Exp 18B: Multistep Synthesis Part 2: Synthesis of Diphenylacetylene from Stilbene

Exp 18B: Multistep Synthesis Part 2: Synthesis of Diphenylacetylene from Stilbene

By Audrey Nguyen and Molica Sieng

Our Experiment

Diphenylacetylene

Trans-stilbene

meso-stilbene dibromide

• The first step involves the bromination of trans-stilbene to produce meso-stilbene dibromide.
• Then, once this product is isolated, it will be dehydrohalogenated to form diphenylacetylene.

Uses:

• Diphenylacetylene is a common reagent used in organic synthesis

Step 1: Bromination of Stilbene

Discussion Questions

• Stilbene (shiny white crystals) and acetic acid are mixed together and heated to reflux until a homogeneous solution is formed
• Pyridinium perbromide (big red crystals) is added to the reaction and heated to reflux further for 10 minutes
• produced an orange solution
• The product, dibromide, is then collected via vacuum filtration
• Mass of dibromide produced: 0.1701g
• Theoretical yield of dibromide: 0.377g
• Percent yield: 45.12%

• The double elimination reaction of HBr in the presence of KOH has a high activation barrier. Therefore, a solvent like ethylene glycol allows for the reaction to proceed forward due to its high boiling point. This means that it would be able to supply the required energy necessary for the reaction to occur without evaporating. Hexanes or ether cannot be used because their boiling points are low, meaning they would evaporate off before the reaction could occur.

• The purpose of adding pyridinium perbromide instead of liquid bromine is because pyridinium perbromide is a stable crystalline salt that is a safer and more convenient alternative to use. Liquid bromine is very volatile and corrosive. It also slowly generates bromine which allows for a more controlled reaction to occur.

• If only 25% of B was formed instead of the 90% as indicated in the convergent synthesis scheme shown below, the total percentage yield of the total synthesis would be 20%.

Introduction

Step 1: Mechanism

Spectra (Continued)

Expected IR bands:

• sp2 C-H (3100-3000)
• (~1600 and ~1500-1430)

• Trans-stilbene undergoes electrophillic addition of bromine (from pyridinium perbromide)
• Followed by backside attack of bromide
• Reaction is stereospecific where it is selective across the double bond (the orientation of the groups on the double bond carbons stay the same)

The purpose of this experiment is to complete a two-step synthesis of diphenylacetylene from trans-stilbene.

In addition, the concepts of retrosynthesis, convergent synthesis, linear synthesis, and overall yield are to be understood.

• Retrosynthesis: the process of combining simple reactions to form a chemical compound
• Convergent Synthesis: the combination of compounds produced from multiple series of reactions to form a final product
• Linear Synthesis: one reagent that is synthesized into a different molecule in a series of steps until a final product is obtained

Step 2: Dehydrobromination

Our Results

• Final yield: 0.0242g
• Theoretical yield: 0.198g
• Percent yield: 12.2%
• Melting point range: 60-62°C
• Literature values: 59-61°C

• Triethylene glycol is added to the dibromide to make a 0.6M solution
• 0.84 mL of triethylene glycol was used
• 10.0 equiv. of 85% KOH is also added
• Reaction is heated using a preheated aluminum block (160-170°C) for 10 minutes
• After cooling, 2 mL of DI water is added for every 1 mL of triethylene glycol added (1.6 mL of DI water used)
• Crystals are collected via vacuum filtration and recrystallized with warmed 95% ethanol

Spectra

Step 2: Mechanism

• The dibromide undergoes two consecutive elimination reactions in which the KOH (strong base) reacts with triethylene glycol to eliminate an HBr
• Triethylene glycol acts as a solvent for the reaction to occur
• Dehydrohalogenation occurs where bond breakage and bond formation are occuring at the same time
• A double elimination reaction is necessary to form the alkyne product