Khan Academy Alkyl Halides- Complete Reaction Guide

What Are Alkyl Halides?

Alkyl halides are organic compounds where a halogen atom (fluorine, chlorine, bromine, or iodine) replaces a hydrogen atom on an alkane. The halogen acts as a leaving group, which makes these molecules reactive in specific ways.

Khan Academy breaks down alkyl halide reactions into two main categories you'll need to master:

• Nucleophilic substitution reactions
• Elimination reactions

These reactions form the backbone of organic chemistry and show up constantly on exams.

SN2 Reactions: One-Step Displacement

SN2 stands for substitution nucleophilic bimolecular. The reaction happens in a single step where the nucleophile attacks the carbon bearing the leaving group from the backside, causing an inversion of configuration.

Key Characteristics

• Rate depends on both the alkyl halide and the nucleophile
• Works best with methyl, primary, and secondary substrates
• Tertiary alkyl halides don't undergo SN2 — steric hindrance blocks the attack
• The stereochemistry inverts at the carbon center

Favorable Conditions for SN2

Strong nucleophiles drive SN2 reactions. Think of species with negative charge or lone pairs that desperately want to donate electrons. Good SN2 nucleophiles include:

• Iodide (I⁻)
• Bromide (Br⁻)
• Cyanide (CN⁻)
• Hydroxide (OH⁻)
• Alkoxides (RO⁻)

Polar aprotic solvents like acetone or DMSO speed up SN2 reactions. They solvate cations but leave nucleophiles naked and reactive.

SN1 Reactions: Two-Step Process

SN1 means substitution nucleophilic unimolecular. The mechanism involves two distinct steps:

1. Step 1: The leaving group departs, forming a planar carbocation intermediate
2. Step 2: The nucleophile attacks from either face of the carbocation

When SN1 Dominates

SN1 reactions occur with tertiary alkyl halides and some secondary substrates. Stable carbocations form readily, which drives the first step.

Weak nucleophiles like water and alcohols work fine here. The nucleophile doesn't need to be strong because it attacks a fully formed carbocation — there's no need to kick out a leaving group simultaneously.

Racemic Mixtures

Because the nucleophile attacks equally from both faces of the planar carbocation, SN1 reactions on chiral centers produce racemic mixtures — equal amounts of both configurations. If you need stereospecificity, SN1 won't deliver it.

Elimination Reactions: E1 and E2

Elimination reactions remove the halogen along with a hydrogen from an adjacent carbon, producing an alkene. The competition between substitution and elimination makes alkyl halides tricky.

E2 Mechanism

E2 is elimination bimolecular — a one-step process where a base abstracts a proton while the leaving group departs simultaneously. The hydrogen and leaving group must be anti-periplanar (on opposite sides of the C-C bond).

E2 competes directly with SN2 and dominates when:

• Strong bulky bases are present (like t-butoxide)
• The substrate is tertiary or sterically hindered
• High temperatures favor elimination over substitution

E1 Mechanism

E1 follows the same first step as SN1 — formation of a carbocation. A base then removes a proton to form the double bond. E1 typically occurs with tertiary substrates in the presence of weak bases and polar protic solvents.

The same carbocation can undergo either substitution or elimination, depending on what nucleophile/base is present.

Substitution vs. Elimination: How to Predict the Outcome

This is where students get stuck. Khan Academy emphasizes three factors that determine which pathway wins:

1. Substrate Structure

• Methyl halides: Only SN2 is possible
• Primary alkyl halides: SN2 dominates unless very strong bases are used
• Secondary alkyl halides: Mixed — substitution and elimination both compete
• Tertiary alkyl halides: SN1 and E1/E2 dominate; SN2 is blocked

2. Nucleophile/Base Strength

Strong nucleophiles favor substitution. Strong bases favor elimination. If you see a bulky strong base (t-BuOK, DBU), elimination is your answer.

3. Solvent Effects

Polar protic solvents (water, alcohols) stabilize carbocations and favor SN1/E1. Polar aprotic solvents favor SN2/E2 by making nucleophiles more reactive.

Comparing SN1, SN2, E1, and E2

Getting Started: How to Solve Alkyl Halide Problems

Follow this sequence when you encounter an alkyl halide reaction problem:

1. Identify the substrate: Is it methyl, primary, secondary, or tertiary? This narrows your options immediately.
2. Identify the reagent: Is it a strong nucleophile, weak nucleophile, strong base, or weak base?
3. Consider the solvent: Polar protic or polar aprotic? This confirms your mechanism choice.
4. Check for competing pathways: Secondary substrates almost always have competition. Pick the most favorable pathway based on conditions.
5. Draw the mechanism: Show electron pushing arrows. For SN2/E2, the arrow goes from nucleophile/base to carbon while the leaving group leaves. For SN1/E1, the leaving group leaves first, then the nucleophile/base attacks.

Common Mistakes Students Make

• Picking SN2 on a tertiary substrate: Impossible due to steric hindrance. If the substrate is tertiary, you're looking at SN1 or an elimination.
• Ignoring Zaitsev's rule: For eliminations, the more substituted alkene usually forms as the major product.
• Forgetting anti-periplanar geometry in E2: The hydrogen and leaving group must be on opposite sides. If they're on the same side, rotation is required first.
• Confusing SN1 with SN2 kinetics: SN1 rate depends only on the alkyl halide. SN2 rate depends on both the alkyl halide and the nucleophile.

Finding This on Khan Academy

Navigate to the Organic Chemistry section, then look for the Reactions of Haloalkanes unit. Khan Academy breaks each mechanism down with clear video explanations and practice problems.

The key videos you'll want:

• Introduction to Substitution Reactions
• SN2 Mechanism
• SN1 Mechanism
• Elimination Reactions Overview
• Practice: Predicting Substitution vs. Elimination

Work through every practice problem until you can predict products without hesitating.

Bottom Line

Alkyl halide reactions follow predictable patterns based on substrate structure, reagent strength, and solvent. Master the four mechanisms — SN1, SN2, E1, E2 — and learn to identify which pathway applies. The patterns repeat throughout organic chemistry, so this unit matters more than it might seem. Work the problems. Draw the mechanisms. There's no shortcut.