Bromination of Arene Study Guide
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43 Terms
Reaction Control
Factors that determine which products form and how quickly reactions occur.
Statistical Factor
The number of equivalent hydrogens available for substitution.
Energy Factor
The relative reactivity of a C-H bond based on bond strength and radical stability.
Free Radical Substitution
A reaction where a hydrogen is replaced by a halogen through a 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 that is not benzylic or allylic.
Benzylic Hydrogen
A hydrogen attached to an sp3 carbon directly next to a benzene ring.
Primary Benzylic Hydrogen
A hydrogen on a benzylic carbon bonded to one other carbon.
Secondary Benzylic Hydrogen
A hydrogen on a benzylic carbon bonded to two other carbons.
Tertiary Benzylic Hydrogen
A hydrogen on a benzylic carbon bonded to three other carbons.
Primary Aliphatic Hydrogen
A hydrogen on a carbon bonded to one other carbon.
Secondary Aliphatic Hydrogen
A hydrogen on a carbon bonded to two other carbons.
Tertiary Aliphatic Hydrogen
A hydrogen on a carbon bonded to three other carbons.
Free Radical
An atom or molecule containing an unpaired electron.
Initiation
Light breaks Br2 into two bromine radicals: Br2 → 2 Br•.
Propagation Step 1
Br• removes a hydrogen from R-H, forming HBr and R•.
Propagation Step 2
R• reacts with Br2, producing R-Br and regenerating Br•.
Termination
Two radicals combine to form a stable molecule, stopping that chain.
Termination Example 1
Br• + Br• → Br2.
Termination Example 2
R• + Br• → R-Br.
Termination Example 3
R• + R• → R-R.
Overall Bromination Reaction
R-H + Br2 → R-Br + HBr, under light.
Why Is Light Used?
Light supplies energy to split Br2 and initiate radical formation.
What Determines Hydrogen Reactivity?
Radical stability and C-H bond strength.
Why Are Benzylic Hydrogens Reactive?
Their removal produces resonance-stabilized benzylic radicals.
Hydrogen Reactivity: Aromatic vs Benzylic vs Aliphatic
Benzylic > ordinary aliphatic > aromatic for this radical bromination.
Hydrogen Reactivity: 1° vs 2° vs 3°
For comparable environments: tertiary > secondary > primary.
Radical Stability Order
Tertiary > secondary > primary; benzylic radicals also gain resonance stabilization.
Why Is a Tertiary Radical More Stable?
More alkyl groups stabilize the radical through hyperconjugation.
Why Is an Aromatic C-H Bond Less Reactive?
It is relatively strong and direct ring substitution is unfavorable under these conditions.
Expected Substrate Reactivity Order
Isopropylbenzene > ethylbenzene > toluene > tert-butylbenzene.
Why Is Isopropylbenzene Fastest?
It has a tertiary benzylic hydrogen that forms a stable radical.
Why Is Ethylbenzene Second?
It has secondary benzylic hydrogens.
Why Is Toluene Third?
It has three primary benzylic hydrogens.
Why Is tert-Butylbenzene Slowest?
It has no benzylic hydrogens available for substitution.
How Is Reaction Speed Measured?
By timing how long the red-orange bromine color takes to disappear.
What Does Faster Color Disappearance Mean?
Faster bromine consumption under the same reaction conditions.
Why Use an Ice-Water Bath?
To keep reaction temperatures consistent and slow the reactions for timing.
What Is the Purpose of DCM?
It serves as a solvent for bromine and the aromatic substrates.
Why Add Cyclohexene?
To consume excess bromine and remove its red-orange color.
What If Ethylbenzene Reacts Faster Than Isopropylbenzene?
Experimental amounts or mixing differences may affect observed reaction times.
What Is the Rate-Influencing Propagation Step?
Hydrogen abstraction by Br•, which depends strongly on radical stability.
What Is the Main Purpose of the Experiment?
Predict and compare bromination rates of different C-H bond environments.