ketones: No silver mirror forms. No reaction occurs.
Addition Reactions
Both aldehydes and ketones undergo addition reactions across the C=O double bond. The double bond can “open up” to add new atoms.
For example, when hydrogen gas is added in the presence of a catalyst (like nickel), aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.
Ethanal + H₂ → Ethanol (primary alcohol)
CH₃CHO + H₂ → CH₃CH₂OH
Propanone + H₂ → Propan-2-ol (secondary alcohol)
CH₃COCH₃ + H₂ → CH₃CH(OH)CH₃
Common Exam Mistakes
Mistakes Students Make:
1. Confusing the positions of functional groups
Many students write ketone structures with the C=O group at the end of the chain. Remember: aldehydes have -CHO at the end, ketones have C=O in the middle.
2. Wrong test results
Students often say ketones give a positive Fehling’s test. Remember: Only aldehydes reduce Fehling’s solution. Ketones do not react.
3. Incorrect naming
Some students write “propan-1-one” for propanone. This is wrong because you don’t need to number the position when there’s only one possible location for the ketone group in a three-carbon chain.
4. Oxidation products
Students forget that aldehydes are oxidized to carboxylic acids, not to ketones. Ethanal becomes ethanoic acid, not a ketone.
5. Mixing up reagents
Don’t confuse Fehling’s solution with Tollen’s reagent. Fehling’s solution is blue; Tollen’s reagent is colourless.
Uses of Carbonyl Compounds
Aldehydes
- Methanal (Formaldehyde): Used to preserve specimens in biology labs and to make plastics
- Ethanal: Used in the manufacture of ethanoic acid and other chemicals
- Vanillin: An aldehyde used as flavouring in foods and perfumes
Ketones
- Propanone (Acetone): Used as a solvent in nail polish remover and paint thinners
- Butanone: Used as a solvent in industries and for making plastics
- Various ketones: Used in perfumes and food flavourings
Practice Questions
Multiple Choice Questions
1. Which of the following is the functional group of an aldehyde?
a) -OH
b) -CHO ✓
c) -COOH
d) C=O (in the middle of chain)
2. What is the product when ethanal is oxidized?
a) Ethanol
b) Propanone
c) Ethanoic acid ✓
d) Methanoic acid
3. Which test produces a silver mirror with aldehydes?
a) Fehling’s test
b) Tollen’s test ✓
c) Benedict’s test
d) Iodine test
4. What colour change occurs when an aldehyde reacts with Fehling’s solution?
a) Blue to green
b) Blue to red-brown ✓
c) Colourless to blue
d) Orange to green
Theory Questions
1. (a) Define the carbonyl group. (2 marks)
(b) State three differences between aldehydes and ketones. (6 marks)
(c) Describe a chemical test to distinguish between ethanal and propanone. (4 marks)
Examiner’s Tip: For part (c), state the reagent, procedure, and observations for both compounds. Don’t just name the test – explain what you would see.
2. (a) Write the structural formula of: (i) propanal (ii) butanone (4 marks)
(b) What is the product when propanal is oxidized? Write the equation. (3 marks)
(c) Explain why aldehydes have higher boiling points than alkanes of similar molecular mass. (3 marks)
Examiner’s Tip: Use structural formulas, not just molecular formulas. Show all bonds clearly. For equations, balance them and show conditions if any.
3. An unknown organic compound X with molecular formula C₃H₆O was tested in the laboratory. When Fehling’s solution was added and heated, a red-brown precipitate formed.
(a) Identify compound X. (2 marks)
(b) Write the structural formula of X. (2 marks)
(c) What type of organic compound is X? (1 mark)
(d) Write the equation for the reaction of X with acidified potassium dichromate. (3 marks)
(e) Name the product formed. (2 marks)
Examiner’s Tip: The positive Fehling’s test tells you it’s an aldehyde. With C₃H₆O, it must be propanal. Show your reasoning clearly.
Memory Aids
Mnemonics to Remember:
“ALCO Ends Here” – ALdehydes have C=O at the END
“KETO in Middle” – KETones have C=O in the MIDDLE
“Aldehydes Are Oxidized” – Remember aldehydes are easily oxidized (both start with A)
“Fehling Favours Aldehydes” – Fehling’s test is positive for aldehydes (both start with F/A sounds)
“Silver Sees Aldehydes” – Tollen’s test (silver mirror) works with aldehydes (both start with S/A)
Quick Comparison Chart:
Aldehydes: -CHO, End position, Oxidized easily, Positive tests
Ketones: C=O, Middle position, Not oxidized, Negative tests
Related Topics
- Alkanoic Acids (Carboxylic Acids) – the oxidation products of aldehydes
- Alkanols (Alcohols) – can be oxidized to form aldehydes and ketones
- Alkanoates (Esters) – formed from carboxylic acids and alcohols
- Functional Groups in Organic Chemistry
- Oxidation and Reduction Reactions