Chapter 13 · Class 11-12 & JEE/NEET

Haloalkanes & Haloarenes

23 topics in 6 parts

Full lessonClass 12 Chemistry · Haloalkanes, Alcohols and Phenols

Key ideas

Diagnostic Comparison & Property Matrix

Parameter / Feature	Standard State / Condition A	Altered State / Condition B
Driving Force	Thermodynamic control (more stable product)	Kinetic control (lower activation barrier)
Solvent Dependency	Polar protic solvent stabilizes charge separation	Polar aprotic solvent accelerates bare nucleophilicity
Temperature Influence	High temperatures favor elimination / endothermic	Mild temperatures favor addition / exothermic
Common Pitfall	Overlooking rearrangement or stereochemical inversion	Neglecting boundary limits or stoichiometric coefficients

Conceptual Foundation & Core Theory

In organic synthesis, the hydroxyl group (-OH) of an alcohol is notoriously difficult to substitute directly because hydroxide (OH⁻) is a strong base and therefore an exceptionally poor leaving group. To overcome this fundamental kinetic barrier, chemists convert the alcohol into a sulfonate ester—most commonly a tosylate (R-OTs) using p-toluenesulfonyl chloride (TsCl) or a mesylate (R-OMs) using methanesulfonyl chloride (MsCl).

Stereochemical Mechanics & Bond Cleavage: During the formation of the tosylate, the oxygen atom of the alcohol attacks the electrophilic sulfur atom of TsCl in the presence of pyridine (which acts as a base to scavenge the liberated HCl). Crucially, the carbon-oxygen (C—O) bond of the alcohol is never broken during this step; only the O—H bond is cleaved. Consequently, if the starting alcohol possesses a stereocenter, the configuration is preserved with 100% retention.

Subsequent Substitution: In the second step, when a nucleophile (Nu:⁻ such as CN⁻, I⁻, or N₃⁻) attacks the alkyl tosylate via an S_N2 mechanism, attack occurs from the backside. The tosylate anion (TsO⁻) is expelled as a phenomenal leaving group because its negative charge is heavily delocalized across three electronegative oxygen atoms via resonance. This backside displacement causes an inversion of stereochemical configuration (Walden Inversion).

Standard Reaction & Reagent Conditions

// Synthesis & Reactions of Tosylates & Mesylates

1. Tosylate Formation (Retention of Configuration):
   R-OH  +  TsCl  ──[ Pyridine / 0°C ]──>  R-OTs  +  [PyH]⁺Cl⁻
   • Note: C-O bond is NOT broken. Stereochemistry is RETAINED.

2. Mesylate Formation:
   R-OH  +  CH3SO2Cl (MsCl)  ──[ (C2H5)3N ]──>  R-OMs  +  Et3NH⁺Cl⁻

3. Nucleophilic Attack (Walden Inversion):
   (R)-R-OTs  +  NaCN  ──[ DMSO / SN2 ]──>  (S)-R-CN  +  NaOTs

4. Relative Leaving Group Ability:
   Triflate (-OTf) > Tosylate (-OTs) > Mesylate (-OMs) >> I⁻ > Br⁻ > Cl⁻ >> F⁻ >> OH⁻

Haloalkanes & Haloarenes

Leaving Group Ability & Stereochemical Comparison

Leaving Group Derivative	Reagents Used	Stereochemistry at C	Relative Leaving Group Rate
Tosylate (R-OTs)	p-Toluenesulfonyl chloride (TsCl) + Pyridine	Retention during formation; Inversion upon SN2 attack	10,000× faster than Br⁻
Mesylate (R-OMs)	Methanesulfonyl chloride (MsCl) + Et3N	Retention during formation; Inversion upon SN2 attack	Comparable to Tosylate (excellent)
Triflate (R-OTf)	Trifluoromethanesulfonic anhydride (Tf2O)	Retention during formation; Inversion upon SN2 attack	Super leaving group (100,000× faster)
Unmodified Alcohol (R-OH)	None (Direct substitution attempts)	Reaction fails without strong acid catalyst	Extremely poor (OH⁻ is strong base)

Step-by-Step Solved Problem (JEE Advanced Standard)

Problem: (R)-butan-2-ol is treated with TsCl in pyridine to form compound A. Compound A is subsequently reacted with NaSH in DMF. Deduce the IUPAC name and stereochemical configuration (R or S) of the final product B.

Step 1: (R)-butan-2-ol + TsCl / Pyridine -> Formation of (R)-sec-butyl tosylate (Compound A). The C-O bond remains intact, so configuration is (R).

Step 2: Reaction with NaSH (strong nucleophile, SH⁻) in polar aprotic solvent (DMF) proceeds via an SN2 pathway.

Step 3: Backside nucleophilic attack causes complete Walden Inversion at the chiral carbon.

Final Result: Compound B is (S)-butan-2-thiol.

Checkpoint

1. When (2R)-octan-2-ol is reacted with TsCl in pyridine followed by reaction with NaBr in acetone, the final product is:

All 23 topics in Haloalkanes & Haloarenes

Preparation Methods of Alkyl Halides
- Free Radical Halogenation
- Addition of Hydrogen Halides to Alkenes
- Halogen Exchange Reactions (Finkelstein & Swarts Reactions)
- Preparation of Alkyl Halides from Alcohols
- Monochlorination Products
Properties
- Physical Properties of Alkyl Halides
- Nature of C-X Bond
- Halide Classification & Structure
- Optical Activity & Stereochemistry
Reactions of Alkyl Halides
- Nucleophilic Substitution Reactions (SN1 & SN2 Mechanisms)
- Stereochemical Aspects of Nucleophilic Substitution Reactions
- Elimination Reactions (Dehydrohalogenation & Saytzeff Rule)
- Reactions with Metals (Grignard Reagents & Wurtz Reaction)
- Nucleophilicity & Leaving Group Ability
- Halide Hydrolysis
- Reactivity of Allylic & Benzylic Halides
- Elimination vs Substitution
Preparation Methods of Aryl Halides
- Electrophilic Substitution (Halogenation of Arenes)
- From Diazonium Salts (Sandmeyer Reaction)
Reactions of Aryl Halides
- Nucleophilic Aromatic Substitution (SNAr Mechanism)
- Electrophilic Substitution Reactions of Aryl Halides
- Reaction with Metals (Wurtz-Fittig & Fittig Reactions)
Polyhalogen Compounds
- Properties, Uses & Environmental Effects