Alkyl Halides

Free Radical Substitution

N-bromosuccinimide

NBS?

Gilman reagents

These are useful because they undergo a coupling reaction with organochlorides, bromides, and iodides (but not fluorides).

They are reactive, versatile, and easy to convert into many other organic compounds.

Why are alkyl halides used as starting materials in organic reactions?

Primary (1°) alkyl halides are preferred in SN2 reactions because they have less steric hindrance, making backside attack easier.

What type of alkyl halide is preferred in an SN2 reaction?

Mechanism: Forces an SN2 pathway (concerted, single-step).

• Stereochemistry: Causes complete inversion of configuration (R —> S).

• Why? The aggressive, charged nucleophile performs a backside attack before the leaving group departs.

For a secondary alkyl halide, how does a strong nucleophile affect the mechanism and stereochemistry?

Mechanism: Allows an SN1 pathway (two-step with intermediate).

• Stereochemistry: Causes racemization (a roughly 50:50 mixture of R and S products).

• Why? The neutral nucleophile waits for the leaving group to leave, forming a planar carbocation intermediate that can be attacked from either face.

For a secondary alkyl halide, how does a weak nucleophile affect the mechanism and stereochemistry?

The greater the number of substituents bonded to the sp² carbon, the more stable the alkene.

What does the Zaitsev Rule state?

substitution

If Nu is a weak base (example CN-) = ?

Elimination

If Base is strong (Example OH-) = ?