SN1 & SN2 Reactions

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Class 12th Chemistry: Haloalkanes and Haloarenes

• Subject is Chemistry, specifically Haloalkanes and Haloarenes for Class 12th.
• Covers topics relevant to board exams.

Nucleophilic Substitution Reactions

• Involve nucleophiles, defined as nucleus-loving species.
• Necessary conditions: Nucleophile must have a complete octet and at least one lone pair.
• Examples of nucleophiles: Cl⁻, OH⁻, SH⁻, N⁻³, H₂O, NH₃, OH⁻
• Reactions can occur in two methods, denoted as Sₙ1 and Sₙ2.
• Sₙ1 Unimolecular Nucleophilic Substitution:
  • Molecularity is 1.
• Sₙ2 Biomolecular Nucleophilic Substitution:
  • Molecularity is 2.
• Sₙ1 reactions:
  • Occur in two steps with an intermediate step.
  • Rate of reaction is proportional to the concentration of the alkyl halide.
  • Molecularity is 1; the order of reaction is 1.
• Sₙ2 reactions:
  • Occur in one step through a transition state.
  • Rate of reaction is proportional to the concentration of both the alkyl halide and the nucleophile.
  • Molecularity is 2; the order of reaction is 2.

Factors Affecting Sₙ1 and Sₙ2 Reactions

• Rate of Reaction:
  • For Sₙ1, the rate depends on the stability of the carbocation intermediate.
  • For Sₙ2, the rate is influenced by the bulkiness of the substituents.
• Effect of Nucleophile:
  • Sₙ1 reactions are not affected by the strength of the nucleophile.
  • For Sₙ2 reactions as nucleophilicity increases from right to left.
  • Down the group increases Nucleophilicity.
• Effect of Solvent:
  • Sₙ1 reactions favor polar protic solvents.
  • Sₙ2 reactions favor polar aprotic solvents like DMSO (dimethyl sulfoxide).
• Solvent Types:
  • Polar solvents have a dipole moment.
  • Non-polar solvents include hydrocarbons and carbon tetrachloride (CCl4).

Leaving Group Effects and Stereochemistry in Sₙ1 vs. Sₙ2

• Leaving Group:
  • Sₙ1 : Good leaving groups are neutral molecules or weak bases
  • Sₙ2 : Down the group, leaving group tendency increases, with the leaving tendency related to stability.
• Stereochemistry:
  • Sₙ1 : Both sides attack with the possibility of forming enantiomers, leading to a racemic mixture.
  • Sₙ2 : Only back-side attack is possible, resulting in Walden inversion.
• Substrate Effects:
  • Sₙ1 reactions occur with bulky groups
  • Sₙ2 reactions prefer less bulky groups.
  • Molecules with structure Ⓒ = C - X do not show Sₙ1 and Sₙ2 reactions

Alkyl Halide Formation and Reactions

• Swarts reaction:
  • Used for alkyl fluoride formation, involving alkyl halides reacting with metal fluorides (AgF, SbF3, Hg₂F₂).
• Alkyl Iodide Formation:
  • Achieved using CH₄ + I₂ ⇌ CH₃-I + HI but the equilibrium must be shifted forward by oxidizing agents.
• Reaction with Alcohols:
  • Alcohols react with PX₃ (where X is a halogen) to form alkyl halides.
  • 3ROH + POCl₃ → 3R-Cl + P(OH)₃ is an alternative reaction.
• Thionyl Chloride (SOCl2):
  • SOC12 (Darzens Method) converts alcohols to alkyl chlorides with retention or inversion of configuration, depending on conditions
• Aryl Halides:
  • Aromatic halides undergo nucleophilic substitution at high temperatures and pressures
  • This is particularly enhanced in the presence of multiple meta-directing groups.
• Preparation Methods
  • Addition of HBr in Alkene, and Bromination in Allylic position using N-Bromosuccinimide (NBS).

Chemical Properties and Reactions of Haloarenes

• Nucleophilic substitution reactions, also known as electrophilic aromatic substitution.
• Examples: Halogenation, Nitration to add NO₂⁺, Sulfonation to add SO₃H, Friedel-Crafts alkylation and acylation.
• Rate of Nucleophilic substitution of chloro-benzene requires high temperatures and high pressure.

Additional Reactions

• Wurtz Reaction: Involves the reaction of alkyl halides to form alkanes
• Wurtz-Fittig Reaction: Combines an aromatic and an alkyl halide to form substituted aromatic compounds.
• Fittig Reaction: Reacts two aromatic halides to form biaryl compounds
• Sandmeyer Reaction:
  • Involves the conversion of aromatic diazonium salts to aryl halides using copper halides.
• Specific Reagents: Converts complex compounds to fluorides with specific reagents (HBF4, HSbF6).
• DDT Formation:
  • DDT is formed from the reaction of chlorobenzene with chloral (0 = CH - CC13) in the presence of concentrated H2SO4.

Physical Properties of Alkyl Halides

• Boiling Point:
  • Boiling point is directly proportional to molecular weight
  • Branching decreases the boiling point.
• Melting Point:
  • Melting point increases with better packing in the solid state.

Test your knowledge of SN1 and SN2 reactions. Learn about nucleophiles, solvents, and reaction rates. Explore stereo-chemical outcomes and reaction mechanisms.