Arenes

Reaction Control

The statistical and energy factors that influence reaction rate and product formation.

Statistical Factor

The number of equivalent hydrogens available for substitution.

Energy Factor

The relative reactivity of a C-H bond, determined by bond strength and radical stability.

Free Radical Substitution of C-H to C-X

hydrogen is replaced by a halogen through a free-radical chain mechanism.

Aromatic Hydrogen

A hydrogen directly attached to a carbon in an aromatic ring.

Aliphatic Hydrogen

A hydrogen attached to an sp3 carbon; classified as primary, secondary, or tertiary.

Benzylic Hydrogen

A hydrogen on an sp3 carbon directly attached to a benzene ring.

Primary Benzylic Hydrogen

Hydrogen on a benzylic carbon attached to one other carbon.

Secondary Benzylic Hydrogen

Hydrogen on a benzylic carbon attached to two other carbons.

Tertiary Benzylic Hydrogen

Hydrogen on a benzylic carbon attached to three other carbons.

Initiation

Light splits Br2 into two bromine radicals: Br2 → 2 Br•.

Propagation

Br• + R-H → HBr + R•; then R• + Br2 → R-Br + Br•.

Termination

Radicals combine to stop a chain: Br• + Br• → Br2; R• + Br• → R-Br; R• + R• → R-R.

Free Radical Mechanism for Our Reaction

Initiation: Br2 → 2 Br•; Propagation: Br• + R-H → HBr + R•, then R• + Br2 → R-Br + Br•; Termination: two radicals combine.

Relative Reactivity: Aromatic vs Aliphatic vs Benzylic

Benzylic > ordinary aliphatic > aromatic under these bromination conditions.

Relative Reactivity: Primary vs Secondary vs Tertiary

Tertiary > secondary > primary for comparable C-H environments.

What Determines Hydrogen Reactivity?

Radical stability and C-H bond strength. More stable radicals generally form more easily.

Expected Reactivity Order in Our Experiment

Isopropylbenzene > ethylbenzene > toluene > tert-butylbenzene.