CBSE Class 12 Organic Chemistry Complete Revision

Organic chemistry fundamentally revolves around hydrocarbons and their derivatives, forming the backbone of the CBSE Class 12 Organic Chemistry Complete Revision curriculum. Haloalkanes, alcohols, aldehydes, and amines are all derivatives of hydrocarbons, formed by replacing hydrogen atoms with specific functional groups. Understanding these basic building blocks is important for mastering organic reactions and their mechanisms, as it allows students to predict how different molecules will interact based on their structural properties.

Chapter 1: Haloalkanes and Haloarenes

Haloalkanes and haloarenes serve as the starting point for understanding how functional groups alter the chemical personality of a hydrocarbon. These compounds are not only essential intermediates in synthetic chemistry but also have significant industrial and medicinal applications, ranging from solvents to fire extinguishers.

Foundational Concepts for Organic Chemistry

Hydrocarbons are compounds composed exclusively of hydrogen and carbon atoms. Derivatives of hydrocarbons are formed when one or more hydrogen atoms in a hydrocarbon are replaced by other atoms or functional groups. For instance, replacing a hydrogen in methane (CH₄) with a chlorine atom yields CH₃Cl, a haloalkane, while replacement with a hydroxyl group forms CH₃OH, an alcohol.

Structural Representation of Organic Compounds

Organic compounds can be represented in four common ways:

1. Molecular Formula: Shows the total number of each atom (e.g., Butane, C₄H₁₀).

2. Expanded Structural Formula: Shows every atom and bond.

3. Condensed Structural Formula: Shows connections between carbon atoms, condensing hydrogen atoms (e.g., CH₃–CH₂–CH₂–CH₃).

4. Bond Line Notation: A shorthand where lines represent bonds, and vertices/endpoints represent carbon atoms, with hydrogens implied to satisfy carbon's four bonds.

Hybridization: A Key Concept

Hybridization determines molecular geometry and properties. For carbon, it is based on the sum of sigma (σ) bonds and lone pairs, excluding pi (π) bonds.

• Sum = 4 → sp³ hybridization

• Sum = 3 → sp² hybridization

• Sum = 2 → sp hybridization

Definition of Haloalkanes and Haloarenes

While both classes of compounds involve the replacement of hydrogen with a halogen atom, the nature of the parent hydrocarbon chain significantly influences their chemical behavior. Haloalkanes are generally derived from open-chain (aliphatic) structures, whereas haloarenes involve halogens bonded directly to a stable, resonance-stabilized aromatic system. The table below summarizes the fundamental structural differences that dictate the physical properties and reactivity patterns of these two categories:

Classification of Haloalkanes and Haloarenes

To simplify the study of countless organic molecules, chemists categorize haloalkanes and haloarenes based on structural similarities. These classifications are not just for naming; they help predict how a molecule will behave during a chemical reaction, such as whether it will follow a substitution or an elimination pathway.

A. Based on the Number of Halogen Atoms

• Mono-haloalkanes: One halogen atom.

• Di-haloalkanes: Two halogen atoms.

• Geminal (Gem) Dihalides: Both halogens on the same carbon. (Memory Tip: Like "Gems" candies in one spot).

• Vicinal (Vic) Dihalides: Halogens on adjacent carbons.

• Tri-haloalkanes: Three halogen atoms (e.g., CHCl₃).

B. Based on the Degree of the Carbon Atom

The degree of carbon (1°, 2°, 3°) refers to the number of other carbons it's bonded to.

• Primary (1°) Haloalkane: Halogen on a 1° carbon.

• Secondary (2°) Haloalkane: Halogen on a 2° carbon.

• Tertiary (3°) Haloalkane: Halogen on a 3° carbon.

C. Based on the Hybridization of the Carbon Atom

1. Compounds with C(sp³)—X Bond:

• Allylic Halides: Halogen on sp³ carbon next to a C=C double bond (C=C—C(sp³) —X).

• Benzylic Halides: Halogen on sp³ carbon attached to a benzene ring (Benzene Ring—C(sp³) —X).

2. Compounds with C(sp²)—X Bond:

• Vinylic Halides: Halogen directly on sp² carbon of a C=C double bond (C=C(sp²) —X).

• Aryl Halides: Halogen directly on sp² carbon of an aromatic ring.

IUPAC Nomenclature

Systematic naming uses a structure: Secondary Prefix – Primary Prefix – Word Root – Primary Suffix – Secondary Suffix.

• Word Root: Number of carbons in the main chain (e.g., prop- for 3).

• Primary Suffix: Type of C-C bonds (-ane, -ene, -yne).

• Secondary Suffix: Principal functional group (not for halogens, treated as prefixes).

• Primary Prefix: Cyclic structure (cyclo-).

• Secondary Prefix: Substituents (e.g., methyl, ethyl) and halogens (Fluoro-, Chloro-, Bromo-, Iodo-).

Example: 2-Bromo-3-chlorobutane (Alphabetical order of substituents: Bromo before Chloro).

Common Names

Common names are used for simpler haloalkanes, with prefixes indicating total carbons and branching.

• n-: Normal, unbranched chain.

• iso-: CH₃ group on the second-to-last carbon.

• neo-: Two CH₃ groups on the second carbon.

• sec-: Functional group on a secondary carbon.

• tert-: Functional group on a tertiary carbon.
  Important Polyhaloalkanes: Chloroform (CHCl₃) – forms poisonous phosgene (COCl₂) on exposure to air/light.

Nomenclature of Haloarenes

• IUPAC: Substituent as prefix to "benzene" (e.g., Chlorobenzene) or numbered for di/polysubstituted (e.g., 1,2-Dichlorobenzene).

• Common: ortho (o-) for 1,2-position, meta (m-) for 1,3-position, para (p-) for 1,4-position.

Isomerism in Haloalkanes

• Chain Isomerism: Same molecular formula, different carbon chain structures.

• Positional Isomerism: Same molecular formula and carbon skeleton, different position of substituent.

Nature of the C-X Bond

The C-X bond is polar. Its properties vary down the halogen group:

Methods of Preparation of Haloalkanes

Preparing alkyl halides involves converting various functional groups—most commonly alcohols and hydrocarbons—into halogenated derivatives. These reactions are fundamental because they allow chemists to introduce a reactive halogen atom onto a carbon skeleton, which can then be used to synthesize more complex organic molecules.

1. From Alcohols (R-OH)

• With Hydrogen Halides (HX): R-OH + HX → R-X + H₂O.

• Conc. HCl + anhydrous ZnCl₂ is the Lucas Reagent (Groove's Process). Reactivity: 3° > 2° > 1°.

• With Phosphorus Halides (PCl₃, PCl₅).

• With Thionyl Chloride (SOCl₂): R-OH + SOCl₂ → R-Cl + SO₂(g) + HCl(g). This is the best and most preferred method as gaseous byproducts escape, leaving pure haloalkane (Darzens Process).

2. From Hydrocarbons

A. From Alkanes by Free-Radical Halogenation: R-H + X₂ → R-X + HX (UV light/heat).

• Mechanism: Three steps: Chain Initiation (homolytic cleavage to form radicals), Chain Propagation (radical reaction continues chain), Chain Termination (radicals combine).

• Disadvantage: Forms a mixture of products, difficult to control. Formation of ethane (C₂H₆) is a key side product from methyl radical termination.
  B. From Alkenes - Addition of Hydrogen Halides (HX):

• Symmetrical Alkenes: Add H and X without ambiguity.

• Unsymmetrical Alkenes: Follow Markovnikov's Rule: (Memory Tip: The negative part of the addendum goes to the carbon with the lesser number of hydrogen atoms.). Example: Propene + HCl → 2-chloropropane (major).

• Anti-Markovnikov's Rule (Peroxide Effect): Applies ONLY to HBr in the presence of peroxide. The negative part (Br) goes to the carbon with the greater number of hydrogen atoms. Example: Propene + HBr (peroxide) → 1-bromopropane (major).
  C. From Alkenes - Addition of Halogens (X₂): R-CH=CH-R + X₂ → R-CHX-CHX-R (vicinal dihalide). This reaction serves as a distinction test for unsaturation (decolorizes Br₂/CCl₄).

3. Halogen Exchange Methods (Named Reactions)

Methods of Preparation of Haloarenes

1. From Diazonium Salts

Step 1: Diazotization: Aniline + NaNO₂ + HCl (273-278 K) → Benzene Diazonium Chloride.

Step 2: Halogen Substitution:

• Iodobenzene: Formed by warming diazonium salt with KI.

• Fluorobenzene (Balz-Schiemann): Formed by reacting diazonium salt with HBF₄.

2. Direct Halogenation (Electrophilic Substitution)

Benzene/Toluene + Cl₂/Br₂ (in presence of Lewis acid like FeCl₃/FeBr₃) → Haloarenes. Toluene yields ortho- and para-halotoluene.

Physical Properties of Haloalkanes and Haloarenes

• Boiling Point:

• Increases with molecular mass: R-I > R-Br > R-Cl > R-F.

• Decreases with branching: Less branching, higher BP.

• Density: Increases with molecular mass: R-I > R-Br > R-Cl.

• Solubility: Practically insoluble in water, despite polarity, due to inability to form strong H-bonds.

• Melting Points of Isomeric Dihaloarenes: Para isomer has a significantly higher MP due to its symmetrical structure allowing efficient crystal packing.

Chemical Properties of Haloalkanes

1. Nucleophilic Substitution Reactions (SN1 vs. SN2)

An incoming nucleophile attacks the partially positive carbon of the C-X bond, replacing the halogen.

SN2 Mechanism: Occurs via backside attack of the nucleophile, forming a single transition state. Leads to Walden Inversion. (Memory Tip: Analogy of an umbrella turning inside out in the wind).

SN1 Mechanism: Step 1 (Slow): C-X bond breaks, forming a planar carbocation. Step 2 (Fast): Nucleophile attacks carbocation from either side, leading to a racemic mixture. (Memory Tip: Banti's mom (nucleophile) can approach him (carbocation) from any side).

2. Reactions with Ambidentate Nucleophiles

Nucleophiles like CN⁻ and NO₂⁻ have two donor sites.

• KCN (ionic): Attacks via carbon to form R-CN (Alkyl Cyanide).

• AgCN (covalent): Attacks via nitrogen to form R-NC (Alkyl Isocyanide).

• KNO₂: Attacks via oxygen to form R-O-N=O (Alkyl Nitrite).

• AgNO₂: Attacks via nitrogen to form R-NO₂ (Nitroalkane).

3. Stereochemical Concepts

• Chirality: A molecule or carbon with four different groups.

• Enantiomers: Non-superimposable mirror images.

• Racemization: Formation of a 50/50 mixture of enantiomers (occurs in SN1).

• Inversion: Flipping of configuration (occurs in SN2).

4. Elimination (Beta-Elimination) Reaction

Also called Dehydrohalogenation, it removes H from β-carbon and X from α-carbon, forming an alkene.

• Reagent: Alcoholic KOH (favors elimination), in contrast to Aqueous KOH (favors substitution).

• Zaitsev's Rule: The more substituted alkene is the major product (more stable).

5. Reaction with Metals

• Grignard Reagent: R-X + Mg (dry ether) → R-Mg-X. Highly reactive, reacts with any proton source (e.g., water).

• Wurtz Reaction: 2 R-X + 2Na → R-R (alkane with double carbons).

• Fittig Reaction: 2 Ar-X + 2Na → Ar-Ar (biphenyl).

• Wurtz-Fittig Reaction: R-X + Ar-X + 2Na → Ar-R (alkylbenzene).

Reactivity of Haloarenes Towards Nucleophilic Substitution

Haloarenes are less reactive than haloalkanes due to:

1. Resonance Effect: C-X bond gains partial double bond character, making it stronger.

2. Hybridization Difference: Halogen attached to sp² carbon, which is more electronegative, making the C-X bond shorter and stronger.

3. Instability of Phenyl Cation: SN1 mechanism would involve an unstable phenyl carbocation.

4. Repulsion: Electron-rich nucleophile is repelled by the electron-rich aromatic ring.
   Reactivity can be forced with electron-withdrawing groups (-NO₂) at ortho and para positions.

Electrophilic Substitution Reactions of Haloarenes

Halogens are deactivating but ortho, para-directing groups.

Chapter 2: Alcohols, Phenols, and Ethers

Nomenclature of Alcohols and Phenols

• Alcohols (IUPAC): Parent alkane -e + -ol (e.g., Ethanol). If higher priority group present, use "hydroxy-" prefix.

• Phenols (IUPAC/Common): Benzene + -OH is "Phenol". Substituted phenols named based on this (e.g., 2-Methylphenol or o-Cresol).

• Dihydroxybenzenes: Benzene-1,2-diol (Catechol), Benzene-1,3-diol (Resorcinol), Benzene-1,4-diol (Quinol).

Structure of Alcohols, Phenols, and Ethers

The bond angle around oxygen influences properties:

• Alcohols (C-O-H): ~108.9°

• Phenols: ~109°

• Ethers (C-O-C): ~111.7° – larger due to significant steric repulsion between two bulky alkyl/aryl groups.

Methods of Preparation of Alcohols

1. Acid-Catalyzed Hydration of Alkenes

Addition of H₂O/H⁺ to alkenes. Follows Markovnikov's Rule for unsymmetrical alkenes. Mechanism involves carbocation formation.

2. Hydroboration-Oxidation

Indirect hydration of alkenes using 1. BH₃/THF and 2. H₂O₂/OH⁻. Follows Anti-Markovnikov's Rule, producing terminal alcohols (e.g., Propene → Propan-1-ol).

3. From Carbonyl Compounds and Grignard Reagents (RMgX)

A versatile method:

• Formaldehyde + RMgX → Primary (1°) Alcohol.

• Other Aldehyde + RMgX → Secondary (2°) Alcohol.

• Ketone + RMgX → Tertiary (3°) Alcohol.

4. Reduction of Carbonyl Compounds and Carboxylic Acids

• Aldehydes → Primary (1°) Alcohols.

• Ketones → Secondary (2°) Alcohols.

• Carboxylic Acids → Primary (1°) Alcohols (using LiAlH₄).

Methods of Preparation of Phenols

1. From Haloarenes

Chlorobenzene + aqueous NaOH (high T/P) → Sodium Phenoxide → Phenol (acidification).

2. From Benzene Diazonium Salts

Diazotization of aniline (NaNO₂/HCl, 273-278 K) to form Benzene Diazonium Salt, followed by warming with water.

3. From Cumene (Commercial Method)

Cumene + O₂ → Cumene Hydroperoxide → Phenol + Acetone (by-product) via acidic hydrolysis.

Methods of Preparation of Ethers

1. Dehydration of Alcohols

Product is temperature-dependent with conc. H₂SO₄:

• 443 K: Alkene (Elimination).

• 413 K: Ether (Nucleophilic Substitution of two alcohol molecules).

2. Williamson Synthesis (SN2 Mechanism)

Most important method. R-ONa (Sodium Alkoxide) + R'-X (Alkyl Halide) → R-O-R' (Ether).

• Critical: Best yield when alkyl halide (R'-X) is primary (1°).

• Limitation: Secondary or tertiary alkyl halides favor elimination (alkene formation).

Physical Properties of Alcohols, Phenols, and Ethers

Boiling Point

• Trend: Increases with molecular mass; decreases with branching.

• Comparison: Carboxylic Acids > Alcohols > Ketones ≈ Aldehydes > Ethers > Hydrocarbons.

• Carboxylic acids form stable dimers (strongest H-bonding).

• Alcohols form strong intermolecular hydrogen bonds.

• Aldehydes/Ketones have dipole-dipole interactions.

• Ethers are weakly polar, no H-bonding with themselves.

• Hydrocarbons have only weak van der Waals forces.

Solubility

Lower molecular weight alcohols/phenols/ethers are soluble in water due to hydrogen bonding with water. Solubility decreases as the hydrophobic alkyl/aryl part increases.

Chemical Properties of Phenol

1. Reaction with Zinc Dust

Phenol + Zn (dust) → Benzene (reduction).

2. Nitration

• Dilute HNO₃: Mixture of ortho- and para-nitrophenol.

• Concentrated HNO₃: Forms 2,4,6-trinitrophenol (picric acid).

3. Halogenation

• Br₂ in non-polar solvent (CCl₄/CS₂): Mixture of ortho- and para-bromophenol.

• Bromine water (Br₂ in H₂O): Forms 2,4,6-tribromophenol (white precipitate, characteristic test).

4. Kolbe's Reaction

Phenol + NaOH → Sodium Phenoxide. Then, with CO₂ followed by H⁺ → ortho-hydroxybenzoic acid (salicylic acid).

5. Reimer-Tiemann Reaction

Phenol + CHCl₃ + aqueous NaOH → intermediate → ortho-hydroxybenzaldehyde (salicylaldehyde) after hydrolysis and acidification.

Chemical Properties of Alcohol

Alcohols can act as both nucleophiles (O-H bond breaks) and electrophiles (C-O bond breaks).

A. Reactions as Electrophiles (C-O bond cleavage)

• Oxidation:

• 1° alcohol: Strong agents (KMnO₄, K₂Cr₂O₇) → Carboxylic Acid; Mild agents (PCC) → Aldehyde.

• 2° alcohol: Any agent → Ketone.

• Dehydrogenation (Cu/573 K): 1° alcohol → Aldehyde; 2° alcohol → Ketone; 3° alcohol → Alkene.

• Dehydration (conc. H₂SO₄): 443 K → Alkene; 413 K → Ether.

B. Reactions as Nucleophiles (O-H bond cleavage)

• Acidity: Alcohols are weakly acidic. Water is more acidic than alcohol due to the +I effect of the alkyl group.

• Esterification: Alcohol reacts with carboxylic acids, acid chlorides, or acid anhydrides to form esters.

Lucas Test for Differentiating Alcohols

Uses Lucas Reagent (anhydrous ZnCl₂ in conc. HCl) to distinguish:

• Tertiary (3°) Alcohols: Immediate turbidity.

• Secondary (2°) Alcohols: Turbidity in 5-10 minutes.

• Primary (1°) Alcohols: No turbidity at room temperature.

Chapter 3: Amines

Introduction to Amines

Amines are derivatives of ammonia (NH₃), formed by replacing one, two, or three hydrogen atoms with alkyl or aryl groups.

• Classification:

• Primary (1°) Amine (R-NH₂): One H replaced.

• Secondary (2°) Amine (R₂NH): Two H replaced.

• Tertiary (3°) Amine (R₃N): Three H replaced.

• Structure: Nitrogen in amines is sp³ hybridized with a pyramidal geometry due to a lone pair.

Nomenclature of Amines

Method of Preparation of Amines

1. Reduction of Nitro Compounds

R-NO₂ → R-NH₂ (using Fe + HCl, Sn + HCl, or H₂/catalyst).

2. Reduction of Nitriles

R-C≡N → R-CH₂-NH₂ (using LiAlH₄ or H₂/Ni). Adds a -CH₂- group.

3. Reduction of Amides

R-CO-NH₂ → R-CH₂-NH₂ (using LiAlH₄). Retains the same number of carbons.

4. Hofmann Bromamide Degradation Reaction

R-CO-NH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O.

• Forms a primary amine with one carbon atom less than the starting amide. (Memory Tip: "Slim" after eating a "brownie").

5. Gabriel Phthalimide Synthesis

• Used only for primary aliphatic amines.

• Cannot prepare secondary, tertiary, or aromatic amines.

• Involves Phthalimide + KOH (forms K-salt) + Alkyl Halide (RX) (forms N-alkylphthalimide) + Hydrolysis → R-NH₂. (Memory Tip: "Gabbar" requires KOH and RX).

Physical Properties of Amines

• Boiling Point: Order for isomeric amines: 1° > 2° > 3° (due to decreasing H-bonding capability).

• Solubility: Lower amines are water-soluble (H-bonding). Solubility decreases with increasing carbon chain length.

• Appearance: Aromatic amines are colorless but develop color on storage due to atmospheric oxidation.

Chemical Properties of Amines

1. Basicity of Amines

Due to the lone pair on nitrogen, amines are basic. Basicity is influenced by electron-donating groups (+I effect).

• Gas Phase Basicity: 3° > 2° > 1° (due to cumulative +I effect).

2. Carbylamine Reaction (Isocyanide Test)

Confirmatory test for primary amines (aliphatic and aromatic).

• Primary Amine + CHCl₃ + Alcoholic KOH → Isocyanide (R-NC), which has a very foul smell.

• Secondary and tertiary amines do not give this test.

3. Hinsberg Test

Distinction test for 1°, 2°, and 3° amines using Benzene sulfonyl chloride (Hinsberg's Reagent).

• 1° Amine: Forms N-alkylbenzene sulfonamide (acidic H on N), which is soluble in alkali.

• 2° Amine: Forms N,N-dialkylbenzene sulfonamide (no acidic H on N), which is insoluble in alkali.

• 3° Amine: Does not react.

4. Electrophilic Substitution on Aniline

• Why no Friedel-Crafts Reaction: Aniline, a Lewis base, reacts with the Lewis acid catalyst (AlCl₃) to form a salt. The resulting -NH₃⁺ group is strongly deactivating, preventing FC reaction.

• Nitration: Aniline, in acidic conditions, forms the anilinium ion (-NH₃⁺), which is meta-directing. This leads to a significant amount of meta-nitroaniline (~47%) along with para- (~51%) and ortho- (~2%) products.

• Bromination: Aniline with bromine water (Br₂(aq)) is highly activated, producing 2,4,6-tribromoaniline (white precipitate).

Diazonium Salts

(Ar-N₂⁺X⁻)

Preparation (Diazotization)

Aromatic primary amine (e.g., aniline) + NaNO₂ + HCl at 273-278 K.

Chemical Reactions

Used to synthesize various benzene derivatives:

• Sandmeyer/Gattermann: Chloro- and bromo-benzene.

• KI: Iodobenzene.

• HBF₄ (Balz-Schiemann): Fluorobenzene.

• Warm water: Phenol.

Coupling Reactions (Azo Dye Test)

Diazonium ion (electrophile) attacks phenols/anilines.

• With Phenol (alkaline medium): Forms p-Hydroxyazobenzene (orange dye).

• With Aniline (acidic medium): Forms p-Aminoazobenzene (yellow dye).