Brominating Alkenes

Brominating Alkenes


The purpose of the experiment is to understand and apply the principles of alkene halogenation by brominating alkenes and synthesizing vicinal dihalides.


The halogenation of an alkenes is a significant reaction in chemistry. Important and useful chemicals are formed by the addition of dihalides such as, chorine (Cl2) or bromine (Br2). The resulting products of an alkene halogen are called vicinal dihalides because the halogen substituents are located on neighboring or adjacent carbons. The bromination of alkenes is the reaction carried out in this experiment. In this mechanism, a bromonium ion is formed from the interaction between the alkene and the bromine dihalide and serves as a bridged intermediate between the reactants and the product. The positive halide in this bridge is unstable and is stabilized when the negative bromine halide attacks it’s positively charge carbons. This attack then breaks the bridge. The resulting products may have different stereochemistry. The most common product is the formation of trans vicinal dihalides which form as a result of anti-addition.

In this experiment, the bromination of alkenes will take place. The alkene utilized in this experiment is trans-Cinnamic acid (C9H8O2) and the bromination reagent is Pyridinium tribromide (C5H6Br3N). Acetic acid and water will also be used in this experiment to further propel this reaction forward and as efficiently as possible. Specifically, acetic acid is used to stabilize the bridged intermediates. At the end of this experiment, a silver nitrate test is performed to verify the that the reaction occurred by the precipitation the bromine halide.

Table of Physical Properties and Hazards:


Physical Properties


trans-Cinnamic acid


Molar Mass: 148.1586 g/mol

Melting Point: 133 °C
Boiling Point: 300°C

Physical State: Solid

trans-Cinnamic acid is irritating. It causes skin corrosion/ irritation, eye damage/ irritation and in some cases, respiratory tract irritation. May also cause irritation if swallowed.

Acetic acid


Molar Mass: 60.052 g/mol

Melting Point: 16.6°C
Boiling Point: 118.1°C

Physical State: Liquid

Acetic acid is corrosive. It causes severe skin burns an eye damage. If inhaled, it can cause coughing, chest pain and irritation of the nose and throat. It is also a flammable liquid and vapor and should be handles in the fume hood at all times.

Pyridinium tribromide


Molar Mass: 319.822 g/mol

Melting Point: -8 °C
Boiling Point: 240 - 242 °C 

Physical State: Solid

Pyridinium tribromide is corrosive and a lachrymator. Prevent eye, skin, and clothing contact. Avoid inhaling or ingesting this compound. Use the fume hood when handling this reagent.

Silver Nitrate


Molar Mass: 169.87 g/mol

Melting Point: 212°C
Boiling Point: 440°C

Physical State: Liquid

Silver nitrate is toxic and oxidizing and can accelerate the burning of combustible material. This compound may also cause headaches, dizziness, nausea and vomiting.



Molar Mass: 46.07 g/mol

Melting Point: -114.1°C
Boiling Point: 78.37°C)

Physical State: Liquid

Ethanol is highly flammable. This compound should be kept away from flames and other heat sources. It may also cause local irritation, but no serious injury.


In this experiment, a 25ml round bottom flask was used to compose of mixture of 450mg of trans-Cinnamic acid, 6 mL of acetic acid, and 1.155g of Pyridinium tribromide. While my partner gathered the chemicals of this mixture, I assembled a reflux apparatus. This technique refers to heating up a solution with an attached condenser. It is an efficient method used to speed up the rate of an organic reaction while also ensuring that they reagents are not lost in the process. Therefore, the reaction proceeds in full. After the reflux apparatus was assembled, the round bottom flask was attached to the bottom with a clamp and the heat was turned on. The mixture was refluxed for about 20 minutes. During this time, a vacuum filtration apparatus was assembled and placed to the side for later use. After approximately 20 minutes, the flask was removed from heating and cooled for several minutes. 8mL of water was added into the mixture and the flask was placed into an ice bath for several minutes.

Once the flask was cool, the mixture was poured through the filter in the vacuum filtration and the crystals were collected at the top. The crystals were clear in color. These crystals were then weighed to be approximately 0.643g. The percent yield was calculated. A silver nitrate test was then conducted to detect the presence of the halogen formed. A small amount of the crystalized product was placed into the test tube. Approximately, 0.5mL of ethanol and 10 drops of silver nitrate was added. Precipitation of the halogen was observed to be a cream-like color – indicating that the reaction had occurred, and bromine had precipitated.


Based on the experiment, the nature of the crystalized material obtained was clear in color. The percent yield of the compound was calculated by first determining the theoretical yield. This was calculated using a balance equation of the reaction and determining the theoretical yield. I did not include acetic acid in the balance equation because it was merely a solvent used to stabilize the bridged intermediates and did not directly influence the final product produced. The theoretical yield was calculated to be 0.939g of the crude compound, C9H8Br2O2. The actual yield obtained from the experiment was 0.643g. And so, the percent yield of this product was approximately 68.5%. Calculations and a detailed mechanism are shown below. Moreover, in the silver nitrate test, the results were positive as precipitation of cream-like substance occurred. This indicated that the alkene used in this experiment, trans-Cinnamic acid, was successfully brominated.

Discussion and Conclusion:

In the final analysis, this experiment allowed us to understand and apply the principles of brominating alkenes. Not only is this reaction important is organic chemistry labs, but many chemical industries utilize this reaction to create useful chemical products. This experiment allowed us to comprehend the process that is used to achieve many of these daily chemical products.  By the end of the experiment, the percent yield of the product was obtained by first balancing the reaction, determining the limiting reagent, and calculating the theoretical yield. To verify the occurrence of the bromination reaction, a silver nitrate test was conducted, and the cream-like precipitate proved that the alkene had in fact been brominated.


National Center for Biotechnology Information. PubChem Database. Cinnamic acid,


National Center for Biotechnology Information. PubChem Database. Acetic acid, CID=176, 

National Center for Biotechnology Information. PubChem Database. Silver nitrate, CID=24470, 

National Center for Biotechnology Information. PubChem Database. Ethanol, CID=702,  (accessed on Mar. 28, 2019)

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