Organic Chemistry: Some Basic Principles and Techniques — NCERT Class 11 Chemistry

• CH₂=C=O: The first C (CH₂=) is sp², the second C (=C=O) is sp (cumulene, two double bonds)
• CH₃CH=CH₂: C₁ (CH₃) is sp³, C₂ (CH=) is sp², C₃ (=CH₂) is sp²
• (CH₃)₂C=O: Both CH₃ carbons are sp³, the central C is sp² (ketone)
• CH₂=CHCN: C₁ (CH₂=) is sp², C₂ (=CH) is sp², C₃ (CN) is sp
• C₆H₆ (benzene): All six carbon atoms are sp² hybridised

(a) CH₃⁺ (carbocation): The carbon is sp² hybridised with an empty p-orbital. Shape is trigonal planar, bond angle ≈ 120°. (b) CH₃⁻ (carbanion): The carbon is sp³ hybridised with a lone pair. Shape is pyramidal (like NH₃), bond angle ≈ 109.5°. (c) CH₃· (free radical): The carbon is sp² hybridised with an unpaired electron in the p-orbital. Shape is trigonal planar, bond angle ≈ 120°.

(CH₃)₃C⁺ (tert-butyl cation) is stabilised by hyperconjugation. The nine α-hydrogen atoms from three methyl groups participate in hyperconjugation (σ C–H → p orbital overlap), delocalising the positive charge. In CH₃⁺ (methyl cation), there are no α-hydrogens for hyperconjugation, so it is the least stable carbocation. More hyperconjugative structures = more stability.

(a) Chain isomers — both are C₄H₁₀ (butane and 2-methylpropane) (b) Functional group isomers — both are C₂H₆O (ethanol and methoxymethane; alcohol vs ether) (c) They are NOT isomers — they have different molecular formulas (C₄H₁₀ and C₅H₁₂); they are homologues (differ by CH₂)

(a) Pentane (b) 2-Methylbutane (c) 2,2-Dimethylbutane (d) 3,3-Dimethylpentane

(a) But-2-ene (b) Propyne (c) Buta-1,3-diene

(a) Ethanol (or Ethan-1-ol) (b) Ethanal (c) Ethanoic acid (d) Methanamine

The correct IUPAC name is butanoic acid. The parent chain has 4 carbons (butane), and the functional group is the carboxylic acid (–COOH), which receives the suffix -oic acid with locant 1 (implied).

Structural isomerism is classified into: 1. Chain isomerism: different carbon skeletons (e.g., n-butane and isobutane) 2. Position isomerism: same functional group at different positions (e.g., propan-1-ol and propan-2-ol) 3. Functional group isomerism: different functional groups (e.g., ethanol and methoxymethane) 4. Metamerism: different alkyl groups on either side of a polyvalent functional group (e.g., ethoxyethane and methoxypropane)

The inductive effect is the permanent polarisation of a σ-bond due to the electron-withdrawing or electron-donating nature of an atom or group, transmitted through σ-bonds.

• +I effect (electron-donating): alkyl groups push electron density towards the bonded carbon, e.g., –CH₃, –C₂H₅. These stabilise carbocations.
• –I effect (electron-withdrawing): electronegative atoms pull electron density away, e.g., –NO₂, –CN, –COOH, halogens. These destabilise carbocations but stabilise carbanions.

Order of +I: –C(CH₃)₃ > –CH(CH₃)₂ > –C₂H₅ > –CH₃ > –H Order of –I: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I

Resonance is the phenomenon in which a molecule cannot be represented by a single Lewis structure but is described as a hybrid of two or more contributing (canonical) structures. The actual molecule (resonance hybrid) is more stable than any individual contributing structure.

In benzene, the two Kekulé structures (with alternating single and double bonds) are the main contributing structures. The actual benzene ring has all C–C bonds of equal length (139 pm, between typical single and double bond lengths), and the π-electrons are delocalised over all six carbon atoms. The resonance stabilisation (delocalisation energy) of benzene is about 150 kJ mol⁻¹.

Hyperconjugation (also called no-bond resonance or σ–π conjugation) is the delocalisation of σ-electrons of C–H bonds into an adjacent empty or partially filled p-orbital or π-orbital. In a carbocation, the empty p-orbital on the positively charged carbon overlaps with the σ-orbital of an adjacent C–H bond, allowing the σ-electrons to partially delocalise. This spreads the positive charge and stabilises the carbocation. More α-hydrogens → more hyperconjugation structures → greater stability: 3° > 2° > 1° > methyl.

Simple distillation: Used when the boiling points of two liquids differ significantly (>25–30 K). The mixture is heated, the more volatile component vapourises first, condenses, and is collected separately.

Fractional distillation: Used when the boiling points of components are close (<25 K). A fractionating column packed with glass beads provides multiple vapourisation-condensation cycles, improving separation efficiency. Each cycle enriches the vapour in the more volatile component.

Chromatography is a technique for separating mixtures based on differences in the rate at which components move through a stationary phase under the influence of a mobile phase. Types include: 1. Column chromatography: stationary phase = alumina/silica gel column; mobile phase = liquid solvent 2. Thin-layer chromatography (TLC): stationary phase = silica gel on glass plate; mobile phase = solvent 3. Paper chromatography: stationary phase = cellulose (paper); mobile phase = solvent 4. Gas chromatography: stationary phase = liquid coated on solid support; mobile phase = inert gas (He, N₂)

Lassaigne's test (sodium fusion test) is used to detect nitrogen, sulphur, and halogens in organic compounds. The organic compound is heated with sodium metal, which converts the elements into ionic form:

• For nitrogen: Na + C + N → NaCN; NaCN + FeSO₄ → Na₄[Fe(CN)₆]; with FeCl₃ → Prussian blue colour
• For sulphur: Na + S → Na₂S; Na₂S + sodium nitroprusside → violet colour
• For both N and S: Na + C + N + S → NaSCN; with FeCl₃ → blood red colour
• For halogens: Na + X → NaX; NaX + AgNO₃ → AgX (precipitate); colour tells which halogen

Electrophiles are electron-seeking, electron-deficient species that accept an electron pair. They are Lewis acids. Examples: H⁺, CH₃⁺, NO₂⁺, BF₃, AlCl₃, SO₃, Br⁺, Cl⁺

Nucleophiles are nucleus-seeking, electron-rich species that donate an electron pair. They are Lewis bases. Examples: OH⁻, CN⁻, CH₃O⁻, NH₃, H₂O, ROH, Cl⁻, Br⁻, I⁻

Some species can act as both: H₂O (lone pairs make it nucleophilic; H can be electrophilic).

• Electrophiles: H⁺, AlCl₃, C₂H₅⁺, BF₃, NO₂⁺, H₃O⁺
• Nucleophiles: CH₃COO⁻, OH⁻, NH₃, CN⁻

(a) Hexane (b) 2,3-Dimethylbutane (or 2,3-dimethylbutane) (c) Pent-2-yne (d) But-2-enal

(a) CH₂=CH–CH(CH₃)–CH₃ (b) CH₃CH₂CH(CH₂CH₃)COOH (c) CH₃–CH(Cl)–CH(CH₃)–CH₂–CH₃ (d) CH₃–CH=CH–CH₂–CH₂–CH₂OH → corrected: CH₃CH=CHCH₂OH (Wait, hex-2-en-1-ol: CH₃CH₂CH₂CH=CHCH₂OH... let me reconsider)

Hex-2-en-1-ol: CH₃–CH₂–CH₂–CH=CH–CH₂OH

(a) Propenal (CH₂=CH–CHO): Structure I: CH₂=CH–CH=O Structure II: CH₂⁻–CH=CH–O⁺ (or ⁻CH₂–CH=CH=O⁺)

(b) Buta-1,3-diene (CH₂=CH–CH=CH₂): Structure I: CH₂=CH–CH=CH₂ Structure II: CH₂⁻–CH=CH–CH₂⁺ (or CH₂=CH–CH=CH₂ with delocalised π-cloud)

(c) Nitrobenzene (C₆H₅–NO₂): The NO₂ group shows resonance with charge separation: N⁺(=O)–O⁻ ↔ N⁺(–O⁻)=O. Combined with benzene ring resonance.

(d) Acetate ion (CH₃COO⁻): Structure I: CH₃–C(=O)–O⁻ Structure II: CH₃–C(–O⁻)=O The two structures are equivalent, making both C–O bonds equal in length (partial double bond character).

Resonance is shown by compounds with conjugated π-systems or groups capable of electron delocalisation:

• CH₃Cl: No resonance (no π-bonds or lone pairs adjacent to π-systems)
• CH₃CH=CH₂: No resonance (isolated π-bond without conjugation)
• C₆H₆: Yes, resonance (delocalised π-electrons over the ring)
• CH₃CH₂OH: No resonance
• CH₂=CH–CH=CH₂: Yes, resonance (conjugated diene)

(a) Inductive effect: Permanent polarisation of σ-bonds due to electronegativity differences, transmitted through the chain. Example: In CH₃–CH₂–Cl, the Cl atom withdraws electron density (–I effect), making C-1 slightly positive and C-2 less positive.

(b) Resonance effect (mesomeric effect): Delocalisation of π-electrons in conjugated systems through π-bonds and lone pairs. +R effect: groups that donate electron density through resonance (e.g., –OH, –NH₂, –OR). –R effect: groups that withdraw electron density through resonance (e.g., –NO₂, –CN, –COOH).

(c) Hyperconjugation: Delocalisation of σ-electrons of C–H bonds into an adjacent empty or partially filled p-orbital or π-orbital. Example: In propene (CH₃–CH=CH₂), the three C–H bonds of the methyl group can overlap with the adjacent π-bond, stabilising the alkene. This is why more substituted alkenes are more stable.

1. Crystallisation: Based on difference in solubilities; impure compound dissolved in hot solvent, filtered, cooled to crystallise. 2. Distillation: For volatile liquids — simple (large BP difference), fractional (close BP), vacuum (high BP or thermally sensitive). 3. Sublimation: For substances that sublime (convert directly from solid to vapour). 4. Chromatography: Separation based on differential adsorption/partition between stationary and mobile phases. 5. Differential extraction: Using two immiscible solvents to separate organic and inorganic components.

Chromatography is based on the differential migration of components of a mixture between a stationary phase and a mobile phase. Components with greater affinity (adsorption or partition) for the stationary phase move slowly, while those with less affinity move faster with the mobile phase. This differential movement leads to separation. The component that is more strongly adsorbed remains near the point of application, while the less adsorbed component moves farther.

Organic compound + Na → NaCN (sodium cyanide) The sodium fusion extract is boiled with FeSO₄ (ferrous sulphate) to form sodium hexacyanoferrate(II): 6NaCN + FeSO₄ → Na₄[Fe(CN)₆] + Na₂SO₄ On adding FeCl₃ and HCl: Na₄[Fe(CN)₆] + 4FeCl₃ → Fe₄[Fe(CN)₆]₃ (Prussian blue) + 12NaCl A Prussian blue colour confirms nitrogen.

In the Dumas method, a known mass of organic compound is heated with copper oxide in a CO₂ atmosphere. Nitrogen is converted to N₂ gas: Compound + CuO → N₂ + CO₂ + H₂O The gaseous mixture passes over heated copper gauze (removes excess O₂), then through a KOH solution (absorbs CO₂). The volume of N₂ gas is measured, and the percentage of nitrogen is calculated: %N = (28 × Volume of N₂ at STP × 100) / (22400 × mass of compound)

Organic compound + Na → Na₂S (sodium sulphide) Two tests: (a) Sodium fusion extract + sodium nitroprusside → violet colour (confirms S) (b) Sodium fusion extract + acetic acid + lead acetate → black precipitate of PbS (confirms S)

1. Organic compound + conc. H₂SO₄ + K₂SO₄ (boiling point elevator) + CuSO₄ (catalyst) → (NH₄)₂SO₄ 2. (NH₄)₂SO₄ + NaOH (excess) → NH₃ gas 3. NH₃ is absorbed in known volume of standard HCl 4. Unreacted HCl is titrated with standard NaOH 5. %N = (1.4 × M × (V₁–V₂)) / W, where M = molarity of HCl, V₁ = volume of HCl initially, V₂ = volume of HCl remaining, W = mass of compound

(a) CH₄: sp³ hybridised, tetrahedral (109.5°) (b) C₂H₄: sp² hybridised at each C, planar (120°) (c) C₂H₂: sp hybridised at each C, linear (180°) (d) CH₃Cl: sp³ hybridised at C, tetrahedral (approx. 109.5°)

A homologous series is a series of organic compounds in which each successive member differs by a –CH₂– unit. Members have the same general formula, similar chemical properties, and a gradual change in physical properties. Example: Alkanes — CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane)... General formula: CₙH₂ₙ₊₂. Boiling points increase regularly; chemical reactions follow the same pattern.

(a) CH₃COOH — Carboxylic acid (–COOH) (b) CH₃CHO — Aldehyde (–CHO) (c) CH₃CH₂OH — Alcohol (–OH) (d) CH₃COCH₃ — Ketone (>C=O)

• Simple distillation: For liquids with a large boiling point difference (>25–30 K). The mixture is heated and the vapour of the more volatile component is condensed.
• Fractional distillation: For liquids with close boiling points (<25 K). Uses a fractionating column for repeated vapourisation-condensation cycles.
• Distillation under reduced pressure (vacuum distillation): For liquids that decompose at or below their boiling points at atmospheric pressure. Reducing the pressure lowers the boiling point, allowing distillation at a lower temperature.

Electrophilic substitution in benzene occurs in two steps: Step 1 — Attack of electrophile (slow, rate-determining): The electrophile (E⁺) attacks the π-electron cloud of benzene, forming a resonance-stabilised arenium ion (sigma complex/Wheland intermediate) in which the carbon bearing the electrophile becomes sp³ hybridised. Step 2 — Loss of proton (fast): A base removes the proton from the sp³ carbon, restoring aromaticity. The overall reaction is substitution (one H replaced by E).

Example — Nitration: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O; NO₂⁺ attacks benzene → arenium ion → loss of H⁺ → nitrobenzene.