Addition of Bromines

Topics: Acetic acid, Electric charge, Atom Pages: 4 (1027 words) Published: February 27, 2012
In this experiment, the stereochemistry of 2,3-dibromo-3-phenylpropanoic was found to determine syn or anti addition of bromines. The melting point was 199-203°C and the NMR J values angle is at 167 which both prove the erythro-2,3-dibromo-3-phenylpropanoic. Determination of the Stereochemistry of 2,3-dibromo-3-phenylpropanoic acid. Introduction

The purpose of this experiment was to determine the mechanism of the reaction of trans-cinnamic acid with the addition of bromines to 2,3-dibromo-3-phenylpropanoic acid. An addition of bromine was added to trans-cinnamic acid and it is necessary to determine whether the intermediates were syn or anti addition or a mixture of them both to find the stereochemistry of the final product. The final product could be erythro-2,3-dibromo-3-phenylpropanoic, threo-2,3-dibromo-3-phenylpropanoic, or a mixture of them both. The erythro-dibromide would occur if the reaction went primarily by syn addition of the bromides and the threo-dibromide would be the result if the reaction went primarily by anti addition. To confirm results and obtain more information, NMR will be used to help determine the final stereochemistry of the product and structural properties.

Anti addition is the predicted favorable outcome for this particular reaction of bromine addition to cyclopentene because a cyclic bromonium ion is formed that is resonance stablilzed. The bromine atom has a strong negative charge and as it starts to approach the pi bond the electrons will move away to the backside of the atom making the bromine atom have a partial positive charge. Once one of the bromine atom has a partial positive charge, the bromine attached to it will break apart and attack the backside of the pi bond making the compound trans . For the determination of the more favorable mechanism there are many factors that must be considered. One of these is steric hindrance. For syn addition, the steric effects of the two bromines might cause the reaction to be...
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