Aldehydes, Ketones and Carboxylic Acids: Carbonyl vs Carboxyl Group
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
Aldehydes and ketones both contain the carbonyl group, which is a carbon double-bonded to oxygen (C=O). In an aldehyde the carbonyl carbon holds at least one H (-CHO); in a ketone it is bonded to two other carbons (>C=O). Carboxylic acids have the carboxyl group -COOH, which is a carbonyl (C=O) and a hydroxyl (-OH) joined on the same carbon. Memory hook: "CARBONYL = C=O only; CARBOXYL = C=O + O-H, so it is acidic."
The C=O carbonyl is shared by all three. An aldehyde adds one H (-CHO), a ketone adds two carbons (>C=O), and a carboxylic acid adds an -OH to make the carboxyl -COOH, which is why only it is acidic.
Your doubts, answered
What is the difference between the carbonyl group and the carboxyl group?
The carbonyl group is just one carbon double-bonded to one oxygen: C=O. It is found in aldehydes and ketones. The carboxyl group is a carbonyl (C=O) PLUS a hydroxyl (-OH) attached to the SAME carbon, written -COOH. So every carboxyl contains a carbonyl, but a carboxyl also has that extra O-H. This O-H is why carboxylic acids can release H+ and are acidic, while plain aldehydes and ketones are not acidic.
How do I tell an aldehyde from a ketone?
Look at the carbon of the C=O group. In an aldehyde, that carbon carries at least one hydrogen atom, so you see -CHO at the end of the chain (aldehydes are always at chain ends). In a ketone, the C=O carbon is bonded to two other carbon atoms, so the C=O sits in the MIDDLE of the chain (>C=O). Simple test: HCHO and CH3CHO are aldehydes (H on the carbonyl carbon); CH3COCH3 (acetone) is a ketone (carbonyl carbon flanked by two CH3).
What are the general formulas of aldehydes, ketones and carboxylic acids?
Aldehyde general form: R-CHO (R can be H or a carbon group). Ketone general form: R-CO-R' (both R and R' are carbon groups). Carboxylic acid general form: R-COOH. All three share the C=O bond, but only the carboxylic acid has the -OH stuck to the carbonyl carbon. The whole -CHO, >C=O and -COOH pieces are the 'functional groups' that decide the chemistry.
Is -CHO an aldehyde group and does it mean the same as C=O?
-CHO is the aldehyde group: it is a carbonyl (C=O) whose carbon also has one H. So -CHO does contain C=O, but C=O by itself is more general. A ketone also has C=O but no H on that carbon. Do not write a ketone as -CHO; write it as >C=O or R-CO-R'. Mixing these up is a common NEET slip.
Why is a carboxylic acid acidic but an aldehyde or ketone is not?
A carboxylic acid can lose the H from its O-H to give a carboxylate ion (R-COO-). This ion is stable because the negative charge spreads over two oxygen atoms (resonance). Aldehydes and ketones have no O-H group to lose, so they do not act as acids in the normal sense. This is why identifying the carboxyl group -COOH instantly tells you the compound is acidic.
⚠️ The NEET trap ✗ Students see the C=O and hastily call every product a 'ketone' or write -CHO for anything with a carbonyl. ✓ C=O alone does not fix the class. Check the neighbours of that carbon: one H present = aldehyde (-CHO); two carbons = ketone (>C=O); an -OH attached = carboxylic acid (-COOH). 🧠 Before naming a class, ask 'what is attached to the C=O carbon?' H, C, or OH decides everything.
Real NEET questions
NEET 2016 Phase 1
The product formed by the reaction of an aldehyde with a primary amine is:
A · Schiff base ✓
B · Ketone
C · Carboxylic acid
D · Aromatic acid
Solution: The aldehyde carbonyl (>C=O) reacts with a primary amine R-NH2 by nucleophilic addition, then loses water to give an imine (>C=N-R). An imine made from a carbonyl compound and a primary amine is called a Schiff base. This is a direct test of recognising the carbonyl group and how it behaves, so the answer is (A).
NEET 2022
Match List-I (products) with List-II (reagent the carbonyl reacts with): (a) Cyanohydrin (b) Acetal (c) Schiff's base (d) Oxime with (i) NH2OH (ii) RNH2 (iii) alcohol (iv) HCN
A · (a)-(iii),(b)-(iv),(c)-(ii),(d)-(i)
B · (a)-(ii),(b)-(iii),(c)-(iv),(d)-(i)
C · (a)-(i),(b)-(iii),(c)-(ii),(d)-(iv)
D · (a)-(iv),(b)-(iii),(c)-(ii),(d)-(i) ✓
Solution: All four products come from the SAME carbonyl group reacting with different partners. Carbonyl + HCN gives a cyanohydrin, so (a)-(iv). Carbonyl + alcohol gives an acetal, so (b)-(iii). Carbonyl + primary amine RNH2 gives a Schiff's base, so (c)-(ii). Carbonyl + hydroxylamine NH2OH gives an oxime, so (d)-(i). Correct match is (D).
NEET 2023 Phase 2
Consider the reaction: CH3COCH3 (acetone) --dil. Ba(OH)2--> X. The functional groups present in compound X are:
A · alcohol and aldehyde
B · alcohol and ketone ✓
C · ketone and double bond
D · double bond and aldehyde
Solution: Acetone (a ketone) undergoes a base-catalysed aldol addition to give diacetone alcohol, (CH3)2C(OH)-CH2-CO-CH3. The product still has the original ketone carbonyl (>C=O) and gains a new -OH (alcohol) group. So the two functional groups in X are an alcohol and a ketone, giving answer (B). This rewards knowing that a ketone keeps its carbonyl while gaining a hydroxyl.
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Do aldehydes, ketones and carboxylic acids all have a carbonyl group?
Yes. All three contain the C=O bond. Aldehydes and ketones have only the carbonyl. In carboxylic acids the carbonyl is part of the bigger carboxyl group -COOH, which also has an O-H attached to the same carbon.
Is the carboxyl group the same as two separate groups?
No, it is one combined group. The -COOH carbon carries both a C=O and an -OH at once. They interact with each other, so carboxyl chemistry is different from a plain alcohol plus a plain ketone.
Which is more reactive in nucleophilic addition, an aldehyde or a ketone?
An aldehyde. Its carbonyl carbon has one H (less crowding and less electron-donation), so nucleophiles reach it more easily. Ketones have two carbon groups that block and stabilise the carbonyl, making them less reactive.
How do I quickly spot each class in a NEET question?
Scan the C=O carbon. See -CHO or an H on it = aldehyde. See C=O between two carbons = ketone. See -COOH (C=O with O-H) = carboxylic acid. This one check answers most 'identify the product' questions fast.
