Friedel-Crafts acylation experiment Setting up the reaction: 1) Put a magnetic s

Question

Friedel-Crafts acylation experiment

Setting up the reaction:

1) Put a magnetic stir bar in your 500 mL round-bottom flask.

2) Take your empty vial, the round-bottom with a stopper, and a graduated cylinder to the hood. Using the vial of 14.0 g of AlCl3 in the hood as a guide, eyeball about the same amount of anhydrous AlCl3 into your empty vial. Cap the vial. Measure out 25 mL of dichloromethane into the graduated cylinder. In the lab's hood, use a POWDER funnel to transfer the AlCl3, a little at a time, into your round-bottom. Remove the powder funnel and rinse any AlCl3 stuck inside the neck of the flask into the flask using the 25 mL of dichloromethane. The neck of the flask must end up clean. Stopper the flask and take it back to your bench. Clamp the round-bottom in an ice-water bath and begin stirring gently.

3) Take your separatory funnel and graduated cylinder to the lab’s hood, bringing the drying tube along as a stopper. Measure out 15 mL of dichloromethane and put it into the funnel. Use the acetyl chloride’s calibrated dispenser (check first that there are no bubbles in the dispensing arm) to deliver the volume needed to put 8.0 g of acetyl chloride into the funnel. Stopper and take the funnel to your fume hood.

4) Finish setting up the rest of the apparatus as shown in Figure 1 (see next page): • The inverted funnel should be at or about 2 mm above the surface of the water in the beaker; it’s purpose is to trap the acidic gases produced by the reaction. •Use the ugliest pieces of rubber tubing you have to connect the condenser to the gas trap; they will only get uglier from the HCl that goes through them. • tubing connectors or pieces of plastic tubing can be used to connect the hoses • You can tape the drying tube and/or hoses to the inside of your hood for support The reaction:

5) Turn on the condenser cooling water. Over a period of 15 minutes, slowly add the acetyl chloride solution in the separatory funnel to the suspension of AlCl3 in the flask. The reaction is very exothermic, and you will understand why adding the solution quickly is a bad idea. Make sure the magnetic stirrer is doing its job. Add more ice to the cooling bath as needed.

6) After the addition is complete, weigh out your calculated mass of unknown into a small beaker and add 10 mL of dichloromethane. Make sure the stopcock on your separatory funnel is closed then pour the solution into the funnel. Slowly add it to the round-bottom flask over a period of 15 minutes. Adding the solution too rapidly will cause excessive bubbling from the rapid release of HCl gas. That’s bad. After the addition is complete, remove the ice-water bath and stir for an additional 30 minutes.

7) Put 50 g of ice and 25 mL of concentrated HCl in a 500-mL Erlenmeyer. Transfer the stir bar from your reaction flask to the Erlenmeyer. In the hood, pour your reaction mixture slowly with stirring into that mixture (fuming will occur). Rinse the reaction flask with a little ice water to improve transfer. Stir the acid mixture thoroughly for 10 minutes. Transfer the mixture to your own 250 mL Erlenmeyer. Remove, clean, and return the 500 mL Erlenmeyer flask and stir bar to the stockroom. Stopper the 250 mL Erlenmeyer and store in a safe place in your drawer or cover with a beaker, label it, and put it in the instrument room hood.

Purifying the product:

8) Transfer the mixture to a clean separatory funnel. Separate the dichloromethane (organic) layer and save it in a flask. Leave the aqueous layer in the separatory funnel. Please, please make sure you know which layer is which. If you throw out your product after all this, ask a friend to hold you while you sob quietly. Put 30 mL of dichloromethane into the original 500 mL Erlenmeyer (rinse the flask) and add it to the aqueous solution in the separatory funnel. Shake and separate layers again, adding the dichloromethane layer to the dichloromethane layer previously saved.

9) Wash your saved dichloromethane solution with 50 mL of saturated sodium bicarbonate. Expect a lot of CO2 to be released and vent frequently. Wash with a second portion of bicarbonate if necessary. Separate the organic layer and dry it over anhydrous sodium sulfate for 10 minutes.

Removing the dichloromethane (Simple Distillation):

We will be doing a simple distillation followed by a vacuum distillation from the same distilling flask. Because we want to use the smallest size flask in the final distillation to minimize product holdup, we will use a flask that doesn’t hold all the solution and add more solution from a separatory funnel as the dichloromethane distills off.

Use a 50-mL round-bottomed distilling flask containing a medium or small magnetic stir bar (instead of boiling chips). Attach a Claisen adapter with a clean dry separatory funnel on the opening above the distilling flask. Put the still head and condenser on the other opening. Attach the digital thermometer to the thermometer adapter with a septum instead of the usual plastic dropper adapter. Fill the flask half full with the dried dichloromethane solution. Put the rest in the separatory funnel. Rinse the drying agent with a little dichloromethane and add the rinse to the funnel. Using hot water as the heat source, distill off the dichloromethane while stirring. Add more solution from the separatory funnel as space becomes available. Distill until all the dichloromethane has been removed.

Vacuum Distillation:

Without washing, reassemble the apparatus as shown in Figure 2, using the same boiling flask and a clean preweighed receiver. Check that all joints are clean and lightly greased (joints look clear, not opaque) and that the thermometer is positioned to obtain accurate boiling points.

----------Question--------

1. Why is it better to do the final distillation in this experiment under reduced pressure rather than at atmospheric pressure?

2. Was the distilled product pure? Was any unreacted starting material present in the distilled product? Give specific evidence.

What could be the evidence for an impure distilled product?

3. Since we used the same mass of all the unknowns in this experiment and the unknowns had different molar masses, the mole ratio of acetyl chloride to aromatic hydrocarbon in some people's experiment was almost 2 to 1. Would diacylation be expected in these cases? Explain.

Unknown: One of the following

toluene

ethylbenzene

o-xylene (1,2-dimethylbenzene)

mesitylene (1,3,5-trimethylbenzene)

m-xylene

cumene (isopropylbenzene)

p-xylene

anisole (methoxybenzene)

p-cymene (1-isopropyl-4-methylbenzene)

tert-butylbenzene

Explanation / Answer

1) After final distillation of dichloromethane , we want to distill out our product. The boiling point of product will be higher than the warm water (100°c). So if we don't put vaccum, we will not be able to reach the boiling point of our product and hence will not get distilled.

Applying vaccum we are reducing the pressure in the round bottom flask. According to the definition of boiling point (Definition - temperature at which vapor pressure equals atmospheric pressure) we can distill of the product under reduced pressure at low temperature by reducing its boiling point.

It is mandatory to put vaccum at the end to distill off the pure product.

2) If the product is distilled at single temperature (its boiling point) it must be pure. If temperature varies while distillation than the impurities might be present.

There might be unreacted starting material too.

The acylation of simple alkyl benzene yields mixture of o-acyl and p-acyl product. E.g. toluene will give o-acyl toluene and p-acyl toluene. Hence two different products at two different boiling points will be distilled.

Tert-butyl benzene will give only p-acylation due to steric hinderance produced by bulky tert-butyl group.

3) Di-acylation is not possible because when first acyl group is introduced, the acyl group being electron withdrawing deactivates the aromatic ring. Hence second acylation is not possible.

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