Lab 4: Bromination of Arenes

Type of reaction?

free radical substitution reaction with a halogen

First step

homolytic cleavage of halogen

Propagation Step

The reactive intermediate reacts with a stable molecule to form a product and another reactive intermediate, allowing the chain to continue

5 chemicals reacting with Bromine in this lab?

1. Toluene

2. Ethyl Benzene

3. Tertiary butyl benzene

4. Cyclohexane

5. Methylcyclohexane

Ethylbenzene

2, benzylic

react QUICKEST

Tertiary butyl benzene

1, aliphatic

SLOWEST

toluene

1, benzylic

2nd quickest

cyclohexane

2, aliphatic

4th quickest

methylcyclohexane

3, aliphatic

3rd quickest

General Reaction in this lab

R-H + Br2 → HBr + R-Br

What TYPE of substituent does bromine react with?

The most reactive hydrogen

3 Types of Hydrogens

1. Benzylic

2. Aliphatic

3. Aromatic

Aromatic Hydrogens

Hydrogens that are attached to a carbon that is in an aromatic ring

Aliphatic Hydrogens

Sp3 hybridized carbon with its hydrogen, which in turn is only attached to sp3 hybridized carbons

Benzylic Hydrogens

On an sp3 hybridized carbon, but that carbon is in an aromatic ring

Order of strength

Aromatic ~ Aliphatic < Benzylic (benzylic will react MOST with bromine)

1 < 2< 3

(3 reacts the MOST)

SOPs

B: Bromine

C: Disposing of waste

D: Wearing of gloves

E: Using acids and bases

F: Pressure

G: Using organic chemicals

H: Glassware

I: Heating reactions

Experiment purpose

the rate at which C-H bonds in different molecular environments undergo radical bromination

C-H bonds

typically unreactive, but they do act as functional groups in reactions that employ radical halides

Type of hydrogen that is attacked directly affects the rate...

at which the bond can undergo homolysis when exposed to a halide radical

Glassware used

Erlenmeyer flask

Test tubes

If bromine is spilled

pour some sodium thiosulfate around the spill, red color slowly disappear, can be treated as halogenated organic waste or wiped up with paper towels and discarded in the trash

Single drop of cyclohexene because...

smells terrible and the less you use the better it is for everyone

DO NOT WASH GLASSWARE THAT CONTAINED BROMINE WITH...

ACETONE