Functional-Group Organic Chemistry and Biomolecules — NEET Chemistry

This is the destination of the organic-chemistry logic you built in the last chapter — and NEET's single highest-yield organic block, 5–6 questions every year. Functional groups are where reactions live: each group has a signature reactivity, a set of named reactions and a diagnostic test. Biomolecules then apply all of it to the molecules of life — the carbohydrates, proteins and nucleic acids a future doctor works with daily. This chapter reasons through each functional group's key reactions and the structure of every biomolecule class, drilling the mechanisms, trends and tests the exam repeats.

Part A — Functional-Group Organic Chemistry

1. Haloalkanes and the substitution mechanisms

Haloalkanes (R–X) undergo nucleophilic substitution — a nucleophile replaces the halide. Two competing mechanisms:

So 3° halides react by SN1, primary/methyl by SN2, and secondary can do either. Reactivity order (C–X bond strength): R–I > R–Br > R–Cl > R–F (weaker bond breaks more easily).

Haloarenes (halogen on a benzene ring) are far less reactive to substitution — the C–X bond has partial double-bond character (resonance) and the ring repels nucleophiles.

Worked example 1.1. Which reacts faster by SN2, CH₃Br or (CH₃)₃CBr, and why? CH₃Br. SN2 needs the nucleophile to attack the carbon from behind; the bulky three methyl groups of the tertiary halide block that approach (steric hindrance). Methyl bromide is unhindered, so it reacts fastest by SN2 (the tertiary halide instead prefers SN1).

2. Alcohols, phenols and ethers

Acidity comparison (a NEET staple) — carboxylic acid > phenol > water > alcohol:

• Phenol is more acidic than alcohol because the phenoxide ion is resonance-stabilised (the negative charge delocalises into the ring); an alkoxide has no such stabilisation. Electron-withdrawing groups (–NO₂) on the ring increase phenol acidity (p-nitrophenol > phenol); electron-donating groups decrease it.
• Carboxylic acid is more acidic than phenol because the carboxylate ion spreads the charge over two equivalent oxygens — more effective delocalisation than phenoxide.

Key reactions: alcohols undergo dehydration (conc. H₂SO₄ → alkene), oxidation (1° → aldehyde → acid; 2° → ketone; 3° resists). Lucas test (conc. HCl + ZnCl₂) distinguishes 1°/2°/3° alcohols by turbidity speed (3° immediate). Ethers (R–O–R′) are relatively unreactive; cleaved by HI.

Worked example 2.1. Why is phenol acidic while ethanol is essentially neutral? Losing H⁺, phenol gives the phenoxide ion whose negative charge is delocalised over the aromatic ring by resonance — a stable anion, so the equilibrium releases the proton. Ethanol gives ethoxide, with the charge localised on one oxygen and no resonance, so it barely ionises.

3. Aldehydes and ketones

The carbonyl (C=O) is polar (δ+ carbon), so aldehydes and ketones undergo nucleophilic addition. Aldehydes are more reactive than ketones (less steric hindrance and less +I electron donation to the carbonyl carbon).

Named reactions:

• Aldol condensation — carbonyls with α-hydrogens react under dilute base to give β-hydroxy carbonyls, then α,β-unsaturated products.
• Cannizzaro reaction — carbonyls with no α-hydrogen (e.g. HCHO, benzaldehyde) disproportionate under conc. base into an alcohol + a carboxylate.

Diagnostic tests (identify the group):

• Tollens' test (ammoniacal AgNO₃) — silver mirror with aldehydes only (not ketones).
• Fehling's / Benedict's — red Cu₂O precipitate with aliphatic aldehydes (not aromatic, not ketones).
• Iodoform test — yellow CHI₃ with methyl ketones and ethanol/acetaldehyde (CH₃CO– or CH₃CH(OH)– group).
• 2,4-DNP (Brady's reagent) — orange precipitate with any aldehyde or ketone.

Worked example 3.1. How would you chemically distinguish acetaldehyde (CH₃CHO) from acetone (CH₃COCH₃)? Tollens' test: acetaldehyde (an aldehyde) gives a silver mirror; acetone (a ketone) does not. Both would give a positive iodoform test (both have the CH₃CO– group), so Tollens' is the distinguishing test.

4. Carboxylic acids and their derivatives

Carboxylic acids (–COOH) are the most acidic common organic group because the carboxylate ion is resonance-stabilised over two oxygens. Electron-withdrawing groups increase acidity (Cl₃C–COOH ≫ CH₃COOH; the −I effect stabilises the anion), and closer groups have more effect.

Acid strength order: HCOOH > CH₃COOH (formic acid is stronger — no electron-donating alkyl group); and among substituted acids, more/closer −I groups → stronger.

Derivatives (acid chloride, anhydride, ester, amide) interconvert; esterification (acid + alcohol, H⁺ catalyst) is reversible (Fischer), while esters hydrolyse back (or saponify with base to soap).

Worked example 4.1. Arrange in increasing acid strength: CH₃COOH, ClCH₂COOH, Cl₂CHCOOH. More electron-withdrawing chlorines stabilise the carboxylate more, raising acidity: CH₃COOH < ClCH₂COOH < Cl₂CHCOOH.

5. Amines

Amines (R–NH₂) are basic — the nitrogen lone pair accepts a proton. Basicity depends on lone-pair availability, and NEET tests two subtly different orders:

• In the gas phase / by +I alone: 3° > 2° > 1° > NH₃ (more alkyl groups donate electron density to N).
• In aqueous solution (where solvation of the protonated ion also matters), the order for the common case is 2° > 1° > 3° > NH₃ — the bulky trialkyl ammonium ion is poorly solvated. This aqueous anomaly is a favourite question.
• Aromatic amines (aniline) are much weaker bases than aliphatic — the nitrogen lone pair is delocalised into the ring, so it is less available to bind a proton.

Distinguishing 1°/2°/3° amines: the carbylamine (isocyanide) test (R–NH₂ + CHCl₃ + KOH → foul-smelling isocyanide) is positive only for primary amines. Hinsberg's test (benzenesulphonyl chloride) separates all three.

Diazonium salts: primary aromatic amines + HNO₂ (NaNO₂/HCl, 0–5 °C) → arenediazonium salt (Ar–N₂⁺), a versatile intermediate that converts to phenols, haloarenes (Sandmeyer) and azo dyes (coupling).

Worked example 5.1. Why is aniline a weaker base than methylamine? In aniline the nitrogen lone pair is delocalised into the benzene ring by resonance, so it is less available to accept a proton. In methylamine the lone pair is fully available and even enhanced by the +I methyl group, making it a much stronger base.

Part B — Biomolecules

6. Carbohydrates

Carbohydrates (polyhydroxy aldehydes/ketones or their polymers) are classified by hydrolysis:

• Monosaccharides — cannot be hydrolysed further: glucose (an aldohexose, aldose) and fructose (a ketohexose, ketose). Glucose exists in ring (pyranose) and open forms; the ring introduces the anomeric carbon (α/β).
• Disaccharides — two units: sucrose (glucose + fructose; non-reducing — no free anomeric OH), maltose and lactose (reducing).
• Polysaccharides — many units: starch (energy store in plants; amylose + amylopectin), glycogen (animal store), cellulose (structural, β-1,4 links — indigestible to humans).

Reducing vs non-reducing sugars: reducing sugars have a free aldehyde/ketone (free anomeric carbon) and give positive Tollens'/Fehling's; sucrose is non-reducing.

Worked example 6.1. Why is sucrose a non-reducing sugar while glucose is reducing? In sucrose the anomeric carbons of both glucose and fructose are joined in the glycosidic bond, leaving no free aldehyde/ketone group to reduce Tollens'/Fehling's reagent. Glucose has a free anomeric carbon (open-chain aldehyde), so it is reducing.

7. Amino acids and proteins

Amino acids contain both –COOH and –NH₂ groups on the same carbon (α-amino acids). They are amphoteric and exist as the internal salt zwitterion (⁺H₃N–CHR–COO⁻) at the isoelectric point (the pH of zero net charge, minimum solubility). There are 20 standard amino acids; essential ones must come from the diet.

Proteins are polymers of amino acids joined by peptide (amide) bonds (–CO–NH–). Four structural levels:

• Primary — the sequence of amino acids.
• Secondary — local folding into α-helix or β-pleated sheet, held by hydrogen bonds.
• Tertiary — the overall 3-D shape (H-bonds, disulphide bridges, ionic and hydrophobic interactions).
• Quaternary — assembly of several polypeptide chains (e.g. haemoglobin's four subunits).

Denaturation (heat, acid, urea) disrupts secondary/tertiary structure — the primary sequence survives, but the protein loses function (a cooked egg white).

Worked example 7.1. What holds the α-helix of a protein together, and what happens on denaturation? The α-helix is held by hydrogen bonds between backbone C=O and N–H groups. Denaturation (heat, pH change) breaks these hydrogen bonds and disrupts the secondary and tertiary structure, so the protein unfolds and loses biological activity — although the primary (peptide-bond) sequence stays intact.

8. Enzymes, nucleic acids, vitamins and lipids

Enzymes are globular protein biocatalysts — highly specific, lowering the activation energy of biochemical reactions with a lock-and-key active site.

Nucleic acids (DNA, RNA) store and express genetic information. Each nucleotide = a nitrogenous base + pentose sugar + phosphate.

In DNA the bases pair A–T and G–C by hydrogen bonds (Chargaff/Watson–Crick), holding the two antiparallel strands in a double helix.

Vitamins: fat-soluble (A, D, E, K — stored in the body) and water-soluble (B-complex, C — excreted, needed regularly). Deficiencies are classic recall: C → scurvy, D → rickets, A → night blindness, B₁ → beri-beri, B₁₂ → pernicious anaemia.

Lipids (fats and oils) are esters of glycerol and fatty acids; oils are unsaturated (liquid), fats saturated (solid). They store energy and build cell membranes.

Worked example 8.1. Which base pairs with adenine in DNA, and by how many hydrogen bonds? Adenine pairs with thymine (A–T) by two hydrogen bonds; guanine pairs with cytosine (G–C) by three. (In RNA, adenine pairs with uracil instead of thymine.)

9. Common traps NEET sets here

• SN1 favours 3° (carbocation), SN2 favours 1°/methyl (steric); SN2 gives inversion, SN1 racemisation.
• Acidity: carboxylic acid > phenol > water > alcohol; −I/−M groups raise acidity, +I lower it.
• Aldehydes more reactive than ketones to nucleophilic addition; Tollens'/Fehling's positive for aldehydes only.
• Iodoform test = methyl ketone or CH₃CH(OH)– group (and ethanol/acetaldehyde).
• Amine basicity: aqueous order 2° > 1° > 3° > NH₃ (solvation); aniline much weaker (ring delocalisation).
• Carbylamine test = primary amines only.
• Sucrose is non-reducing; glucose, maltose, lactose are reducing.
• DNA: deoxyribose + thymine, double helix; RNA: ribose + uracil, single strand. A–T, G–C.
• Vitamin deficiencies: C scurvy, D rickets, A night blindness, B₁ beri-beri.

10. Memory aids

• "3° goes SN1, 1° goes SN2" — substitution mechanism by substitution level.
• "COOH > phenol > water > alcohol" — the acidity ladder.
• "Tollens' mirrors an aldehyde" — silver mirror test.
• "Iodoform loves a methyl ketone" — CH₃CO– yellow precipitate.
• "Aniline's lone pair is busy in the ring" — why aromatic amines are weak bases.
• "Sucrose keeps no free carbon" — why it is non-reducing.
• "A–T, G–C; DNA has T, RNA has U" — base pairing and nucleic-acid difference.
• "Scurvy-C, Rickets-D, Night-A, Beri-B₁" — vitamin-deficiency recall.

11. Exam protocol

1. Haloalkanes: choose SN1 (3°, carbocation, racemisation) vs SN2 (1°/methyl, inversion); reactivity R–I > R–Br > R–Cl.
2. Rank acidity (carboxylic > phenol > alcohol) using resonance and −I/−M substituent effects.
3. Aldehyde/ketone: nucleophilic addition (aldehyde more reactive); pick the right test (Tollens', Fehling's, iodoform, 2,4-DNP).
4. Amines: apply the aqueous basicity order and aniline's weakness; use carbylamine for primary amines; diazonium for aromatic-amine synthesis.
5. Carbohydrates: classify (mono/di/poly), and identify reducing vs non-reducing (free anomeric carbon).
6. Proteins: peptide bond; the four structure levels and what denaturation destroys; enzymes as protein catalysts.
7. Nucleic acids: nucleotide parts; DNA vs RNA differences and A–T/G–C pairing.
8. Vitamins/lipids: fat- vs water-soluble and the classic deficiency diseases.