Reaction of Cyclohexene with Bromine and Potassium Permanganate

Chemicals: cyclohexene benzene 10 -2 M aqueous bromine  (1.28 g bromine / 800 mL H2O) 10 -4 M aqueous KMnO4  (12.6 mg KMnO4 / 800 mL H2O) Apparatus and glass wares: 4 graduated cylinders with stopper, 500 mL 2 pipettes 1 mL, graduated in 0.1 mL 2 pipette bulbs 4 snap-cap vials 10 mL Hazards and safety precautions: Benzene is known to be a human carcinogen on sufficient evidence of carcinogenicity in humans. Breathing benzene can cause drowsiness, dizziness, and unconsciousness; long-term benzene exposure causes effects on the bone marrow and can cause anemia and leukemia. Severe eye irritant. Skin and respiratory irritant. Benzene and cyclohexene are highly flammable. Bromine is highly toxic if inhaled, ingested or comes in contact with the skin. Bromine water is harmful if ingested or inhaled. Prolonged skin contact can cause burns. Eye irritant - lengthy contact will lead to eye damage. Safety goggles and protective gloves must be worn. The experiment should be performed under a portable fume cupboard giving all-round visibility! Experimental procedure: Two snap-cap vials contain 0.75 mL benzene. 0.8 mL cyclohexene are pipetted into each of two further snap-cap vials. Two collecting cylinders are each filled with 10 -2 M aqueous bromine and two further cylinders are filled with 10-4 M aqueous KMnO4. The solutions are mixed with benzene and cyclohexene, respectively. Cylinder 1 400 mL aqueous bromine 0.75 mL benzene Cylinder 2 400 mL aqueous bromine   0.8 mL cyclohexene   Cylinder 3   400 mL aqueous permanganate   0.75 mL benzene Cylinder 4 400 mL aqueous permanganate 0.8 mL cyclohexene The stoppered cylinders are vigorously shaken. Results: Benzene reacts neither with aqueous bromine nor with permanganate solution. Cyclohexene reacts with both aqueous bromine and permanganate. The solution of bromine is decolorized and purple permanganate turns brown.

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Discussion:

The decolorization of bromine water is often used as a test for a C = C double bond. Bromine undergoes electrophilic addition to the double bond of alkenes. In non-aqueous solvents such as carbon tetrachloride, this gives the di-bromo product. For example, reaction with ethylene will produce 1,2-dibromoethane. When used as bromine water, the corresponding bromohydrin is formed instead.


Fig. 1 Bromination of Cyclohexene

On approaching the electron dense area of the p bond of cyclohexene, the bromine molecule becomes polarized. The electron density of the bromine is shifted, so that one bromine is partially positive and the other is partially negative charged. The Br - Br bond is heterolytically cleaved. The positively charged bromine atom acts as an electrophile, reacting with the C = C double bond. A cyclic bromonium ion is formed. The subsequent attack of the bromide ion on the three-membered ring can proceed only from the back-side, because the front-side attack is sterically hindered. The result is formation of a mixture of two enantiomeric compounds of trans-1,2-dibromocyclohexane  (1). When other nucleophiles such as water or alcohol are existing, these may attack the cyclic bromonium ion to give an alcohol or an ether  (2).


Also the permanganate hydroxylation is used as a qualitative test for the presence of an alkene (Bayer test). Permanganate converts cyclohexene into a diol. In the course of the reaction purple permanganate is reduced to the brown manganese dioxide, where Mn is in a +4 oxidation state  (3).



Fig. 2 Hydroxylation of Cyclohexene

Since a syn-hydroxylation takes place, the reaction is thought to involve the formation of an intermediate cyclic hypomanganate ester which is readily hydrolyzed under the reaction conditions to yield the glycol  (4).  Experiments with ¹⁸ O-labelled permanganate demonstrate, that the two oxygen atoms of glycol originate on the permanganate and not on the solvent.