Halogenoalkanes

Introduction

• Halogenoalkanes, also known as alkyl halides, are compounds where one hydrogen atom of alkanes is replaced by a halogen atom.

• Known as halogen derivatives of alkanes.

• General formula for halogenoalkanes is R-X, and for halogenoarenes is Ar-X (where Ar represents an aromatic group).

Classification of Alkyl Halides

• Alkyl halides can be classified as:
    - Primary Alkyl Halides:
      - Definition: Alkyl halides where the halogen atom is attached to a primary carbon atom.
      - Primary carbon is bonded to only one other carbon atom or none (e.g., methane).
      - Example: Methyl chloride (Chloromethane, $CH_3Cl$) and n-Propyl chloride (Chloropropane, $CH_2CH_2CH_2Cl$).
  
    - Secondary Alkyl Halides:
      - Definition: Alkyl halides where a halogen is attached to a secondary carbon atom.
      - Secondary carbon is bonded to two other carbon atoms.
      - Example: Iso-Propyl chloride (2-Chloropropane, $CH_3CHClCH_3$) and Sec-Butyl chloride (2-Chlorobutane, $CH_3C(Cl)(CH_3)CH_2CH_3$).
  
    - Tertiary Alkyl Halides:
      - Definition: Alkyl halides where a halogen atom is attached to a tertiary carbon.
      - Tertiary carbon is bonded to three alkyl groups.
      - Example: T-Butyl chloride (2-Chloro-2-methylpropane, $C_4H_9Cl$).

• Physical Properties: Alkyl halides have higher melting and boiling points compared to alkanes.

Student Learning Outcomes (SLOs)

• Describe the production of halogenoalkanes, e.g., the reaction of benzene with Cl₂ and Br₂ in the presence of a catalyst.

Preparations of Halogenoarenes

Halogenation of Benzene

• Halogenation occurs with halogens (X₂) in the presence of a Fe or Lewis acid catalyst (e.g., $FeX_3$).

• Reaction equation:
    $C_6H_6 + X_2 <br>ightarrow C_6H_5X + HX$
      - Where X can be Br or Cl, yielding products like Bromobenzene and Chlorobenzene with by-products HBr or HCl respectively.

Reactivity of Halogenoalkanes

Factors Influencing Reactivity

• The strength of the C-X bond significantly affects the reactivity of alkyl halides and aryl halides.

• Due to the higher electronegativity of halogens compared to carbon, the C-X bond is polarized:
    - Carbon receives a partial positive charge ($C^+$)
    - Halogens receive a partial negative charge ($X^-$)

• This polarization makes halogen an electron-attracting (nucleophilic) species which can be replaced during reactions by stronger nucleophiles.

Example: Reactivity of Chloroethane vs. Chlorobenzene

• Chloroethane:
    - Undergoes nucleophilic substitution reaction via the mechanism:
      $RCH_2CH_2Cl + Nu^- <br>ightarrow RCH_2CH_2Nu + Cl^−$
    - Nucleophile attacks the carbon attached to the chlorine atom, replacing chlorine.

• Chlorobenzene:
    - The resonance effect delocalizes electrons influencing the C-C bond character as partial double bond (makes breaking C-Cl bond more difficult).
    - Chlorobenzene undergoes electrophilic substitution instead.
    - Example: Nitration of chlorobenzene yields o-nitrochloro-benzene and p-nitrochloro-benzene:
      - $C_6H_4Cl + HNO_3 + H_2SO_4 <br>ightarrow C_6H_4(NO_2)Cl + H_2O$

Nucleophilic Substitution Mechanisms (SN1 and SN2)

General Concepts

• Alkyl halides undergo two main types of reactions:
    - Nucleophilic Substitution Reactions (SN Reactions)
    - Elimination Reactions

Nucleophilic Substitution (SN) Explanation

• A nucleophilic substitution reaction involves replacing the halogen atom in an alkyl halide
  aided by a strong nucleophile.

Definitions:

  - Substrate: Alkyl halide undergoing nucleophilic attack.
  - Nucleophile: Species with lone pairs that donates a pair of electrons to the electrophilic carbon of the alkyl halide.
  - Leaving Group (LG): The departing halogen atom, which is also a nucleophile.

Nucleophiles Examples

• Weak Nucleophiles: H₂O, NH₃, Alcohols, etc.

• Strong Nucleophiles: OH⁻ (Hydroxide), Br⁻ (Bromide), CN⁻ (Cyanide), etc.

Mechanism Types (SN1 vs. SN2)

SN1 Mechanism

• Definition: Unimolecular nucleophilic substitution occurs in two steps.

• Steps:
    - 1st Step: Ionization of alkyl halide forming a carbocation intermediate and halide (this is the rate-determining step).
      $R-X <br>ightarrow R^+ + X^-$
    - 2nd Step: Nucleophile attacks the carbocation.

• Rate Expression:
    $ext{Rate} = k[R-X]$ (depends only on the substrate)

SN2 Mechanism

• Definition: Bimolecular nucleophilic substitution mechanism occurs in a single concerted step.

• Steps:
    - Nucleophile attacks the electrophilic carbon and simultaneously, the leaving group departs.
      $Nu + R-X <br>ightarrow R-Nu + X^-$

• Rate Expression:
    $ext{Rate} = k[Nu][R-X]$ (depends on both the substrate and nucleophile)

Factors influencing SN1 vs. SN2

1. Electrophile Nature:
      - Primary carbon→favours SN2.
      - Tertiary carbon→favours SN1.

2. Nucleophile Strength: Strong nucleophiles favour SN2; weaker nucleophiles favour SN1.

3. Solvent:
      - Polar Protic solvents favour SN1 (stabilize carbocations).
      - Polar Aprotic solvents favour SN2.

Elimination Reactions

E1 and E2 Mechanisms

• Elimination reactions lead to the formation of double bonds.

• E1 Mechanism:
    - Two-step process, starts with the formation of a carbocation (similar to SN1).
    - $R-X <br>ightarrow R^+ + X^- <br>ightarrow Alkene + H^+$

• E2 Mechanism:
    - One-step process where a base abstracts a proton as the leaving group leaves.
    - $R-X + B^- <br>ightarrow Alkene + B^+ + X^-$

Comparing SN vs. E Reactions

• Reactivity conditions differ based on substrate type (primary, secondary, tertiary) and specific nucleophile/base strength.

• Primary haloalkanes: Generally undergo SN2 reactions.

• Secondary haloalkanes: Can undergo SN2 or E2 depending on the reactants.

• Tertiary haloalkanes: Favor E2 when a strong base is present but SN1 or E1 when a weaker base/nucleophile is involved.

Reactivity on the Order of C-X Bonds

• Reactivity of haloalkanes assessed through their reaction with aqueous silver nitrate (AgNO₃).

• Reactivity Order: RI > RBr > RCl > RF (based on the strength of C-X bonds).

• Observations:
    - $R-X + Ag^+ <br>ightarrow R-NO_3 + AgX$ yielding precipitates (white, cream, or yellow depending on halide).

Retro-Synthesis with Halogenoalkanes

Definition

• Retrosynthesis: Method used in organic chemistry to deduce starting materials by working backward from the product.

Example of Retro-Synthesis Pathway

• For the synthesis of ethanol:
    - Start with Ethanol $CH_3CH_2OH$ and propagate back to identify halogenoalkane.
    - $CH_3CH_2Br + OH^- <br>ightarrow CH_3CH_2OH + Br^-$
  

Summary of Learning Points

• Understanding mechanisms of halogenoalkanes and their reactivity patterns is crucial for organic synthesis.

• Various factors including solvent, nucleophile strength, and carbon structure influence reaction outcomes.

Multiple Choice Questions (MCQs)

Sample Questions

1. Which catalyst is commonly used in the halogenation of benzene with Cl₂ or Br₂?
      - a) H2SO4
      - b) FeCl3
      - c) AlCl3
      - d) NaOH
      Answer: b)

2. What is the major product of the reaction between benzene and Br₂ with FeBr3 as a catalyst?
      - a) Benzene bromide
      - b) Bromobenzene
      - c) Chalorobenzene
      - d) Dichlorobenzene
      Answer: b)

3. Which mechanism do primary halogenoalkanes typically follow in nucleophilic substitution reactions?
      - a) SN1
      - b) SN2
      - c) Both SN1 and SN2
      - d) None of these
      Answer: b)

Short Answer Questions

1. Describe the halogenation of benzene.
      - It involves benzene reacting with halogens in presence of a Lewis acid catalyst which facilitates the electrophilic substitution.

2. Explain the difference between SN1 and SN2 mechanisms.
      - SN1 is unimolecular and involves a carbocation intermediate, whereas SN2 is bimolecular and occurs in one concerted step.

Long Answer Questions

1. Explain the mechanisms of SN1 and SN2 reactions in detail, including the factors that affect each mechanism and examples of substrates that prefer each pathway.
      - SN1 involves carbocation formation and is influenced by stability, while SN2 prefers less steric hindrance and strong nucleophiles.

2. Compare and contrast the reactivity of halogenoalkanes and halogenoarenes with examples. Discuss the factors contributing to their differing reactivities.
      - Halogenoalkanes react easily via nucleophilic substitution due to bond polarity, in contrast to halogenoarenes, which are stabilized by resonance effects affecting reactivity patterns.