Halogenoalkanes: SN1/SN2 and Elimination

Detailed Explanations

A. Why nucleophiles attack the carbon (the key causal chain)

Because the C-X bond is polarised (C is δ +), therefore electron-rich nucleophiles donate a lone pair to the carbon to form a new C–Nu bond.

B. Substitution: what the product depends on

Common nucleophiles and what they form:

• OH⁻ → alcohol
• CN⁻ → nitrile (adds one carbon to the chain)
• NH₃ → amine (often tested as “ammonia in ethanol”)

Example (hydrolysis): CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

Example (chain extension): CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻

C. SN2 (one-step substitution)

SN2 is a single-step mechanism:

• nucleophile attacks as the halide leaves
• it works best when the carbon is not crowded (primary > secondary >> tertiary)

What to say in words:

• “The nucleophile donates a lone pair to the δ + carbon as the C-X bond breaks, forming X⁻.”

If the reacting carbon is chiral, backside attack turns the tetrahedral arrangement inside out. This inversion of configuration is the stereochemical result expected for SN2.

D. SN1 (two-step substitution)

SN1 typically happens for tertiary halogenoalkanes:

• Step 1: C-X bond breaks to form a carbocation (slow step)
• Step 2: nucleophile attacks the carbocation (fast)

What to say in words:

• “A carbocation forms first; then the nucleophile attacks.”

The carbocation is trigonal planar, so a nucleophile can attack from either face. Starting from one enantiomer therefore gives both configurations and, when the two paths are equally likely, a racemic mixture.

E. SN1 vs SN2 (comparison table)

F. Elimination vs substitution (conditions rule)

To form an alkene, use:

• ethanolic KOH or NaOH
• heat (often reflux)

Example: CH₃CH₂CH₂Br + OH⁻ → CH₃CH = CH₂ + H₂O + Br⁻

Key idea to write:

• “The base removes a proton from a carbon adjacent to the halogen (a β-hydrogen), and the double bond forms as the halide leaves.”

G. Workflow: decide substitution vs elimination quickly

1. Read the conditions: aqueous vs ethanolic? heat/reflux?
2. If nucleophile is OH⁻:
   • aqueous OH⁻ → substitution
   • ethanolic OH⁻ + heat → elimination
3. If the question asks SN1 vs SN2, use the halogenoalkane type (primary/secondary/tertiary) and rate information.

Mini example: “Ethanolic KOH, heat under reflux” is an elimination trigger phrase.

H. Halogenoarenes are different

In a halogenoarene such as chlorobenzene, a lone pair on chlorine overlaps with the benzene π system. The C–Cl bond has partial double-bond character and is shorter and stronger than the corresponding bond in a halogenoalkane. The carbon is also sp²-hybridised. Chlorobenzene therefore resists the usual SN1 and SN2 hydrolysis conditions; do not transfer halogenoalkane mechanisms to it.

I. Fluorinated compounds and environmental choices

C–F bonds are strong, which makes many fluorinated compounds stable. This is useful in applications such as non-stick coatings, refrigerants and some medicines, but stability can also make compounds persistent.

CFCs release chlorine radicals in the stratosphere and catalyse ozone destruction. HCFCs contain C–H bonds and tend to break down sooner, so their ozone-depletion effect is lower but not zero. HFCs contain no chlorine and do not deplete ozone, although many are strong greenhouse gases. Compare ozone depletion and global warming as separate environmental effects.