Haloalkanes & Haloarenes –
Complete Detailed Notes

6.1 Classification of Haloalkanes & Haloarenes

When one or more hydrogen atoms of an aliphatic or aromatic hydrocarbon are replaced by halogen atoms (F, Cl, Br, I), the resulting compounds are called organohalogen compounds or organic halides.

6.1.1 Based on Number of Halogen Atoms

Visual Examples: Haloalkanes

Visual Examples: Haloarenes

6.1.2 Compounds with sp³ C–X Bond

These are the most important class. Subdivided as:

6.1.3 Compounds with sp² C–X Bond

Special Categories for Dihalides

6.2 Nomenclature

Common vs IUPAC Names — Master Table

NCERT Solved Examples & Practice Questions

Example 6.2 IUPAC Names of Vinylic / Allylic Bromides

Write IUPAC names of the following compounds (structural diagrams from textbook):

▼ Show Solution

(i)

4-Bromopent-2-enepent-2-ene backbone • Br at C4

(ii)

3-Bromo-2-methylbut-1-enebut-1-ene • CH₃ at C2 • Br at C3

(iii)

4-Bromo-3-methylpent-2-enepent-2-ene • CH₃ at C3 • Br at C4

(iv)

1-Bromo-2-methylbut-2-enebut-2-ene • CH₃ at C2 • CH₂Br at C1

(v)

1-Bromobut-2-enebut-2-ene • CH₂Br at C1

(vi)

3-Bromo-2-methylpropenepropene • CH₃ at C2 • Br at C3

6.3 Nature of the C–X Bond

Since halogen atoms are more electronegative than carbon, the C–X bond is polar covalent. The carbon atom carries a partial positive charge (δ+) and the halogen carries a partial negative charge (δ−).

The C–X bond is polar; C is δ+ and X is δ−. The arrow shows direction of electron drift.

Comparison: Haloalkane vs Haloarene C–X Bond

6.4 Methods of Preparation of Haloalkanes

6.4.1 From Alcohols (Most Important)

The –OH group is replaced by halogen using concentrated halogen acids, phosphorus halides, or thionyl chloride. Order of reactivity of alcohols: 3° > 2° > 1°

6.4.2 From Hydrocarbons

6.5 Preparation of Haloarenes

(I) Electrophilic Aromatic Substitution (Halogenation)

Aryl chlorides and bromides are prepared by electrophilic substitution in presence of a Lewis acid catalyst (Fe or FeCl₃/FeBr₃). The halogen acts as the electrophile after activation by Lewis acid.

(II) Sandmeyer's Reaction — From Diazonium Salts

This is the most versatile method for preparing aryl halides from primary aromatic amines (anilines). The amine is first converted to a diazonium salt, then the diazonium group is replaced by halogen.

6.6 Physical Properties

State & Colour

• Methyl chloride (CH₃Cl), methyl bromide (CH₃Br), ethyl chloride (C₂H₅Cl) and some chlorofluoromethanes are gases at room temperature.
• Higher members are liquids or solids.
• Alkyl halides are colourless when pure.
• Bromides and iodides develop colour on exposure to light (due to free halogen formation).
• Many volatile organohalogen compounds have a sweet smell.

Boiling Points

Solubility & Density

• Solubility in water: Very slightly soluble. Energy required to break H-bonds in water > energy released by new intermolecular attractions between haloalkane and water.
• Miscible in organic solvents (like CCl₄, ether, benzene) because new solvent–haloalkane attractions ≈ energy needed to break them apart.
• Density: Bromo, iodo and polychloro compounds are heavier than water. Density increases with number of C atoms, halogen atoms, and atomic mass of halogen. E.g., n-C₃H₇Cl (0.89 g/mL), n-C₃H₇Br (1.335), n-C₃H₇I (1.747).

6.7 Chemical Reactions of Haloalkanes

Three main categories of reactions:

1. Nucleophilic Substitution (SN1 and SN2)
2. Elimination Reactions (β-elimination → alkene formation)
3. Reactions with Metals (Grignard, Wurtz)

6.7a Nucleophilic Substitution: SN1 & SN2

Common Nucleophilic Substitution Products

(a) SN2 Mechanism

The S_N2 reaction (Substitution Nucleophilic Bimolecular) follows second-order kinetics, where the rate depends on the concentration of both the alkyl halide and the nucleophile:
Rate = k [R–X] [Nu⁻] Proposed by Hughes and Ingold in 1937, this mechanism involves a single concerted step with no intermediates.

Factors Affecting S_N2 Rate: Steric Hindrance

The rate of S_N2 reactions is highly sensitive to the size of alkyl groups. Since the nucleophile attacks from the backside, bulky groups nearby create steric hindrance, making it harder for the nucleophile to reach the reaction center. This leads to a dramatic decrease in reactivity:

• Methyl (CH₃X): 30 (Highest rate)
• Primary (1°): 1
• Secondary (2°): 0.02
• Tertiary (3°): ~0 (Almost negligible)

Structure (A) is the mirror image of Structure (B).

If the reactant is optically active, this inversion results in a product with a different spatial orientation. This fundamental principle explains why S_N2 reactions are stereospecific.

(b) SN1 — Substitution Nucleophilic Unimolecular

Rate = k [alkyl halide] — first order kinetics, depends on concentration of only the alkyl halide.

Occurs preferably with tertiary alkyl halides in polar protic solvents (water, alcohol, acetic acid).

Mechanism:

Comparison: SN1 vs SN2 — Master Summary

6.7b Stereochemical Aspects of Nucleophilic Substitution

Optical Activity

Certain compounds rotate the plane of plane-polarised light (produced by a Nicol prism). Such compounds are optically active. The angle of rotation is measured by a polarimeter.

• Dextrorotatory (+) or d-form: Rotates light clockwise.
• Laevorotatory (−) or l-form: Rotates light anticlockwise.
• Such isomers are called optical isomers; the phenomenon is optical isomerism.

Chirality & Asymmetric Carbon

• J. Van't Hoff & C. Le Bel (1874): Independently argued that the spatial arrangement around a central carbon is tetrahedral.
• If all four substituents attached to that carbon are different, the mirror image is non-superimposable.
• Such a carbon is called an asymmetric carbon or stereocentre.

The above test of molecular chirality can be applied to organic molecules by constructing models and its mirror images or by drawing three dimensional structures and attempting to superimpose them in our minds. Let us consider two simple molecules, propan-2-ol and butan-2-ol, and their mirror images.

Propan-2-ol — Achiral Molecule

Observation: Structure A (Propan-2-ol) has two identical groups (–CH₃) on the central carbon. Its mirror image B, when rotated 180° to give C, is superimposable on A. Therefore, Propan-2-ol is achiral (optically inactive).

• No asymmetric carbon: Has two identical groups attached to the central C.
• Mirror image (B), when rotated 180° (structure C), completely overlaps with (A).

Butan-2-ol — Chiral Molecule

Observation: Structure D (Butan-2-ol) has four different groups on the central carbon (–CH₃, –C₂H₅, –OH, –H). Its mirror image E, rotated 180° to give F, is non-superimposable on D.

• Therefore, Butan-2-ol is chiral, and the forms D and F are enantiomers.
• Common Chiral Examples: 2-chlorobutane, 2,3-dihydroxypropanal, bromochloro-iodomethane, 2-bromopropanoic acid.

The stereoisomers related to each other as non-superimposable mirror images are called enantiomers. Enantiomers possess identical physical properties (melting point, boiling point, refractive index) but differ in the direction of optical rotation.

Pair of Chiral Enantiomers — Non-superimposable mirror images

Racemic Mixture & Racemisation

A racemic mixture (or racemic modification) is an equimolar mixture of two enantiomers. It has zero optical rotation because rotations cancel each other out. Represented by prefixing dl or (±). Example: (±)-butan-2-ol.

Racemisation = conversion of an enantiomer into a racemic mixture.

Example 6.8 Identify Chiral and Achiral Molecules

Identify chiral and achiral molecules in each of the following pair of compounds. (Wedge and Dash representations according to Class XI).

▼ Show Solution

Stereochemical Outcomes — 3 Possibilities

SN2 → Inversion of Configuration

Nucleophile attacks from the back of the C–X bond. The three remaining groups flip to the other side (like umbrella inversion). Product has opposite configuration at the asymmetric centre.

• Product has inverted configuration.
• Nucleophile attaches on the side opposite to the leaving halogen.
• Example: (–)-2-bromooctane + ^–OH → (+)-octan-2-ol.

SN1 → Racemisation

Carbocation intermediate is sp² hybridised, planar and achiral. Nucleophile can attack from either face with equal probability → equimolar mixture of (+) and (–) products → racemic mixture.

• Accompanied by racemisation.
• Carbocation is sp² hybridised and planar (achiral).
• Nucleophile attacks from either face → equimolar mixture of (+) and (–) products.
• Example: Hydrolysis of optically active 2-bromobutane → (±)-butan-2-ol.

Retention of Configuration

• Definition: Preservation of the spatial arrangement of bonds to an asymmetric centre during a reaction.
• Occurs when no bond to the stereocentre is broken.
• Example: (–)-2-methylbutan-1-ol + HCl → (+)-1-chloro-2-methylbutane.

• Key Note: Spatial configuration remains the same, but the sign of optical rotation can change (e.g., from – to +).

Important distinction: Same configuration ≠ same sign of rotation. The sign depends on the actual groups and their priority, not just the spatial arrangement.

Three Outcomes when a bond to an asymmetric carbon IS broken:

• If only product (A) is obtained → Retention of configuration
• If only product (B) is obtained → Inversion of configuration
• If a 50:50 mixture of (A) and (B) is obtained → Racemisation

6.7c Elimination Reactions & Reactions with Metals

β-Elimination (Dehydrohalogenation)

When a haloalkane with a β-hydrogen is heated with alcoholic KOH (not aqueous), elimination occurs: H is removed from the β-carbon and X from the α-carbon → alkene is formed.

General β-Elimination:

Example — 2-Bromopentane:

Elimination vs Substitution — Competition

Reactions with Metals

6.7d Reactions of Haloarenes

1. Nucleophilic Substitution — Why Very Difficult?

Haloarenes are extremely unreactive towards nucleophilic substitution. Four reasons:

Nucleophilic Substitution — When It CAN Occur

The presence of strongly electron-withdrawing groups (like –NO₂) at the ortho and para positions significantly activates haloarenes towards nucleophilic substitution.

Example 1 — Chlorobenzene (requires harsh conditions):
C₆H₅Cl  +  NaOH(aq)  623 K, 300 atm  C₆H₅OH (Phenol)  +  NaCl

Example 2 — o-Nitrochlorobenzene (milder conditions):
o-NO₂–C₆H₄–Cl  +  NaOH  443 K  o-NO₂–C₆H₄–OH

Example 3 — 2,4-Dinitrochlorobenzene (even milder):
2,4-(NO₂)₂–C₆H₃–Cl  +  NaOH  368 K, warm H₂O  2,4-(NO₂)₂–C₆H₃–OH

Example 4 — 2,4,6-Trinitrochlorobenzene (mildest conditions):
2,4,6-(NO₂)₃–C₆H₂–Cl  +  H₂O  warm  2,4,6-(NO₂)₃–C₆H₂–OH (Picric acid)

Why ONLY ortho/para NO₂ activates — NOT meta?

When –NO₂ is at ortho or para position to the leaving Cl, the negative charge on the Meisenheimer complex (intermediate carbanion) can be stabilised by the –NO₂ group through resonance. The negative charge at the carbon bearing Cl is delocalised onto the electronegative oxygens of the nitro group.

When –NO₂ is at the meta position, the negative charge in the intermediate does not appear on the carbon bearing the nitro group, so the nitro group cannot stabilise it through resonance → no rate enhancement.

Mechanism of the Reaction

2. Electrophilic Aromatic Substitution (EAS)

Haloarenes readily undergo the usual EAS reactions. Halogen is:

• o,p-directing (due to resonance — lone pairs donated into ring at ortho and para positions)
• Ring deactivating (due to –I inductive effect — withdraws electrons from ring)

(i) Halogenation

Reagent: Cl₂ / Anhydrous FeCl₃
Products: 1,4-Dichlorobenzene (Major) + 1,2-Dichlorobenzene (Minor)

(ii) Nitration

Reagent: HNO₃ / conc. H₂SO₄ (room temp)
Products: 1-Chloro-4-nitrobenzene (Major) + 1-Chloro-2-nitrobenzene (Minor)

(iii) Sulphonation

Reagent: Conc. H₂SO₄ (heat / Δ)
Products: 4-Chlorobenzenesulphonic acid (Major) + 2-Chlorobenzenesulphonic acid (Minor)

(iv) Friedel-Crafts Alkylation

Reagent: CH₃Cl / Anhydrous AlCl₃
Products: 1-Chloro-4-methylbenzene (Major) + 1-Chloro-2-methylbenzene (Minor)

(v) Friedel-Crafts Acylation

Reagent: CH₃COCl / Anhydrous AlCl₃
Products: 4-Chloroacetophenone (Major) + 2-Chloroacetophenone (Minor)

Example 6.9 Although chlorine is an electron withdrawing group, yet it is ortho-, para- directing in electrophilic aromatic substitution reactions. Why?

▼ Show Solution

Chlorine withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution.

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the inductive effect for the attack at ortho- and para- positions and hence makes the deactivation less for ortho- and para- attack.

Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.

3. Reactions with Metals

Wurtz-Fittig Reaction

A mixture of aryl halide + alkyl halide with sodium in dry ether gives an alkylarene.

Ar–X  +  R–X  +  2Na  dry ether  Ar–R  +  2NaX

Example: C₆H₅Br + CH₃Br + 2Na → C₆H₅CH₃ (toluene) + 2NaBr

Fittig Reaction

Two aryl halide molecules with sodium in dry ether join together (two aryl groups coupled).

2Ar–X  +  2Na  dry ether  Ar–Ar  +  2NaX

Example: 2C₆H₅Br + 2Na → C₆H₅–C₆H₅ (biphenyl) + 2NaBr

6.8 Polyhalogen Compounds

Carbon compounds containing more than one halogen atom. Many are industrially and medicinally important but some are environmental hazards.

Chloroform Oxidation — Important Reaction

DDT Structure

✏️ Practice Questions