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Friday, April 27, 2012

Bromination of trans-cinnamic acid

In this lab we'll look at what its like to add bromine to transcinamic acid via adding methods of stereo-chemistry. Additionally we'll take a look at trying to prove the reaction was successful and that we actually did get 2,3-dibromo-3-pheylpropanoic acid.Bromination are a type of Halogenation procedures using bromine, read more about it here.

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And Now BACK TO ORGO!!!!!

 Below is kind of a generalized look at the addition of the Bromine via a Trans Addition, which sorry to spoil the surprise is the way its going to add.

Trans-cinnamic acid (E-3-phenyl-2-propanoic acid) has a molecular weight of 148 g/mol and a melting point of 133 degrees Celsius.

There are several enantiomers of 2,3-dibromo-3-phenylpropanoic acid, they all have a molecular weight of 308 g/mol (Molecular Weight: 307.96662 to be specific).

The (2R, 3S) and (2S, 3R) have a melting point of 202-204 degrees Celsius.

The (2R, 3R) and (2S, 3S) have a melting point of 93.5 to 95 degrees Celsius.

 Things that were done:

150 mg trans-cinnamic acid and .6 mL glacial acetic acid were put acid in a test tube. The tube was put in a 50 degree celcius water bath till all the trans-cinnamic acid dissolves. Next 1.0mL of 1.0M Br2 in acetic solution. the mix was stirred via agitation at 50 degrees Celsius till the color faded from red/brown to light orange and was heated for another 15 minutes. Next a drop of cyclohexane was added. The mix was cooled in ice water bath for 10 minutes (crystals formed). Meanwhile distilled water was cooled (in an ice bath). Note if the crystal didn't form the side would have been scratched with a siring rod and left in the ice water bath for another ten minutes.

The product was collected via vacuum filtration. The crystals were washed with ice water ( note its a good idea to do it till the smell of acetic acid is gone. Next was the process of recrystallization. The sample retrieved from vacuum filtration was placed in .5mL ethanol in an 10mL Erlenmeyer flask. After the sample had dissolved boiling stick was added and the mix was heated via a steam water bath. If the sample doesn't dissolve add more ethanol. Next a volume of water equal to the amount of ethanol was added. The sample was removed from the heat and allowed to cool for 10 minutes in ice. Crystals were removed from the solution via vacuum filtration (note a new clean Hirsch funnel and new filter paper was used)

 Things you may ask...

What is a mixed solvent recrystallization? What is the reason for using mixed solvent recrystallization?

 Mixed solvent recrystalization refers to a process of crystallization used for the purpose of purification. A solvent is added to a mixture with desired and undesired components creating a solution. The solution is heated so that all of its components are able to dissolve, this solvent chosen because of its known properties of solubility. At a higher temperature everything should remain dissolved in the solvent as it becomes supersaturated. However as it cools crystals of your desired compound should form being that at lower temperatures the solvent can no longer remain super saturated, meanwhile impurities and undesired compounds should remain dissolved in the solvent. In this experiment the crystals could be isolated via vacuum filtration.

 Should the product exhibit optical activity?

 No the product would not exhibit optical activity because while each enantiomer is chiral they exist as a racemic mixture such that the specific rotation of one enantionmer would cancel the rotation of the other being that they should have the same magnitude but in the opposite direction.

 What is the mechanism of the addition like?

It can happen via Radicals
What can the melting point/range say about the purity of the sample?

 If the observed melting point or range is near the reported melting range from the literature it suggests that  the product is that of the enantiomer set. For this particular experiment the  (2R,3S) and (2S, 3R) enantiomers of 2,3-dibromo-3-phenylproanoic acid are expected. Indicating that the bromine addition occurred via an anti method. Had bromine of added via syn addition the melting point would have been near 95 °C (provided in the literature).  The difference of several degrees between the literature and the experimental values may be the result of impurities and the possible existence of other enantiomers or compounds in the product. Recall that impurities cause depression and extend the range of the melting points.


  1. "There are several enantioinmers of 2,3-dibromo-3-phenylpropanoic acid, they all have a molecular weight of 208 g/mol". This is incorrect, they are 308 g/mol.

  2. Thanks for finding that! I will update to reflect the error.