Organic Chemistry

Organic chemistry carries about 7 of 22 questions (our estimate). The official model question "in Williamson's synthesis the haloalkane is treated with ... sodium alkoxide" shows the style: the reagent, the product or the test for a named reaction. Naming, isomer counts and acidity / basicity orders fill the rest. The six sub-areas follow one logic: what is the functional group, what attacks it, and what is formed.

3.1 Introduction: principles, purification, nomenclature, isomerism, mechanism

Hybridisation and bonding: C in alkane sp³ (109.5°), alkene sp² (120°), alkyne sp (180°); a C–C triple bond is shortest; the % s-character (25, 33, 50) makes alkynes the most acidic hydrocarbons. Classification: acyclic / cyclic, aliphatic / aromatic, homologous series (differ by CH₂). Functional groups: –OH alcohol, –O– ether, –CHO aldehyde, >C=O ketone, –COOH acid, –COOR ester, –CONH₂ amide, –NH₂ amine, –NO₂ nitro, –X halo.

IUPAC rules: longest chain containing the principal functional group; lowest locants to the principal group, then to multiple bonds, then to substituents; alphabetical order. Priority: acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkyne > halide / nitro. CH₃CH(CH₃)CH₂CH₃ is 2-methylbutane; CH₃COCH₃ propanone; CH₃CH₂CHO propanal; (CH₃)₂CHOH propan-2-ol; CH₃CH=CHCH₃ but-2-ene.

Isomerism. Structural: chain, position, functional, metamerism (diethyl ether and methyl propyl ether), tautomerism. Number of structural isomers: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9; C₂H₆O 2 (ethanol, dimethyl ether); C₄H₈ (alkenes) 3 (+2 cycloalkanes). Stereo: geometrical (cis/trans, E/Z) and optical (a chiral C has four different groups; enantiomers rotate light equally and oppositely; a racemic mixture is inactive). Degree of unsaturation =\frac{2C+2+N-H-X}{2} (benzene C₆H₆: 4).

Electronic effects: inductive (–I: NO₂, CN, halogens, COOH; +I: alkyl); resonance (+R: OH, NH₂, OR; –R: NO₂, CHO, CN); hyperconjugation (explains the stability order of alkenes and carbocations: 3° > 2° > 1° > CH₃⁺). Reagents: electrophiles (H⁺, NO₂⁺, carbocations, AlCl₃) and nucleophiles (OH⁻, CN⁻, NH₃, H₂O); free radicals; carbanions. Reaction types: substitution (SN1, SN2), addition, elimination (E1, E2; Saytzeff: the more substituted alkene), rearrangement. SN1: two steps, carbocation, racemisation, 3° > 2° > 1°; SN2: one step, inversion (Walden), 1° > 2° > 3°.

Purification: crystallisation, sublimation (naphthalene), distillation (simple, fractional, steam, vacuum), chromatography (R_f = distance of solute / of solvent). Detection (Lassaigne): sodium extract: N as NaCN gives Prussian blue, S as Na₂S gives violet with nitroprusside and black PbS, halogen as AgX (white AgCl, pale-yellow AgBr, yellow AgI). Quantitative: Dumas (N), Kjeldahl (N), Carius (halogens and S).

3.2 Hydrocarbons

Alkanes C_nH_{2n+2} (sp³, σ only): preparation by hydrogenation, Wurtz (2RX + 2Na → R–R + 2NaX; 2CH₃Br → ethane), decarboxylation (soda lime), Kolbe electrolysis. Reactions: combustion; free-radical halogenation (initiation, propagation, termination; reactivity 3° > 2° > 1° H; Cl₂ less selective than Br₂); cracking and reforming. Boiling point rises with chain length and falls with branching.

Alkenes C_nH_{2n}: dehydration of alcohols, dehydrohalogenation (alc. KOH). Electrophilic addition: Markovnikov (H adds to the carbon with more H: propene + HBr → 2-bromopropane); anti-Markovnikov (Kharasch, with peroxides, HBr only); Br₂/CCl₄ decolourised (test of unsaturation); Baeyer's test (cold dil. alkaline KMnO₄ decolourised, diol formed); hydration; ozonolysis (O₃ then Zn/H₂O: C=C → two C=O; 2-methylpropene gives acetone and formaldehyde); hydrogenation; polymerisation (polythene).

Alkynes C_nH_{2n-2}: from CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂. Acidic terminal H: with ammoniacal AgNO₃ (white) / Cu₂Cl₂ (red) precipitates; hydration (Hg²⁺/H₂SO₄): ethyne → ethanal, propyne → propanone; Lindlar's catalyst gives cis-alkene, Na/liquid NH₃ gives trans; trimerisation of ethyne gives benzene.

Aromatic hydrocarbons. Benzene is planar, six π electrons, Hückel (4n+2)\pi electrons (benzene, naphthalene, the cyclopentadienyl anion; not cyclooctatetraene). It prefers electrophilic substitution: nitration (HNO₃/H₂SO₄, electrophile NO₂⁺), halogenation (Cl₂/FeCl₃), sulphonation, Friedel–Crafts alkylation and acylation (AlCl₃). Directing groups: –OH, –NH₂, –R, –X are ortho/para directors (halogens deactivate); –NO₂, –CN, –CHO, –COOH, –SO₃H are meta directors and deactivate; Friedel–Crafts fails on strongly deactivated rings. Toluene oxidises to benzoic acid.

3.3 Haloalkanes and haloarenes

Preparation: from alcohols (SOCl₂ best; HX/ZnCl₂), addition of HX, halogenation, Finkelstein (NaI/acetone). Reactions: nucleophilic substitution with aq. KOH (alcohol), alc. KOH (elimination to alkene), KCN (nitrile), NH₃ (amine), AgNO₂ (nitroalkane) vs KNO₂ (alkyl nitrite), NaOR (Williamson ether synthesis), Na (Wurtz), Mg in dry ether (Grignard reagent RMgX; with HCHO gives primary, other aldehydes secondary, ketones tertiary alcohols; with CO₂ gives acids; reacts with water to give an alkane). Reactivity R–I > R–Br > R–Cl; SN1: (CH₃)₃CBr > (CH₃)₂CHBr > CH₃CH₂Br > CH₃Br. Haloarenes are inert to SN (resonance, sp² C–X bond shorter) unless activated by ortho / para nitro groups; Sandmeyer and Fittig reactions; DDT, chloroform (CHCl₃, anaesthetic; stored in dark bottles because it forms phosgene), CCl₄, freons.

3.4 Alcohols, phenols and ethers

Alcohols. Preparation: hydration of alkenes, hydrolysis of RX, reduction of carbonyls (LiAlH₄, NaBH₄), Grignard addition. Lucas test (conc. HCl + anhydrous ZnCl₂): 3° turbid immediately, 2° in about 5 minutes, 1° only on heating. Reactions: with Na (H₂), esterification, dehydration (conc. H₂SO₄, 170 °C gives alkene; 140 °C gives ether), oxidation (1° → aldehyde (PCC) → acid; 2° → ketone; 3° resists), halide formation. Boiling points are high due to hydrogen bonding. Methanol is poisonous; ethanol from fermentation of sugar (zymase); denatured alcohol; iodoform test (I₂/NaOH, yellow CHI₃) is given by CH₃CH(OH)– and CH₃CO– groups: ethanol, propan-2-ol, ethanal, acetone — not methanol.

Phenols. More acidic than alcohols (phenoxide is resonance-stabilised) but weaker than carboxylic acids and H₂CO₃; electron-withdrawing groups raise acidity (p-nitrophenol > phenol > p-cresol > ethanol). Reactions: FeCl₃ gives a violet colour; bromine water gives white 2,4,6-tribromophenol; Reimer–Tiemann (CHCl₃/NaOH gives salicylaldehyde); Kolbe (CO₂ on sodium phenoxide gives salicylic acid, then aspirin); nitration to picric acid; coupling with diazonium salt. Phenol is made industrially from cumene.

Ethers. R–O–R′; Williamson synthesis R–X + R′ONa → R–O–R′ + NaX (best with a primary halide; tertiary halides give alkenes); low reactivity, peroxides on standing; cleaved by HI; diethyl ether anaesthetic and solvent.

3.5 Aldehydes, ketones, carboxylic acids and derivatives

Preparation: oxidation of 1° / 2° alcohols; Rosenmund (RCOCl, H₂, Pd/BaSO₄) for aldehydes; ozonolysis; Friedel–Crafts acylation for aromatic ketones; Stephen reduction. Reactions: nucleophilic addition to C=O (HCN gives cyanohydrin; NaHSO₃; Grignard; reactivity HCHO > RCHO > RCOR′ > ArCHO); condensation with NH₂OH (oxime), NH₂NH₂ (hydrazone), 2,4-DNP (orange precipitate for all carbonyls). Distinguishing tests: Tollens' (silver mirror) and Fehling's (red Cu₂O) only for aliphatic aldehydes (benzaldehyde does not respond to Fehling); iodoform for CH₃CO–; ketones are not oxidised easily. Reduction: Clemmensen (Zn–Hg/HCl) and Wolff–Kishner give CH₂; aldol condensation (needs α-H: CH₃CHO gives 3-hydroxybutanal; HCHO and benzaldehyde have none); Cannizzaro (no α-H, conc. NaOH: 2HCHO → CH₃OH + HCOONa; C₆H₅CHO → alcohol + benzoate). Formalin is 40 % HCHO.

Carboxylic acids. Acidity: HCOOH > CH₃COOH (alkyl +I weakens); –I substituents strengthen (ClCH₂COOH > CH₃COOH; CCl₃COOH very strong); benzoic acid is stronger than acetic acid. Preparation: oxidation of alcohols, hydrolysis of nitriles, Grignard + CO₂. Reactions: with NaHCO₃ (CO₂ effervescence — distinguishes acids from phenols), esterification (acid + alcohol, H⁺: CH₃COOH + C₂H₅OH ⇌ ethyl acetate + H₂O), acid chloride (SOCl₂), amide, reduction with LiAlH₄, Hell–Volhard–Zelinsky (α-bromination), decarboxylation (soda lime). Derivatives reactivity: RCOCl > anhydride > ester > amide; ester hydrolysis in base is saponification (soap); esters smell fruity. Acid anhydrides, amides (Hofmann bromamide: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O, one carbon less).

3.6 Nitro compounds and amines

Nitroalkanes: from RX + AgNO₂; reduction gives amines; tautomerism (aci-form). Nitrobenzene: benzene + HNO₃/H₂SO₄; Sn/HCl gives aniline; in neutral medium gives phenylhydroxylamine; used for dyes.

Amines. Preparation: ammonolysis, reduction of nitro compounds / nitriles / amides, Hofmann bromamide, Gabriel (pure 1° amines). Basicity: in water the order is (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ > C₆H₅NH₂ (aniline's lone pair is delocalised into the ring, so it is weakest). Tests: carbylamine (CHCl₃ + KOH, foul isocyanide) only for primary amines; Hinsberg (benzenesulphonyl chloride) separates 1°, 2°, 3°; nitrous acid: 1° aliphatic gives alcohol + N₂, 2° gives nitrosamine (yellow oil), 3° forms a salt; aniline + NaNO₂/HCl at 0–5 °C gives benzenediazonium chloride. Diazonium salts: Sandmeyer (CuCl, CuBr, CuCN replace N₂⁺ by Cl, Br, CN), Gattermann, replacement by OH (warm water), I (KI), H (H₃PO₂); coupling with phenol (alkaline) or aniline gives coloured azo dyes (–N=N–); aniline with Br₂ water gives 2,4,6-tribromoaniline; acetylation controls reactivity; aniline is o/p-directing, but nitration in strong acid gives meta (anilinium ion).

Formula and fact sheet

Practice questions

Set A — Recall (14 questions)

1. The IUPAC name of CH₃CH(CH₃)CH₂CH₃ isa2-methylbutaneb3-methylbutanec2-ethylpropanedn-pentane
2. The number of structural isomers of C₄H₁₀ isa1b2c3d4
3. Propene reacts with HBr (no peroxide) to give mainlya1-bromopropaneb3-bromopropanec2-bromopropaned1,2-dibromopropane
4. A test for unsaturation using cold dilute alkaline KMnO₄ isaTollens'bFehling'scLucasdBaeyer's
5. The Wurtz reaction of methyl bromide with sodium givesaethanebmethanecpropanedethene
6. Lucas reagent isaconc. HCl + anhydrous AlCl₃bconc. HCl + anhydrous ZnCl₂cconc. H₂SO₄ + ZnCl₂dHNO₃ + HCl
7. In Williamson's synthesis a haloalkane is treated withaammoniabalcoholcsodium alkoxidedsilver salt of a fatty acid
8. Phenol gives which colour with neutral FeCl₃?aredbgreencyellowdviolet
9. Tollens' reagent gives a silver mirror withaaldehydesbketonescethersdalcohols
10. The iodoform test is given byamethanolbethanolcpropan-1-oldbutan-1-ol
11. Acetic acid reacts with ethanol (H⁺) to giveamethyl acetatebethyl formatecethyl acetateddiethyl ether
12. Aniline with NaNO₂ and HCl at 0–5 °C givesanitrobenzenebphenolcazobenzenedbenzenediazonium chloride
13. In aqueous solution the strongest base among these isa(CH₃)₂NHbCH₃NH₂c(CH₃)₃NdNH₃
14. Which of these is aromatic?acyclohexanebbenzeneccyclooctatetraenedcyclohexene

Set B — Understanding (8 questions)

1. Ozonolysis of 2-methylpropene followed by reductive work-up givesatwo molecules of formaldehydebacetone and formaldehydectwo molecules of acetonedacetaldehyde and acetone
2. Hydration of propyne (HgSO₄/H₂SO₄) givesapropanalbpropan-1-olcpropanonedpropanoic acid
3. The compound most reactive in an SN1 reaction isaCH₃BrbCH₃CH₂Brc(CH₃)₂CHBrd(CH₃)₃CBr
4. The electrophile in the nitration of benzene isaNO₂⁺bNO₂⁻cNO₂ radicaldHNO₃ molecule
5. Conversion of an amide into an amine with one carbon atom less usesaLiAlH₄bBr₂ and NaOHcNaBH₄dHNO₂
6. Formaldehyde with concentrated NaOH givesamethanol onlybformic acid onlycmethanol and sodium formatedCO₂ and H₂
7. Which compound undergoes the aldol condensation?aHCHObbenzaldehydecCCl₃CHOdCH₃CHO
8. Which is the most acidic?ap-nitrophenolbphenolcp-cresoldethanol

Set C — Application (6 questions)

1. The number of moles of water formed on complete combustion of one mole of propane isa3b4c5d8
2. The percentage of carbon in methane isa80b25c75d85
3. The mass of acetic acid obtained by completely oxidising 9.2 g of ethanol isa6 gb9.2 gc18 gd12 g
4. A hydrocarbon contains 85.7 % C and 14.3 % H and has vapour density 28. Its molecular formula isaC₄H₈bC₂H₄cC₃H₆dC₅H₁₀
5. The number of structural isomers of C₅H₁₂ isa2b4c3d5
6. The degree of unsaturation of benzene (C₆H₆) isa3b5c2d4