Halogen Derivatives
Easy Overview
Take a regular hydrocarbon and slap a halogen (Cl, Br, I, F) on it. That's a halogen derivative. These compounds show up in everything from anaesthetics to pesticides to Teflon pans. And the reactions they do? SN1 vs SN2 is the eternal board-exam debate.
Classification and properties
Alkyl halides (R-X), vinyl halides (C=C-X), aryl halides (X on benzene). The C-X bond is polar — halogen pulls electrons. That makes the carbon slightly positive and vulnerable to attack. Boiling points increase with the size of the halogen. Iodo compounds boil higher than chloro compounds.
SN1 reactions
Substitution Nucleophilic Unimolecular. This happens in two steps. First, the halogen leaves on its own (slow step). Then the nucleophile attacks the carbocation (fast). Works best with tertiary carbons (3 alkyl groups). It's like an old building — first the roof caves in, then new tenants move in.
SN2 reactions
Substitution Nucleophilic Bimolecular. One step — the nucleophile attacks from behind while the halogen leaves from the front. Like a revolving door — one person goes in, the other comes out. Works best with primary carbons. The reaction inverts the molecule (Walden inversion) — like turning an umbrella inside out.
Elimination reactions (E1 and E2)
Sometimes instead of replacing the halogen, you just remove it along with a neighboring hydrogen — forming a double bond. E1 is like SN1 (carbocation intermediate). E2 is like SN2 (one step). Strong bulky bases favor elimination. Small strong bases favor substitution. It's a competition.
Important halogen compounds
Chloroform (CHCl3) was used as an anaesthetic — now it's a lab solvent. DDT is a controversial pesticide. Freons (CFCs) were in fridges but wrecked the ozone layer. Teflon (PTFE) is polymerized CF2=CF2 — non-stick, heat-resistant. Halothane is still used as an anaesthetic in medicine.
Key Points
- •SN1: 2 steps, carbocation intermediate, tertiary best, racemization
- •SN2: 1 step, backside attack, primary best, inversion
- •E1: elimination via carbocation, favors bulky bases
- •E2: elimination in one step, requires strong base
- •Polar solvents favor SN1; aprotic solvents favor SN2
- •Alkyl halides react with Mg to form Grignard reagents (R-Mg-X)
- •CFCs deplete ozone; now replaced by HFCs
Practice Questions
- Distinguish between SN1 and SN2 reactions with examples.
- Explain the mechanisms of E1 and E2 elimination reactions.
- Why do tertiary alkyl halides undergo SN1 reactions more readily than primary?
- What are Grignard reagents? How are they prepared?