1. Chapter at a Glance
This chapter is the grammar of organic chemistry. It explains the special nature of carbon, how organic compounds are named (IUPAC), the kinds of isomerism, the electronic effects that control reactivity (inductive, resonance, hyperconjugation, electromeric), reaction intermediates (carbocations, carbanions, free radicals), and the types of reagents and reactions. It also covers methods of purifying and analysing organic compounds. NEET asks about two questions from here every year, and its ideas are needed for every later organic chapter.
- IUPAC names of compounds with functional groups and branches.
- Structural and stereo-isomerism; counting isomers.
- Stability order of carbocations, carbanions and free radicals.
- Inductive, resonance (±R) and hyperconjugation effects; acidity and basicity comparisons.
- Purification (distillation types, chromatography) and quantitative analysis (Kjeldahl, Carius, Dumas).
2. Tetravalence of Carbon and Structural Representation
- Carbon forms four covalent bonds. Its hybridisation decides the shape: sp³ (tetrahedral, 109.5°), sp² (trigonal planar, 120°), sp (linear, 180°).
- Electronegativity of carbon rises with s-character: sp > sp² > sp³. That is why the terminal H of ethyne is weakly acidic.
- Structures can be written as complete (dash) formulas, condensed formulas (CH₃CH₂OH) or bond-line (zig-zag) formulas, where each corner or end is a carbon.
- Homologous series: members differ by –CH₂–, have the same general formula and similar chemical properties.
3. IUPAC Nomenclature
- Choose the longest carbon chain that contains the principal functional group and the maximum number of multiple bonds.
- Number the chain to give the principal functional group the lowest locant, then multiple bonds, then substituents.
- Name substituents alphabetically as prefixes (ignoring di-, tri- when alphabetising).
- Add the suffix for the principal functional group.
| Priority (high → low) | Suffix | Prefix |
|---|---|---|
| Carboxylic acid | -oic acid | carboxy |
| Sulphonic acid | -sulphonic acid | sulpho |
| Ester | -oate | alkoxycarbonyl |
| Acid chloride | -oyl chloride | chlorocarbonyl |
| Amide | -amide | carbamoyl |
| Nitrile | -nitrile | cyano |
| Aldehyde | -al | formyl / oxo |
| Ketone | -one | oxo |
| Alcohol | -ol | hydroxy |
| Amine | -amine | amino |
| Alkene / alkyne | -ene / -yne | — |
Groups always named as prefixes: halo (–X), nitro (–NO₂), alkoxy (–OR). Example: CH₃CH(OH)CH₂COOH is 3-hydroxybutanoic acid.
4. Isomerism
- Structural isomerism: chain (different carbon skeleton), position (different position of a group), functional (different functional group, e.g. ethanol and dimethyl ether), metamerism (different alkyl groups around the same functional group, e.g. diethyl ether and methyl propyl ether), and tautomerism (keto–enol).
- Stereoisomerism: same connectivity, different arrangement in space.
- Geometrical (cis–trans): needs restricted rotation (C=C or a ring) and two different groups on each doubly bonded carbon.
- Optical: a chiral molecule (usually with a carbon bearing four different groups) is non-superimposable on its mirror image. Enantiomers rotate plane-polarised light equally in opposite directions; a 50 : 50 mixture (racemic) is optically inactive.
- Maximum number of optical isomers = 2ⁿ for n different chiral centres (fewer if a meso form exists).
5. Electronic Effects
| Effect | What happens | Examples | NEET Note |
|---|---|---|---|
| Inductive (−I) | Electron withdrawal through σ bonds | –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I > –OH | Weakens with distance |
| Inductive (+I) | Electron release through σ bonds | (CH₃)₃C– > (CH₃)₂CH– > CH₃CH₂– > CH₃– | Alkyl groups |
| Resonance (+R/+M) | Lone pair donated into a π system | –OH, –OR, –NH₂, –X | Makes ring electron-rich |
| Resonance (−R/−M) | π electrons pulled out of the system | –NO₂, –CHO, –COOH, –CN | Makes ring electron-poor |
| Hyperconjugation | C–H σ electrons delocalise into an adjacent empty p or π* orbital | Alkyl carbocations, alkenes | More α-H ⇒ more stable |
| Electromeric | Temporary shift of π electrons when a reagent attacks | C=O, C=C | Disappears when reagent removed |
- Acidity increases when the conjugate base is stabilised: electron-withdrawing groups raise acidity (Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH).
- Basicity of amines rises with electron-donating groups and falls when the lone pair is delocalised (aniline is weaker than methylamine).
6. Reaction Intermediates and Reagents
- Homolytic cleavage gives free radicals; heterolytic cleavage gives carbocations and carbanions.
- Carbocation (sp², planar, electron-deficient): stability 3° > 2° > 1° > CH₃⁺, because of +I and hyperconjugation. Allyl and benzyl carbocations are extra stable because of resonance. Carbocations can rearrange (hydride or methyl shift) to a more stable form.
- Carbanion (sp³, pyramidal, electron-rich): stability CH₃⁻ > 1° > 2° > 3° (alkyl groups push in more electrons).
- Free radical (sp², one unpaired electron): stability 3° > 2° > 1° > CH₃•.
- Carbene: neutral, divalent carbon (:CH₂).
- Electrophiles are electron-seeking (H⁺, NO₂⁺, Br⁺, AlCl₃, BF₃, carbocations); nucleophiles are electron-rich (OH⁻, CN⁻, NH₃, H₂O, RO⁻).
- Reaction types: substitution, addition, elimination and rearrangement.
7. Purification and Analysis
| Method | Used for | Example |
|---|---|---|
| Sublimation | Solids that change directly to vapour | Camphor, naphthalene, benzoic acid |
| Crystallisation | Solid with an impurity of different solubility | Sugar, benzoic acid from water |
| Simple distillation | Liquids with a large difference in boiling point | Chloroform (334 K) and aniline (457 K) |
| Fractional distillation | Liquids with close boiling points | Crude oil fractions |
| Distillation under reduced pressure | Liquids that decompose below their boiling point | Glycerol from spent lye |
| Steam distillation | Steam-volatile, water-immiscible liquids | Aniline, essential oils |
| Differential extraction | Compound more soluble in an organic solvent | Separating funnel |
| Chromatography | Separation by different adsorption or partition | Column, TLC (Rf), paper chromatography |
- Lassaigne's test (sodium fusion) detects N (Prussian blue), S (violet with sodium nitroprusside; black PbS), and halogens (AgX precipitates). If both N and S are present, a blood-red colour with Fe³⁺ appears (thiocyanate).
- Kjeldahl's method (% N): N is converted to ammonium sulphate, NH₃ is released and absorbed in a known volume of standard acid, and %N = 1.4 × (normality × volume in mL of acid used by NH₃)/mass of compound. It does not work for nitro, azo compounds or N in rings (pyridine).
- Dumas method (% N): N converted to N₂ gas and its volume measured.
- Carius method: halogens and sulphur estimated as AgX and BaSO₄.
- Rf = distance moved by the substance/distance moved by the solvent front.
8. Formula and Rule Master Table
| Quantity | Formula / rule |
|---|---|
| Optical isomers | 2ⁿ (n = different chiral centres) |
| Carbocation / radical stability | 3° > 2° > 1° > CH₃ |
| Carbanion stability | CH₃ > 1° > 2° > 3° |
| Electronegativity of C | sp > sp² > sp³ |
| Rf value | distance of spot/distance of solvent front |
| % halogen (Carius) | (atomic mass X/M of AgX) × (mass AgX/mass compound) × 100 |
| % S (Carius) | (32/233) × (mass BaSO₄/mass compound) × 100 |
| % N (Kjeldahl) | 1.4 × (meq of acid used by NH₃)/mass of compound |
9. Master Summary Table
| Concept | Key idea | NEET Note |
|---|---|---|
| IUPAC | Principal group gets lowest locant | Acid > ester > aldehyde > ketone > alcohol |
| Geometrical isomerism | Restricted rotation | Each C of C=C needs two different groups |
| Optical isomerism | Chirality | Racemic mixture inactive |
| −I groups | Increase acidity | NO₂ strongest |
| Hyperconjugation | More α-H, more stable | Also called no-bond resonance |
| Carbocation | sp², planar | Rearranges to more stable ion |
| Steam distillation | Immiscible, steam-volatile | Aniline |
| Kjeldahl | % N | Not for nitro or ring N |
10. Mind Map
- IUPAC priority
- Structural
- Stereo
- ±I
- ±R
- Hyperconjugation
- Carbocation
- Carbanion, radical
- Nu⁻ and E⁺
- Purification
- Lassaigne
- Estimation
11. Tips and Tricks
- For acid strength, look at how stable the anion is: resonance > −I groups close to COOH > −I groups far away.
- Number of hyperconjugation structures = number of α-hydrogens.
- To decide chirality, look for carbon with four different groups; check for a mirror plane (meso) if there are two chiral centres.
- Degree of unsaturation = (2C + 2 + N − H − X)/2 — each ring or π bond counts 1.
- Giving the carbanion stability order the same as carbocations.
- Numbering from the end nearer a substituent instead of the principal functional group.
- Calling a racemic mixture optically active.
- Applying Kjeldahl's method to nitrobenzene.
12. Important One-liners
- Carbocations are sp² hybridised and planar.
- The tert-butyl carbocation is more stable than the isopropyl carbocation.
- Benzyl and allyl carbocations are stabilised by resonance.
- Ethanol and methoxymethane are functional isomers.
- Aniline is purified by steam distillation.
- Glycerol is purified by distillation under reduced pressure.
- A blood-red colour in Lassaigne's test shows that both N and S are present.
- BF₃ and AlCl₃ are electrophiles; NH₃ and H₂O are nucleophiles.
13. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
The IUPAC name of CH₃–CH(CH₃)–CH₂–OH is:
- 2-Methylpropan-1-ol
- Butan-1-ol
- 2-Methylpropan-3-ol
- Isobutanol-1
Show solution
The longest chain containing the carbon with the OH group has 3 carbons. Numbering from the OH end puts the OH on C-1 and the methyl on C-2. (C) numbers from the wrong end, (B) ignores the branch, and (D) is not an IUPAC name.
Answer: (A)
The most stable carbocation among these is:
- CH₃⁺
- CH₃CH₂⁺
- (CH₃)₂CH⁺
- (CH₃)₃C⁺
Show solution
The tertiary carbocation has the most +I donation and 9 hyperconjugating α-hydrogens, so it is the most stable. CH₃⁺ is the least stable.
Answer: (D)
Which is the strongest acid?
- CH₃COOH
- ClCH₂COOH
- Cl₂CHCOOH
- Cl₃CCOOH
Show solution
Each Cl pulls electrons away (−I) and stabilises the carboxylate anion. Three Cl atoms give the greatest stabilisation, so trichloroacetic acid is the strongest acid.
Answer: (D)
Which compound shows geometrical isomerism?
- CH₂=CHCl
- CH₃CH=CHCH₃
- (CH₃)₂C=CH₂
- CH₂=CH₂
Show solution
In but-2-ene, each double-bonded carbon carries two different groups (H and CH₃), so cis and trans forms exist. In the other three compounds, at least one carbon has two identical groups.
Answer: (B)
The number of hyperconjugation structures possible for the isopropyl carbocation, (CH₃)₂CH⁺, is:
- 3
- 6
- 9
- 2
Show solution
The number of structures equals the number of α-hydrogens. Two CH₃ groups give 6. The tert-butyl cation would have 9 and the ethyl cation 3.
Answer: (B)
A mixture of chloroform (b.p. 334 K) and aniline (b.p. 457 K) is best separated by:
- Simple distillation
- Steam distillation
- Sublimation
- Crystallisation
Show solution
The boiling points differ by more than 100 K, so simple distillation works. Steam distillation is used to separate aniline from water-insoluble, non-volatile impurities, which is a different situation.
Answer: (A)
The number of optical isomers of 2,3-dihydroxybutanedioic acid (tartaric acid) is:
- 4
- 3
- 2
- 1
Show solution
There are two chiral centres, but they carry identical groups. So instead of 2² = 4 isomers, there are two optically active enantiomers (d and l) plus one meso form, which is inactive. That makes 3 stereoisomers.
Answer: (B)
Kjeldahl's method cannot be used to estimate nitrogen in:
- Urea
- Acetamide
- Nitrobenzene
- Aniline
Show solution
Nitrogen in nitro groups, azo groups and ring positions (pyridine) is not converted into ammonium sulphate under Kjeldahl conditions. Urea, acetamide and aniline all give NH₃ properly.
Answer: (C)
14. Quick Revision Summary
- IUPAC: longest chain with principal group; lowest locant to it; prefixes alphabetical.
- Isomers: chain, position, functional, metamer, tautomer; cis–trans; optical (2ⁿ, minus meso).
- −I/−R raise acidity; +I/+R and hyperconjugation stabilise cations.
- Carbocation/radical 3° > 2° > 1°; carbanion reverse.
- Purification by b.p. difference, steam, reduced pressure, chromatography; Lassaigne, Kjeldahl, Carius.