1. Chapter at a Glance
General Organic Chemistry (GOC) — NCERT's "Organic Chemistry: Some Basic Principles and Techniques" — is the foundation of every organic chapter. It explains how carbon forms bonds, how organic compounds are named (IUPAC), the types of isomerism and, most importantly, the electronic effects (inductive, resonance, hyperconjugation) that decide the stability of intermediates and the acidity or basicity of compounds. JEE Main asks two or three questions based directly on GOC concepts every year.
- Stability order of carbocations, carbanions and free radicals.
- Acidity of carboxylic acids and phenols; basicity of amines.
- IUPAC names of polyfunctional compounds.
- Hybridisation, isomer counting and identification of nucleophiles/electrophiles.
2. Hybridisation and Representation
| Carbon type | Hybridisation | Geometry, angle | % s-character |
|---|---|---|---|
| Single bonds only (CH₄) | sp³ | Tetrahedral, 109.5° | 25 |
| One double bond (C=C) | sp² | Trigonal planar, 120° | 33.3 |
| Triple bond or two double bonds (C≡C, =C=) | sp | Linear, 180° | 50 |
More s-character makes an atom more electronegative, which is why an sp carbon (ethyne) is more acidic than sp² or sp³. Compounds are drawn as complete, condensed or bond-line (skeletal) structures; in bond-line formulas each corner and line end is a carbon.
3. IUPAC Nomenclature
- Choose the longest chain that contains the principal functional group and the most multiple bonds.
- Number the chain to give the lowest locant to the principal group, then to multiple bonds, then to substituents (lowest set rule).
- Write substituents as prefixes in alphabetical order, then the parent name with the suffix.
Priority of functional groups (high to low): carboxylic acid > sulphonic acid > anhydride > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne. Halo, nitro, alkoxy and alkyl groups are always prefixes.
| Structure | IUPAC name |
|---|---|
| CH₃CH(CH₃)CH₂CH₂OH | 3-Methylbutan-1-ol |
| CH₂=CH–CHO | Prop-2-enal |
| HOOC–CH₂–CH(OH)–COOH | 2-Hydroxybutanedioic acid |
| CH₃COCH₂CH₂COOH | 4-Oxopentanoic acid |
| CH₃–C≡C–CH(CH₃)₂ | 4-Methylpent-2-yne |
4. Isomerism
- Structural: chain, position, functional (alcohol/ether, aldehyde/ketone, acid/ester), metamerism (different alkyl groups on either side of a functional group, e.g. diethyl ether and methyl propyl ether) and tautomerism (keto–enol).
- Stereoisomerism: geometrical (cis/trans or E/Z, needs restricted rotation and two different groups on each carbon) and optical (a chiral centre makes non-superimposable mirror images).
- Number of stereoisomers for n different chiral centres = 2n (fewer if a meso form exists).
- Degree of unsaturation (DoU) = C − H/2 − X/2 + N/2 + 1. Benzene C₆H₆ has DoU 4 (one ring + three double bonds).
5. Electronic Effects
Inductive effect (I)
Permanent shift of σ electrons along a chain because of electronegativity difference; it weakens quickly with distance. −I groups pull electrons: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I > –OCH₃ > –C₆H₅. +I groups push electrons: alkyl groups, (CH₃)₃C– > (CH₃)₂CH– > CH₃CH₂– > CH₃–.
Resonance (mesomeric) effect (R or M)
Delocalisation of π electrons or lone pairs through a conjugated system. +R groups donate (–OH, –NH₂, –OR, –X); −R groups withdraw (–NO₂, –CHO, –COOH, –CN). The real molecule is a resonance hybrid, more stable than any single contributor.
Hyperconjugation
Delocalisation of σ electrons of a C–H bond on the carbon next to (α to) a carbocation, free radical or double bond. More α-hydrogens mean more stabilisation — this is "no-bond resonance". Example: (CH₃)₃C⁺ has 9 α-H.
Electromeric effect
A temporary, complete transfer of π electrons to one atom of a multiple bond when an attacking reagent approaches; it disappears when the reagent is removed.
6. Reactive Intermediates and Reagents
- Homolytic cleavage gives free radicals; heterolytic cleavage gives ions (carbocation + carbanion).
- Carbocation (sp², planar, 6 electrons) stability: benzyl ≈ allyl > 3° > 2° > 1° > CH₃⁺ (resonance and hyperconjugation). Rearrangements (hydride or methyl shifts) give a more stable carbocation.
- Free radical (sp², 7 electrons): same order as carbocations.
- Carbanion (sp³, pyramidal, 8 electrons): opposite order: CH₃⁻ > 1° > 2° > 3°; stabilised by −I and −R groups.
- Carbenes (:CH₂) are neutral, divalent carbon species.
- Nucleophiles (electron-rich): OH⁻, CN⁻, NH₃, H₂O, RO⁻. Electrophiles (electron-poor): H⁺, NO₂⁺, Br⁺, carbocations, BF₃, AlCl₃.
7. Acidity and Basicity
- Anything that stabilises the conjugate base increases acidity: −I and −R groups increase acidity; +I and +R decrease it.
- Acid strength: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH; FCH₂COOH > ClCH₂COOH; HCOOH > CH₃COOH.
- Phenols: o/p-nitrophenol are more acidic than phenol; p-cresol is less acidic. 2,4,6-Trinitrophenol (picric acid) is almost as strong as a carboxylic acid.
- General order: RCOOH > PhOH > H₂O > ROH > HC≡CH > NH₃ > CH₂=CH₂ > CH₃CH₃.
- Basicity of amines in water: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃; for methyl: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. Aniline is much weaker (lone pair delocalised into the ring).
8. Master Table
| Concept | Rule / order |
|---|---|
| Degree of unsaturation | C − H/2 − X/2 + N/2 + 1 |
| Stereoisomers | 2n (n chiral centres, no meso) |
| Carbocation / radical | benzyl ≈ allyl > 3° > 2° > 1° > CH₃ |
| Carbanion | CH₃⁻ > 1° > 2° > 3° |
| −I strength | NO₂ > CN > COOH > F > Cl > Br > I |
| Hyperconjugation | stabilisation ∝ number of α-H |
9. Mind Map
- sp³, sp², sp
- IUPAC priority
- Isomerism
- Inductive
- Resonance
- Hyperconjugation
- Electromeric
- Carbocation, radical
- Carbanion, carbene
- Rearrangement
- Acidity
- Basicity
- Nucleophile/electrophile
10. Tips and Tricks
- When resonance and induction compete, resonance usually wins — except for halogens, where −I outweighs +R.
- Count α-hydrogens quickly: each CH₃ next to the charged carbon gives 3.
- For acidity questions, draw the conjugate base and ask: "Is the negative charge spread out?"
- The ortho effect: almost any ortho-substituted benzoic acid is stronger than benzoic acid.
- Numbering a chain from the wrong end — the principal group gets priority over substituents.
- Thinking hyperconjugation is possible without α-hydrogens (e.g. in (CH₃)₃C–CH=CH₂ the t-butyl gives none).
- Using the gas-phase amine basicity order for aqueous solutions.
11. Important One-liners
- The central carbon of allene (CH₂=C=CH₂) is sp hybridised.
- Carbocations are planar; carbanions are usually pyramidal.
- Resonance structures differ only in the position of electrons, not atoms.
- Tautomers are real, separate compounds; resonance structures are not.
- Electrophiles attack electron-rich sites; nucleophiles attack electron-poor sites.
- Trichloroacetic acid is stronger than acetic acid because of the −I effect of three Cl atoms.
12. PYQ-pattern Questions with Solutions
These questions are original and modelled on concepts frequently tested in JEE Main.
The hybridisation of the central carbon in CH₂=C=CH₂ is:
- sp³
- sp²
- sp
- sp³d
Show solution
The central carbon forms two σ bonds and two π bonds (two double bonds), so it is sp hybridised and linear.
Answer: (C)
The IUPAC name of CH₃CH(CH₃)CH₂CH₂OH is:
- 2-Methylbutan-4-ol
- 3-Methylbutan-1-ol
- Isopentyl alcohol
- 3-Methylbutan-4-ol
Show solution
Number from the end nearer the –OH group: C1 carries OH, the methyl is on C3. Name: 3-methylbutan-1-ol. "Isopentyl alcohol" is a common name, not IUPAC.
Answer: (B)
The most stable carbocation among the following is:
- CH₃CH₂⁺
- (CH₃)₂CH⁺
- (CH₃)₃C⁺
- C₆H₅CH₂⁺
Show solution
The benzyl cation spreads its positive charge over the ring by resonance (four resonance structures). This is stronger stabilisation than the hyperconjugation in the t-butyl cation.
Answer: (D)
The correct order of acid strength is:
- CH₃COOH > ClCH₂COOH > Cl₂CHCOOH
- Cl₂CHCOOH > ClCH₂COOH > CH₃COOH
- ClCH₂COOH > Cl₂CHCOOH > CH₃COOH
- All are equal
Show solution
Each Cl pulls electron density (−I) and stabilises the carboxylate ion. More Cl atoms, stronger acid.
Answer: (B)
How many α-hydrogens are available for hyperconjugation in the tert-butyl cation, (CH₃)₃C⁺?
Show solution
Three methyl groups are attached to the positive carbon; each has 3 H. Total = 9.
Answer: 9
Which of the following is an electrophile?
- NH₃
- OH⁻
- NO₂⁺
- CN⁻
Show solution
NO₂⁺ (nitronium ion) is electron-deficient and attacks electron-rich benzene in nitration. The other three have lone pairs and are nucleophiles.
Answer: (C)
How many structural isomers (alcohols and ethers together) have the formula C₄H₁₀O?
Show solution
Alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol (4). Ethers: methoxypropane, 2-methoxypropane, ethoxyethane (3). Total = 7.
Answer: 7
The correct order of basic strength in aqueous solution is:
- (CH₃)₃N > (CH₃)₂NH > CH₃NH₂
- (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N
- CH₃NH₂ > (CH₃)₂NH > (CH₃)₃N
- NH₃ > CH₃NH₂ > (CH₃)₂NH
Show solution
In water, basicity depends on +I effect, solvation of the protonated amine (by H-bonding) and steric hindrance. For methylamines these balance to give (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. The tertiary amine is poorly solvated because its cation has only one N–H.
Answer: (B)
13. Quick Revision Summary
- sp: 50% s, most electronegative; sp³: 25% s.
- IUPAC: longest chain with the principal group; lowest locants; alphabetical prefixes.
- Effects: I (σ, permanent), R (π, permanent), hyperconjugation (σ–π), electromeric (temporary).
- Carbocation: benzyl ≈ allyl > 3° > 2° > 1° > CH₃⁺; carbanion: reverse.
- −I/−R increase acidity and decrease basicity.
- DoU = C − H/2 − X/2 + N/2 + 1.