Stereospecificity: Electrophilic addition and bromohydrin formation

chemical equation showing the reaction of trans-stillbene with bromine and water that asks which product will form

During this activity students investigate the stereochemistry of an electrophilic addition reaction. Students will carry out the reaction, purify the product by recrystallization, and identify the product based on it's melting point.

The Reaction

Carefully dispense 0.25 g trans-stillbene onto a piece of weighing paper.

flow chart showing the starting point and the decision box for whether to add more dimethylsulfoxide.

Add a stir bar, the trans-stillbene, 0.12 mL water, and 7 mL dimethyl sulfoxide (DMSO).

mathmatical expression showing how to determine the mass of NBS to add.

a flow chart showing the steps for the addition of the NBS and the chilling of water

Add 2 molar equivalents of N-bromosuccinimide (NBS) in equal portions over 5 minutes.

a flow chart showing the add to ice-cold water step

Pour the bright orange reaction mixture into 20 mL of ice-cold water.

steps in a flowchart for setting up a vacuum filtration apparatus to filter the reaction mixture through a pad of celite

Place a piece of filter paper in the Buchner funnel, wet it with a small amount of water, and cover the filter paper with a shallow layer (~ 0.5 cm) of Celite.

two steps of a flow chart that say to add ether and filter

Place a piece of filter paper in the Buchner funnel, wet is with a small amount of water, and cover the filter paper with a shallow layer (~ 0.5 cm) of Celite.

TIP: Press down on the funnel to make certain that a good seal has formed between the funnel and the filtering flask.

TIP 2: Keep the vacuum on a low setting. A strong vacuum will cause the ether to evaporate.

TIP 3: Pour slowly and slowly move the beaker while pouring.

Check: Before moving to the separatory funnel step, make certain that you have two layers.

The steps of a flowchart describing the separtation and second extraction of the aqueous layer.

Using a separatory funnel, separate the organic and aqueous layers.

Return the aqueous layer to the separatory funnel and extract the aqueous layer with an additional 7 mL of diethyl ether.

Combine the organic layers.

A flowchart showing the steps of the washing of the ether with water and saturated salt water.

Return the ether layer to the separatory funnel. Wash the ether layer with 10 mL of water; that is, add 10 mL of water to the ether layer, shake vent a few times, and pour of the aqueous layer.

Wash the ether layer with 10 mL of a saturated aqueous sodium chloride solution (the satruated solution will draw some water out of the ether layer).

A flowchart describing the drying and filtering of the ether layer.

Add a couple of scoops of anhydrous magnesium sulfate to the ether solution. After waiting a few minutes, if the magnesium sulfate still rolls around freely, continue with the activity. If the ether appears cloudy or none of the magnesium sulfate rolls around freely, add another scoop.

TIP: UCLA's Prof. Bacher has a very good webpage on drying agents.

A flowchart describing the steps used for evaporating the ether solvent.

Assemble a simple distillation apparatus and trransfer the ether solution to the still. Turn on the condenser water and lover the apparatus into the hot water bath.

Evaporate the ether until approximately 3 mL of solution remains and transfer the 3 mL of solution to a conical vial.

Evaporate the remaining 3 mL of ether using a hot water bath and a gentle stream of compressed air.

TIP: To save time heating the water bath start with hot water from the tap.
TIP 2: Make certain to adjust the flow of air to a gentle stream before it is positioned about the product solution.

A flowchart describing the steps of a recrystallization.

Warm some high-boiling petroleum ether (pet. ether) to near boiling.

Add a small amount of the hot pet. ether to the crude product, swirl the solution warm for a few minutes. Slowly add small amounts of the pet. ether to the crude product until it dissolves.

TIP: Avoid adding more pet. ether than absolutely necessary. Using to much pet. ether will lower the yield of the product.
TIP 2: Make certain to wet the filter paper with a small amount of pet. ether and make certain that the vacuum source is on before transferring the product mixture to the funnel.
TIP 3: Remember to make certain that there is a good seal between the flask and the funnel.

The Product

Mass

Determine the mass of the recrystallized product.

Melting Point

Determine the melting point of the recrystallized product.

A graphic showing the chemical structures of the erythro---(R,S) and (S,R)---enantiomers and the threo---(R,R) and (S,S)---enantiomers of 2-bromo-1,2-diphenylethanol. The melting point fo the threo enantiomer is 51 to 52 degrees Celcius and the melting point for the erthyro enantiomers is 83 to 84 degrees celcius.