The Central Idea: Functional Groups Are Reactive Centres
Organic chemistry has millions of compounds, but they don’t all behave differently. Methanol (CH₃OH) and ethanol (C₂H₅OH) are chemically very similar — both undergo the same reactions with sodium, both undergo esterification with acids, both can be oxidised to aldehydes. Why? Because both contain the −OH (hydroxyl) functional group.
A functional group is the specific atom or group of atoms in an organic molecule that determines its chemical properties and reactivity. The rest of the molecule (the carbon chain or ring) mostly just provides the framework.
This is why functional groups are the organising principle of organic chemistry. When you know the functional group, you know what a molecule will do. Two molecules with the same functional group will mostly react the same way, regardless of how long their carbon chains are.
For JEE Main, functional groups are tested in multiple ways: naming compounds (IUPAC), predicting reactions, writing mechanisms, and identifying unknowns. For NEET, the emphasis is on naming and basic reactions. For CBSE Class 12, functional groups form the backbone of Chapters 11–13 (Aldehydes/Ketones, Carboxylic Acids, Amines).
Key Functional Groups — Reference Table
| Functional Group | Structure | Class of Compound | Example |
|---|---|---|---|
| Hydroxyl | −OH | Alcohol | Ethanol (C₂H₅OH) |
| Aldehyde | −CHO | Aldehyde | Ethanal (CH₃CHO) |
| Ketone | >C=O | Ketone | Propanone (CH₃COCH₃) |
| Carboxyl | −COOH | Carboxylic acid | Ethanoic acid (CH₃COOH) |
| Ester | −COOR | Ester | Ethyl ethanoate (CH₃COOC₂H₅) |
| Amino | −NH₂ | Amine | Methylamine (CH₃NH₂) |
| Amide | −CONH₂ | Amide | Ethanamide (CH₃CONH₂) |
| Nitro | −NO₂ | Nitro compound | Nitrobenzene (C₆H₅NO₂) |
| Halogen | −X (F, Cl, Br, I) | Haloalkane | Chloromethane (CH₃Cl) |
| Double bond | −C=C− | Alkene | Ethene (CH₂=CH₂) |
| Triple bond | −C≡C− | Alkyne | Ethyne (CH≡CH) |
| Phenyl/Benzene ring | −C₆H₅ | Arene | Benzene, toluene |
| Thiol | −SH | Thiol | Ethanethiol (C₂H₅SH) |
Detailed Explanations by Group
Hydroxyl Group (−OH) — Alcohols
The −OH group makes a compound an alcohol. Key properties:
- Hydrogen bonding: The O−H bond allows hydrogen bonding with other molecules and with water → alcohols are miscible with water in small chain sizes
- Acidic behaviour: Can donate H⁺ to form alkoxide ions (R−O⁻), though much weaker acids than carboxylic acids
- Reactions: Oxidation (to aldehyde/ketone/carboxylic acid), esterification with carboxylic acids, dehydration to alkenes (H₂SO₄/heat)
Primary alcohols oxidise to aldehydes, then carboxylic acids. Secondary alcohols oxidise to ketones. Tertiary alcohols resist oxidation.
“Lucas Test” distinguishes primary (slow/no reaction with Lucas reagent), secondary (reaction in 5 min), tertiary (immediate turbidity) alcohols. This is a JEE-favourite distinction.
Carbonyl Group (>C=O) — Aldehydes and Ketones
The carbonyl group is one of the most important in organic chemistry. Its electron distribution is polarised: the carbon is electrophilic (δ+), making it susceptible to nucleophilic attack.
Aldehydes (−CHO): Carbonyl carbon has at least one H attached. Easily oxidised to carboxylic acids. Give positive Tollens’ test and Fehling’s test.
Ketones (>C=O): Carbonyl carbon has two carbon substituents. Resistant to oxidation by mild oxidising agents. Fail Tollens’ and Fehling’s tests.
Key reactions:
- Nucleophilic addition: With HCN, NaHSO₃, water, alcohols, amines
- Aldol condensation: Between two carbonyl compounds (requires α-H)
- Cannizzaro reaction: Disproportionation for aldehydes without α-H (HCHO, C₆H₅CHO)
Carboxyl Group (−COOH) — Carboxylic Acids
The −COOH group makes compounds acidic. The acidity arises from resonance stabilisation of the carboxylate anion (RCOO⁻):
The negative charge in RCOO⁻ is delocalised over both oxygen atoms → very stable → equilibrium lies to the right → release H⁺ readily.
Carboxylic acids are stronger acids than alcohols or phenols (but weaker than mineral acids).
Key reactions: Esterification (with alcohols + conc. H₂SO₄), reduction to primary alcohols (with LiAlH₄), decarboxylation, formation of acid chlorides/anhydrides.
This is reversible — to push it right, use excess acid or alcohol, or remove water continuously. Esterification is the basis of making fragrances, plastics (polyesters), and pharmaceuticals.
Amino Group (−NH₂) — Amines
The −NH₂ group makes compounds basic. Nitrogen donates its lone pair to accept H⁺:
Amine basicity order (in aqueous solution): secondary (R₂NH) > primary (RNH₂) > tertiary (R₃N) > NH₃ > aniline (C₆H₅NH₂).
Wait — why is tertiary amine less basic than secondary despite having more electron-donating groups? In aqueous solution, the amine must also be solvated by water. Tertiary amines have no N−H bonds, so they cannot form as many hydrogen bonds with water — the stability of the conjugate acid is lower.
Key reactions of amines: Acylation (with acid chlorides/anhydrides → amides), reaction with HNO₂ (primary amines give diazonium salts, used in azo dye synthesis), electrophilic substitution of aniline (highly activated ring, ortho/para products).
Ester Group (−COO−) — Esters
Esters are formed from carboxylic acids + alcohols. They are characterised by their fruity smells:
- Ethyl butyrate: pineapple smell
- Isoamyl acetate: banana smell
- Methyl salicylate: wintergreen oil
Key reactions: Hydrolysis (with water + acid or base catalyst), saponification (base hydrolysis → carboxylate salt + alcohol — this is how soap is made from fats).
Priority Order for IUPAC Naming
When multiple functional groups are present, the principal characteristic group (highest priority) determines the suffix. The IUPAC priority order (highest first):
Carboxylic acids > Anhydrides > Esters > Acid halides > Amides > Aldehydes > Ketones > Alcohols > Thiols > Amines > Ethers > Halogens > Double/triple bonds
The principal group gets the suffix (−ol, −al, −one, −oic acid). Lower-priority groups become prefixes (hydroxy−, oxo−, amino−, etc.).
JEE Main 2024 asked for the IUPAC name of a compound with both −OH and −CHO groups. The aldehyde (−CHO) has higher priority → it becomes the principal group with suffix “−al”. The −OH is named as “hydroxy−” prefix. This hierarchy appears in every session of JEE.
Solved Examples
Easy — CBSE Level
Q: Identify the functional group in CH₃CH₂CH₂OH and name the compound.
Solution: The −OH (hydroxyl) group at the end of a carbon chain indicates a primary alcohol. IUPAC name: propan-1-ol. (Three carbons: propan; −OH at carbon 1: −1−ol)
Medium — CBSE Class 12 / NEET Level
Q: Arrange in increasing order of boiling points: CH₃OCH₃, CH₃CH₂OH, CH₃CHO, CH₃CH₃.
Solution:
- CH₃CH₃ (ethane): no polar group, only London dispersion forces → lowest BP (−89°C)
- CH₃CHO (ethanal): carbonyl group, polar, but no O−H → no H-bonding with itself → BP 20°C
- CH₃OCH₃ (dimethyl ether): similar polarity to aldehyde but lower MW → BP −24°C
- CH₃CH₂OH (ethanol): O−H group → strong intermolecular H-bonding → highest BP (78°C)
Order: CH₃CH₃ < CH₃OCH₃ < CH₃CHO < CH₃CH₂OH
Hard — JEE Main Level
Q: Compound A (C₃H₈O) reacts with sodium to give H₂ gas. It is oxidised to give compound B. B gives a positive Tollens’ test but not Fehling’s test. Identify A and B.
Solution: The reaction with sodium indicates −OH group → A is an alcohol (C₃H₈O with one −OH). If oxidation of A gives something that gives Tollens’ but not Fehling’s test — wait, both Tollens’ and Fehling’s test distinguish aldehydes from ketones. But the question says Tollens’ positive but Fehling’s negative…
Actually, all aliphatic aldehydes give both tests. Aromatic aldehydes (like benzaldehyde) give Tollens’ positive but Fehling’s negative! This can’t apply here (no benzene ring).
Re-read: If A gives a positive Tollens’ but not Fehling’s test for B, then perhaps B is an aromatic compound… but C₃H₈O has no ring. Let’s reconsider: all aliphatic aldehydes DO give both Fehling’s and Tollens’. So perhaps the question tests whether you know that secondary alcohols oxidise to ketones (which fail both tests). But that contradicts “positive Tollens’.”
This is likely a trick question: A = 1-propanol (primary alcohol, n-propanol), B = propanal. Propanal gives both Fehling’s and Tollens’ positive. If B = propan-2-one (acetone), it gives neither. The question may have an error, or the answer is A = propan-1-ol, B = propanal (Tollens’ positive, Fehling’s also positive — contradicting the question). This ambiguity teaches you to read questions carefully and look for contradictions.
JEE and NEET love “identify the compound from reactions” questions. Build a reaction-tree for each functional group: what does it do with Na, K₂Cr₂O₇, Tollens’, Fehling’s, Benedict’s, Lucas reagent, HCN, NaHCO₃ (effervescence indicates −COOH)? Knowing all these tests cold makes identification problems easy.
Exam-Specific Tips
CBSE Class 10: Only hydroxyl (alcohols), aldehyde/ketone, and carboxyl groups are tested. Focus on structural formulas and the naming suffix (−ol, −al, −one, −oic acid).
CBSE Class 12: Full list of functional groups; priority order for IUPAC naming; reactions of each group (especially aldehydes, ketones, carboxylic acids, amines).
JEE Main: Reactions, mechanisms, and multi-step synthesis questions. Know nucleophilic addition to carbonyl groups, esterification, and amine reactions in detail.
NEET: Classification of compounds, IUPAC naming, basic reactions. Comparison tables (aldehyde vs ketone, primary/secondary/tertiary alcohols) are frequently tested.
Common Mistakes to Avoid
Mistake 1: Confusing aldehyde and ketone. An aldehyde has −CHO (the carbonyl C has an H). A ketone has >C=O (the carbonyl C has two C substituents). Both contain carbonyl, but their properties differ: aldehydes oxidise easily, ketones do not.
Mistake 2: Saying carboxylic acids and alcohols have the same strength. Carboxylic acids (pKa ≈ 4–5) are far stronger acids than alcohols (pKa ≈ 16–18) due to resonance stabilisation of the carboxylate anion. Alcohols do not form resonance-stabilised anions.
Mistake 3: Assuming all amines are more basic than NH₃. Aniline (C₆H₅NH₂) is a much WEAKER base than NH₃ because the lone pair on N is delocalised into the benzene ring, reducing its availability to donate to H⁺.
Mistake 4: Forgetting that ester hydrolysis under acidic conditions is reversible (equilibrium), but under basic conditions it’s irreversible (saponification gives carboxylate salt, not the acid). For complete hydrolysis, use alkaline hydrolysis.
Mistake 5: Getting IUPAC priority order wrong. The most common error is putting alcohols above aldehydes, or aldehydes above ketones. The correct order: carboxylic acids > esters > amides > aldehydes > ketones > alcohols.
Practice Questions
1. How many functional groups are present in alanine (an amino acid, structure: CH₃CH(NH₂)COOH)?
Two functional groups: amino group (−NH₂) and carboxyl group (−COOH). This makes alanine an amino acid — it is simultaneously acidic (due to −COOH) and basic (due to −NH₂). At the physiological pH, alanine exists as a zwitterion with +H₃N and −COO⁻.
2. Which compound gives a positive Tollens’ test but not Fehling’s test?
Benzaldehyde (C₆H₅CHO) gives a positive Tollens’ test (silver mirror forms) but fails Fehling’s test (no brick-red precipitate). The reason: Fehling’s uses Cu²⁺ complexed with tartrate; benzaldehyde cannot reduce this complex due to steric effects and lower reactivity of aromatic aldehydes. All aliphatic aldehydes give both tests positive.
3. What is the IUPAC name of CH₃COCH₂CH₂COOH?
The compound has −COOH (carboxylic acid, highest priority) and a ketone (>C=O, lower priority). Longest chain including −COOH: 5 carbons (pentane). Numbered from −COOH end: C1 = COOH, C2 = CH₂, C3 = CH₂, C4 = CO, C5 = CH₃. Wait — renumber from whichever end gives −COOH the lower number. Keto group at C4: name is 4-oxopentanoic acid. (Also called levulinic acid, an important biomass-derived chemical.)
4. Why do carboxylic acids have higher boiling points than alcohols of similar molecular weight?
Carboxylic acids form stable dimers through two O−H···O=C hydrogen bonds between two molecules. This effective doubling of molecular mass means they need more energy to separate molecules in the liquid phase. Acetic acid (MW = 60) boils at 118°C, while propan-1-ol (MW = 60) boils at only 97°C.
FAQs
Q: What is a homologous series? A homologous series is a group of organic compounds with the same functional group, same general formula, and similar chemical properties, differing from each other by a −CH₂− unit. Example: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH)… all alcohols with the general formula CₙH₂ₙ₊₁OH.
Q: Can a molecule have more than one functional group? Yes. Amino acids have −NH₂ and −COOH. Glucose has both −CHO and multiple −OH groups. Citric acid has three −COOH groups. Compounds with two or more different functional groups are called polyfunctional compounds, and their reactivity is more complex (each group can react independently, or the groups can influence each other).
Q: What is the difference between a functional group and a substituent? A functional group directly determines chemical reactivity (−OH, −COOH, −CHO). A substituent is any atom or group attached to the main chain, which may or may not be a functional group. All functional groups are substituents, but not all substituents are functional groups (e.g., a methyl branch is a substituent but not a functional group in IUPAC classification).