245 * Lab #9
Addition Reaction of HBr to an Alkene mixture

Before beginning, be sure that you have:
· Read the MSDS for Chemical Hazards
· Determined the percent yield of your alkenes,
· Performed a G.C. Analysis on them,

Place about 3.5 g, measured precisely, of the alkenes from the previous week into a 50 mL round bottom flask. Cool the flask in an ice bath. Then slowly add, In a Hood, 15.0 mL of 48% (9M) HBr (Corrosive). Then add two drops of detergent.

 Equip the flask with a magnetic stirrer and reflux condenser. Place a sand bath on top of a hot plate and under the flask. With rapid stirring, bring the mixture to a gentle reflux for 30 minutes.

(NOTE: sections meeting twice a week will not have sufficient time to finish this procedure. Bring the mixture to a gentle reflux for as long a time as you can. Cool the system. Securely stopper the RBFlask, label the flask, and store it as your instructor tells you. The next period, continue as follows)

 After cooling to room temperature, transfer the liquid to a separatory funnel. Add 10.0 mL of water and 10.0 mL of methylene chloride (listed). Carefully drain off the organic layer (listed solvent) and save it. Discard the aqueous layer into a labeled Waste Bottle. If an emulsion forms in this, or subsequent steps, add a little NaCl and/or stir the emulsion to break the emulsion up.

 Wash the organic layer with an approximately equal volume of sodium bicarbonate solution. If considerable foaming occurs, repeat the bicarbonate wash. Discard the wash into a Waste Bottle.

 Carefully, wash the organic layer twice with equal volumes of water. Again save the organic layer. Dry the organic layer with a small amount of anhydrous sodium sulfate. Transfer the organic layer to a clean, dry, preweighed 25mL RBFlask, and distill off the methylene chloride. The small amount (less than 5mL) of liquid remaining in the pot is your bromoalkane. (Distillation and recovery of such small quantities of liquid are not convenient, nor efficient. However, if time permits, consult your instructor about short-path distillation of the product.)

 Characterization of the product could include, in order of priority:

· Percent yield determination,

· Refractive index,

· IR

Chemicals:

48% Hydrobromic Acid
t-Amyl Alcohol
Sodium Chloride
Methylene Chloride (Dichloromethane)
Sodium Bicarbonate

WASTE Spill & Disposal B: HydroBromic Acid (Corrosive)

WASTE Spill & Disposal F: Methylene Chloride (carcinogen suspect), Alkenes, , Water & Bicarbonate Washes.
DO NOT PUT USED WEIGH BOATS IN THE HW CHEMICAL CONTAINER. CONTAMINATED PAPER TOWELS AND WEIGH BOATS ARE PUT INTO THE CLEAR PLASTIC HW BOX IN THE HOOD. PAPER ENVELOPES W/CHEMICALS SHOULD BE FOLDED MULTIPLE TIMES AND SEALED BEFORE BEING PUT INTO THE SPECIFIED HW CONTAINER.

Refractometer

Pre lab Questions

1. What is the expected structure of the sole product of the hydrobromination?

2. What is the limiting reagent in this reaction? Show your work!

3. 48% HBr (one of the starting materials) and the bromoalkane (the product) have about the same densities. What are two factors that should ensure the organic and aqueous layers should separate cleanly, based on their densities?

4. What are the boiling points of:

a. methylene chloride

b. the higher boiling alkene

c. the bromoalkane

5. Given that we wish to distill off the methylene chloride, as well as any unreacted alkenes, but leave the bromoalkane in the still pot, what would you use as a heat source in the final distillation? (See question 4)