Aldehydes, Ketones & Carboxylic Acids
Chemistry – SRMJEEE
(Beginner‑friendly, everyday analogies, short sentences, tiny examples)
1. What is a “carbonyl group”?
- The carbonyl is a central part of many organic molecules.
- It looks like a tiny “gate” made of a carbon atom (
C) double‑bonded to an oxygen atom (O). - Think of the gate as a sticky spot that attracts other parts of the molecule.
O
||
C … (C = carbon, O = oxygen)
2. Aldehydes
| Feature | What it means | Everyday analogy |
|---|---|---|
| C‑terminus | The carbonyl carbon is attached to at least one hydrogen (–H) |
Like a door that opens to air (H) |
| General formula | R–CHO | R = any carbon chain |
| Name | Add ‑al to the parent alkane name |
e.g., Ethane → Ethanal (ethanal = acetaldehyde) |
| Typical smell | Fruity, sweet | The smell of ripe apples (acetaldehyde) |
| Common source | Oxidation of alcohols | Glucose → Ethanal (in fruit) |
| Simple test | Fehling’s test – turns blue to red | A quick “red‑water” test |
Tiny Example
Acetaldehyde (ethanal)
- Formula:
CH₃–CHO - Found in fresh apples.
- When you cut an apple, you can smell it because of acetaldehyde.
3. Ketones
| Feature | What it means | Everyday analogy |
|---|---|---|
| C‑terminus | The carbonyl carbon is attached to two other carbons (no hydrogen) | Like a door that leads to two rooms |
| General formula | R–CO–R' | R and R' can be the same or different |
| Name | Add ‑one to the parent alkane name |
e.g., Propane → Propanone (acetone) |
| Typical smell | Strong, sharp | The smell of nail polish remover (acetone) |
| Common source | Dehydration of secondary alcohols | 2‑Propanol → Propanone |
| Simple test | Tollens’ test – turns silver mirror | A quick “mirror” test |
Tiny Example
Acetone (propanone)
- Formula:
CH₃–CO–CH₃ - Common solvent, smells like nail polish remover.
4. Carboxylic Acids
| Feature | What it means | Everyday analogy |
|---|---|---|
| C‑terminus | The carbonyl carbon is attached to a hydroxyl group (–OH) |
Like a door with a doorbell (OH) |
| General formula | R–COOH | R = any carbon chain |
| Name | Add ‑oic acid to the parent alkane name |
e.g., Ethane → Ethanoic acid (acetic acid) |
| Typical smell | Sour, pungent | The smell of vinegar (acetic acid) |
| Common source | Oxidation of aldehydes or alcohols | Ethanol → Acetic acid |
| Simple test | Litmus paper turns red | Red litmus = acidic |
Tiny Example
Acetic Acid (ethanoic acid)
- Formula:
CH₃–COOH - Found in vinegar.
- Gives a sour taste and smells sharp.
5. How are they made? (Short & simple)
| Starting material | Reaction | Product |
|---|---|---|
| Alcohol (primary) | Oxidation | Aldehyde → Carboxylic Acid |
| Alcohol (secondary) | Oxidation | Ketone |
| Aldehyde | Further oxidation | Carboxylic Acid |
- Oxidation = adding oxygen or removing hydrogen.
- In the lab, we use chemicals like
KMnO₄orCrO₃to do it.
6. Quick “What to remember” Cheat Sheet
| Class | Formula pattern | Common name ending | Typical smell | Simple test |
|---|---|---|---|---|
| Aldehyde | R–CHO | ‑al |
Fruity | Fehling’s (red) |
| Ketone | R–CO–R' | ‑one |
Sharp | Tollens’ (mirror) |
| Carboxylic Acid | R–COOH | ‑oic acid |
Sour | Litmus red |
7. Mini‑Quiz (to test yourself)
-
What is the common name of the acid found in vinegar?
Answer: Acetic acid (ethanoic acid). -
Which compound smells like nail polish remover?
Answer: Acetone (propanone). -
If you oxidise
2‑propanol, what do you get?
Answer: Propanone (acetone). -
Which test turns a blue solution to red?
Answer: Fehling’s test (for aldehydes).
8. Final Thought
- Think of the carbonyl group as a gate that can lead to three families: aldehydes (one side open), ketones (both sides closed), and carboxylic acids (doorbell added).
- All of them are everywhere: in fruit, in cleaning supplies, in food, and in the body.
- Knowing their simple names, smells, and tests helps you spot them in