Photobromination of alkylbenzenes

The yellow color of the reaction mixture disappears slowly under the effect of bright light with different rates in different beakers

Scientific name: Radical bromination of alkylbenzenes has different rates depending on the stability of the corresponding alkyl radical

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Safety

Wear eye protection goggles. Use gloves. Bromine forms a poisonous vapour! Prepare the solution of bromine in a hood or outdoors.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

R-H + X2 → R-X + HX

C6H6 + Br2 → no reaction

C6H5CH3 + Br2 → C6H5CH2Br + HBr

C6H5CH2CH3 + Br2 → C6H5CH(Br)CH3 + HBr

C6H5CH(CH3)2 + Br2 → C6H5CBr(CH3)2 + HBr

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. Prepare solutions of alkylbenzenes in dichloromethane. Pour 50 ml of dichloromethane into 4 beakers. Add 4.5 ml of benzene into the first beaker, 5.3 ml of toluene into the second beaker, 6.1 ml of ethylbenzene into the third beaker, 7.0 ml of cumene into the fourth beaker.
  2. Mix the beakers.
  3. Prepare a solution of bromine in dichloromethane. Pour 100 ml of dichloromethane and 8.2 ml of bromine in a 200-milliliter beaker. Mix thoroughly.
  4. Add 25 ml of the bromine solution into each of the four beakers.
  5. Shine bright light on the beakers.
  6. The yellow color of bromine slowly disappears in the following order: cumene, ethylbenzene, toluene. Benzene doesn't react with bromine under these conditions.

Scientific background

Alkylbenzenes can be brominated to the ring or to the side chain depending on the reaction conditions. In the presence of an electrophilic reaction catalyst (Lewis bases), such as aluminum chloride, electrophilic substitution of the hydrogen atom with bromine in the aromatic ring occurs. In this case, the catalyst is absent, and radical reactions proceed under the influence of light to the side chain. Benzene has no side chain, therefore it can not be brominated under these conditions. Light is a good free-radical initiator.

The first stage. Initiation.

Formation radicals of bromine under the action of light.

Br2 → 2 Br*

The second stage. Chain propagation.

Bromine radicals may displace hydrogen atoms of the alkyl group, thus forming an alkyl radical

Br + RH → R + HBr

that is highly reactive and may further react with bromine, forming new radicals of bromine

R + Br2 → RBr + Br

etc.

The third stage. Chain termination

It occurs by the reaction of two radicals.

Br + Br → Br2

R + Br → RBr

R + R → R-R

Various reaction rate can be explained by different resistance of hydrocarbon radicals. Their stability increases in the series: the primary - secondary - tertiary. This effect can be explained by the presence of electronic effects.

Danger:
Coolness:
Difficulty:

Published on 15 May 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst