Introduction
A dilute solution of bromine is added to samples of heptane and cyclohexene. The bromination reactions and mechanisms are compared.
Equations
Alkanes: Bromination (substitution reaction)
R-H + Br2 → R-Br + HBr
( colorless) (amber) (colorless)
UV light splits the bromine molecule into two reactive radicals, resulting in a very slow loss of amber bromine color. The radical bromine atoms can then go on to react with other species in solution (including each other) in a chain reaction.
Alkanes: Oxidation (no reaction)
Alkenes: Bromination (addition reaction)
RHC = CHR + Br2 → RHBrC-CBrHR
(colorless) (amber) (colorless)
This reaction proceeds at room temperature and does not require sunlight or UV radiation to initiate it. The rapid disappearance of the amber color of the bromine at room temperature is characteristic of reaction with alkenes.
Alkenes: Oxidation (reaction with KMnO4)
RHC = CHR + KMnO4 → RHOHC-CHROH + MnO2
(colorless) (purple) (colorless) (brown ppt.)
Reaction of an alkene with aqueous potassium permanganate results in addition of two hydroxyl groups to each side of the double bond to form a glycol.
Prior to Lecture (prep time ~ 20 min.)
1. Prepare the following chemicals:
- Heptane
- Cyclohexene
- 1% bromine in dichloromethane (1mL Br2 + 99mL CH2Cl2)
- 5% potassium permanganate (dissolve 5g KMnO4 in 95g H2O)
2. Assemble the following equipment:
- 4 200mm test tubes in lighted rack
- gloves
- goggles
To Conduct Demonstration:
- Pour two samples of heptane and two samples of cyclohexene into separate test tubes in a lighted rack.
- Add one or two drops of 1% bromine in dichloromethane to one sample of each hydrocarbon. Compare the rate of the reactions and the conditions.
- Add one or two drops of 5% KMnO4 to the second sample of each hydrocarbon. Compare the reactions.
Safety and Disposal
Bromine in dichloromethane and potassium permanganate can cause burns; avoid contact with skin.