Bromination of Trans- Stilbene to Form 1,2- Dibromo- 1,2-Diphenylethane

Topics: Acid, Bromine, Chlorine Pages: 4 (1080 words) Published: April 27, 2012
Bromination of trans- Stilbene to form 1,2- Dibromo- 1,2-diphenylethane

Abstract

1,2-dibromo-1,2-diphenylethane was produced by the bromination of trans-stilbene through the addition of hydrobromic acid (HBr) and hydrogen peroxide (H2O2). This experiment was a greener bromination of stilbene because bromine was generated in situ and ethanol was used as the solvent. The melting point (243.30°C), mass (.427g), and percent yield (45.54) of the crystals were recorded. The FTIR was used to confirm that the product was produced. The peaks on the HNMR revealed the presence of aliphatic and aromatic hydrogens and were used to verify the identity of the product. In addition, the melting point of the crystals was also used to verify the synthesis of 1,2-dibromo-1,2-diphenylethane.

Introduction

Bromination of an alkene is an example of an addition reaction in which bromine adds across the double bond to form a vicinal dibromide as shown in Figure 1.

[pic]

Figure 1: The bromination of trans-stilbene

In this experiment, 1,2-dibromo-1,2-diphenylethane is synthesized from the bromination of trans-stilbene through the addition of hydrobromic acid (HBr) and hydrogen peroxide (H2O2). This is a green reaction because bromine is generated in situ from the reaction of hydrobromic acid and hydrogen peroxide as shown in Figure 2.

2 HBr + H2O2 ( Br2 + 2 H2O

Figure 2: The formation of bromine from the reaction of hydrobromic acid and

hydrogen peroxide

The alkene, trans-stilbene, acts as a nucleophile and the bromine acts as an electrophile. The Br–Br bond becomes polarized and the more positively charged Br atom is transferred to the alkene to yield a bromonium ion and a bromonium anion. The bromonium anion attacks a carbon atom to open the three-membered ring and produce vicinal dibromide. The net result of this reaction is anti addition of bromine as shown in Figure 3. [pic]

Figure 3: The anti...
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