10.6. Chemical Reactions

A. Nucleophilic Substitution Reactions

• SN1 (Unimolecular) Mechanism

  • Two steps: formation of carbocation, then attack by nucleophile.
  • Favoured in tertiary haloalkanes.
  • Rate depends on the concentration of the substrate.

• SN2 (Bimolecular) Mechanism

  • One-step mechanism; nucleophile attacks while the halide leaves.
  • Favoured in primary haloalkanes.
  • Rate depends on both substrate and nucleophile.

• Common Nucleophilic Substitution Reactions:

  • RX + OH⁻ → ROH (Alcohol)
  • RX + CN⁻ → RCN (Nitrile)
  • RX + NH₃ → RNH₂ (Amine)

Nucleophilic substitution reactions involve the replacement of a halogen (X) in haloalkanes with another nucleophile (a molecule that donates an electron pair). There are two main mechanisms:

1. SN1 (Substitution Nucleophilic Unimolecular): This mechanism occurs in two steps. First, the halogen leaves, creating a carbocation (a positively charged carbon). Then, a nucleophile attacks this carbocation. This mechanism is favored in tertiary haloalkanes because they form stable carbocations. The rate of the reaction depends only on the concentration of the substrate (the haloalkane).

2. SN2 (Substitution Nucleophilic Bimolecular): This is a one-step process where the nucleophile attacks the molecule as the halide simultaneously leaves. This mechanism is favored in primary haloalkanes because they do not form stable carbocations. The rate of the reaction depends on the concentrations of both the substrate and the nucleophile.

Common nucleophiles include hydroxide (OH⁻), cyanide (CN⁻), and ammonia (NH₃), which can replace the halogen with functionalities like alcohols, nitriles, and amines, respectively.

Think of the SN1 mechanism like a game of musical chairs. The halogen (the one who leaves) is like a player who gets eliminated while the music (the nucleophile) starts playing. In the SN2 mechanism, it’s like a relay race: as one runner (the halogen) passes the baton (the carbon), the next runner (the nucleophile) is already on the way to grab it, resulting in a seamless handoff.

B. Elimination Reactions

• Dehydrohalogenation: RX + alcoholic KOH → Alkene + KX + H₂O

Elimination reactions involve the removal of a halogen and a hydrogen atom from adjacent carbon atoms in a haloalkane, resulting in the formation of an alkene. A common method of this reaction is dehydrohalogenation, where using a strong base like alcoholic potassium hydroxide (KOH), the reaction proceeds to form an alkene, a salt (KX), and water (H₂O). This process is important for synthesizing alkenes, which are crucial for further chemical reactions.

You can think of dehydrohalogenation as cleaning a messy desk. Imagine the halogen and hydrogen as clutter on your desk, and using a strong base (the cleaning tool), you clear away the clutter to reveal a clean surface (the alkene) ready for new activities.

C. Reaction with Metals

• Wurtz Reaction: 2RX + 2Na → R–R + 2NaX (In dry ether)

The Wurtz reaction is a coupling reaction where two haloalkanes react with sodium metal (Na) in a dry ether solvent, resulting in the formation of a new carbon-carbon bond between the two haloalkanes. This reaction is particularly useful for creating larger alkane molecules from smaller ones by combining them. However, the reaction often yields a mixture of products because the reaction can occur between different haloalkane pairs, affecting the final outcome.

Imagine you're building a tower using blocks. Each block represents a haloalkane. When you use a special glue (sodium), you can connect blocks together to create larger structures (new alkanes). But sometimes, you might accidentally stick the wrong blocks together, resulting in a mix of structures (different products).

D. Reactions of Haloarenes

• Nucleophilic Substitution

  • Much less reactive than haloalkanes due to resonance and partial double bond character.
  • React under drastic conditions (heat, pressure).
  • Example: C₆H₅Cl + NaOH (300°C, 200 atm) → C₆H₅OH

• Electrophilic Substitution

  • Halogen is electron-withdrawing but ortho/para directing.
  • Reactions:
    • Nitration: C₆H₅Cl + HNO₃ → o- and p-chloronitrobenzene
    • Sulphonation, Friedel-Crafts alkylation/acylation follow similar patterns.

Haloarenes (halo compounds attached to benzene rings) have unique properties that affect their reactivity.

1. Nucleophilic Substitution: Compared to haloalkanes, haloarenes are much less reactive because of resonance stabilization; the presence of double bonds in the aromatic ring makes the carbon-halogen bond harder to break. These reactions often require extreme conditions like high heat and pressure. An example is when chlorobenzene reacts with sodium hydroxide under such conditions to form phenol (C₆H₅OH).

2. Electrophilic Substitution: Whereas nucleophilic substitution is more difficult with haloarenes, they can easily undergo electrophilic substitution reactions where electrophiles (electron-deficient species) attack the aromatic ring. The halogen decreases the electron density but directs new substituents to the ortho and para positions. Common reactions include nitration and sulfonation, where new functional groups replace hydrogen atoms in the ring.

You can think of nucleophilic substitution in haloarenes like trying to take a toy out of a tightly packed box (the aromatic ring) where everything is interlocked. It requires a lot of effort (high conditions) to remove it. For electrophilic substitution, imagine that the box has openings (ortho/para positions) where new toys (substituents) are added. While it's a little easier compared to removal, the toys can only go into those specific spots.

--