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Organic Compounds Containing Halogens

Organic Compounds Containing Halogens

Halogen-containing organic compounds are categorized into two main groups:

  • Alkyl Halides: These are aliphatic carbon chains where one or more hydrogen atoms are substituted by halogen atoms.
    Example: Chlorobutane.
  • Aryl Halides: These are aromatic rings with halogen atom(s) directly attached to the ring.
    Example: Chlorobenzene.

Methods of Preparation of Alkyl Halides

1. Grove’s Process

  • Involves replacing the -OH group in primary and secondary alcohols with a halogen (X) using zinc chloride (ZnCl₂).

R−OH + HX → ZnCl2R−X + H2O

Where:

  • R-OH = Alcohol (Primary or Secondary)
  • HX = Hydrogen halide (HCl, HBr, HI)
  • ZnCl₂ = Catalyst
  • R-X = Alkyl halide

The reaction follows the Sₙ2 mechanism when ZnCl₂ concentration is low.

2. Darzens Process

  • Involves the reaction of thionyl chloride (SOCl₂) with primary alcohols.
  • Can proceed with or without pyridine.

Reaction Steps:

With pyridine:

ROH + SOCl₂ → HCl + ROSOCl

HCl + C₅H₅N → C₅H₅NH⁺Cl⁻

  • ROSOCl + Cl⁻ → RCl + SO₂ (Sₙ2 mechanism)

Other Methods for Preparing Alkyl Halides

These reactions are fundamental in organic chemistry and widely applied in synthesis and industrial processes.

  • Using Phosphorus Halides:
    ROH + PCl₅ → RCl + HCl + POCl₃
  • Halogen Addition to Alkenes:
    R-CH=CH₂ + Br₂/CCl₄ → R-CH(Br)-CH₂Br
  • Photohalogenation:
    CH₄ + Cl₂ + hv → CH₃Cl + HCl
  • Halogen Exchange (Finkelstein Reaction):
    RCl + NaI → RI + NaCl
  • Borodine-Hunsdiecker Reaction:
    RCOOAg + Br₂ → RBr + CO₂ + AgBr
  • Hydrohalogenation of Unsaturated Hydrocarbons:
    Without peroxide:
    RCH=CH₂ + HBr → RCH(Br)CH₃
  • With peroxide (Anti-Markovnikov’s Rule):
    RCH=CH₂ + HBr + Peroxide → RCH₂CH₂Br

Methods of Preparing Aryl Halides

1. Halogenation

  • Reaction:
    Ar-H+X2+Lewis Base→Ar-X+HX
  • In this process, an aromatic hydrocarbon (Ar-H) reacts with a halogen (X₂) in the presence of a Lewis acid to form an aryl halide (Ar-X).

2. From Diazonium Salts

Aryl halides can also be synthesized from diazonium salts using the following reactions:

  • Schiemann Reaction:
    C6H5N2Cl+HBF4→C6H5F
  • Sandmeyer Reaction:
    C6H5N2Cl + CuCl→C6H5Cl
  • Gattermann Reaction:
    C6H5N2Cl + Cu powder→C6H5Cl

Sₙ1 and Sₙ2 Mechanism Comparison

Feature

Sₙ1 (Unimolecular Nucleophilic Substitution)

Sₙ2 (Bimolecular Nucleophilic Substitution)

Steps

Two steps:

(1) R-X → R⁺ + X⁻

(2) R⁺ + Nu⁻ → R-Nu

One-step:

R-X + Nu⁻ → R-Nu + X⁻

Rate Equation

Rate = k [RX] (First-order reaction)

Rate = k [RX] [Nu⁻] (Second-order reaction)

Transition State (TS) of Slow Step

Formation of carbocation (R⁺) in the rate-determining step

Single-step reaction with a transition state involving both R-X and Nu⁻ (backside attack)

Stereochemistry

Leads to inversion and racemization

Results in inversion (backside attack)

Molecularity

Unimolecular (depends only on RX)

Bimolecular (depends on both RX and Nu⁻

Reactivity of Alkyl Halide (R-X)

3° > 2° > 1° > CH₃ (favored due to carbocation stability)

CH₃ > 1° > 2° > 3° (hindered by steric effects)

Determining Factor

Stability of carbocation (R⁺)

Steric hindrance in R group

Reactivity of Halide (X)

RI > RBr > RCl > RF

RI > RBr > RCl > RF

Effect of Solvent

Rate increases in polar protic solvents

Rate increases in polar aprotic solvents

Effect of Nucleophile

No effect on rate (nucleophile attacks in second step)

Stronger nucleophile increases the reaction rate (I⁻ > Br⁻ > Cl⁻ > RS⁻ > RO⁻)

Catalysis

Lewis acids (e.g., Ag⁺, AlCl₃, ZnCl₂) may promote ionization

None required

Competing Reactions

Elimination, rearrangement

Elimination

Reactions of Alkyl Halides

  • Hydrolysis:
    RX + OH⁻ → ROH + X⁻
  • Williamson Synthesis:
    R-ONa + R'X → R-O-R' + NaX
  • Reaction with Dry Silver Oxide:
    2R-X + Ag₂O → R-O-R
  • Reaction with Sodio-Alkynides:
    R-C≡C-Na + X-R → R-C≡C-R + NaX
  • Reaction with Potassium Cyanide:
    KCN + R-X → RCN + KX
  • Reaction with Silver Cyanide:
    AgCN + R-X → RNC + AgX
  • Reaction with Silver Nitrite:
    AgNO₂ + R-X → RNO₂ + AgX
  • Reaction with Potassium Nitrite:
    KNO₂ + R-X → R-O-N=O + KX
  • Friedel-Crafts Reaction:
    R-X + C₆H₆ + AlCl₃ → C₆H₅-R
  • Malonic Ester Synthesis:
    R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
  • Acetoacetic Ester Synthesis:
    R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
  • Reaction with Ammonia:
    R-X + NH₃ → R-NH₂ + HX
  • Wurtz Reaction:
    2R-I + 2Na → R-R + 2NaI
  • Dehydrohalogenation:
    CH₃CH₂CH₂Br + alco.KOH → CH₃-CH=CH₂ + KBr + H₂O
  • Reaction with Alcoholic AgNO₃:
    R-X + AgNO₃ → R⁺ + AgX↓ + HNO₃

Haloform Reaction (Trihalide Reaction)

Preparation

  • Chloroform (CHCl₃) can be prepared from alcohols containing the –CH(OH)CH₃ group or from aldehydes and ketones with three α-hydrogen atoms, using halogen (X₂) and an alkali or Na₂CO₃.

Substitution / Elimination

CH₃X

RCH₂X (1°)

R₂CHX (2°)

R₃CX (3°)

Methyl

Reaction Type

Bimolecular reactions only

Bimolecular and unimolecular

Sₙ1/E1 or E2

Reactivity

Gives Sₙ2 reactions

Mainly Sₙ2, but with hindered strong bases (e.g., (CH₃)₃CO⁻), E2 dominates.

Mainly Sₙ2 with weak bases (I⁻, CN⁻, RCO₂⁻), and E2 with strong bases (RO⁻).

Frequently Asked Questions