Orgo9 - Organic Chemistry Lab Experiment title: Preparation...

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Organic Chemistry Lab Experiment title: Preparation of Diphenylacetylene from 1,2-Dibromo-1,2- diphenylethane (Meso Stilbene Dibromide) Introduction/Purpose of experiment: The main purpose of this experiment was to demonstrate the preparation of an alkyne by a double dehydrohalogenation. The dehydrohalogenation of alkyl halides is a general method of producing a double bond. The sequential elimination of two molecules of a hydrogen halide will lead to the formation of a triple bond. During the first stage of the reaction, there is a base-induced elimination of a hydrogen halide to give a haloalkene (aka a vinyl halide), and this process occurs by a concerted E2 mechanism, with the base abstracting the proton on the carbon atom to the leaving halide ion. This reaction involves a transition state, at which point the hydrogen and the departing halogen are in the anti-periplanar orientation. During the second step of the sequence, there needs to be a strong base and more forcing conditions (i.e., heating). In this experiment, the combination of bromination of an alkene followed by the double dehydrobromination of the intermediate vicinal dibromide (vicinal referring to the presence of two functional groups bonded to two adjacent carbon atoms) represents an appropriate means of converting an alkene into an alkyne. A temperature of 200˚C is essential to effect the elimination of hydrogen bromide from the intermediate vinyl bromide, and the use of a high-boiling solvent such as triethylene glycol becomes necessary. General Reaction: Main reaction: Reagents: Components Molecular Formula Formula Weight Amount used MP/BP Density
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Meso stilbene dibromide C 14 H 12 Br 2 340.056 g/ mol 150 mg 241 ˚ C/NA NA Triethylene glycol C 6 H 14 O 4 150.17 g/ mol 1 mL -7 ˚ C/ 285˚C 1.1 g/mL Potassium Hydroxide KOH 56.105 g/ mol 1 pellet
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Orgo9 - Organic Chemistry Lab Experiment title: Preparation...

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