Question

2. You are about to do the following experimental procedure:

a. Add 0.35 ml of 1-hexene to a 15 ml reaction tube. Working in the hood, add 1.0 ml of 30% HBr in acetic acid (5 M) tothe tube. Cap the tube and shake it frequently for 10 min. Occasionally loosen the cap to release any buildup of pressure.Write a chemical equation that representsthe reaction that occurs. Draw the structure of reactants and products. What is the function of the acetic acid?

b. After the reaction period, allow the phases to separate and remove the lower acetic acid layer ith a Pasteur pipet. Place the acid layer in a 150 ml beaker containing 50 ml of water. What substance is or is in the top layer?

c. Add 2.0 ml of ether and 2.0 ml of water to the organic phase remaining in the reaction tube. Cap the tube and shake it

to mix the phases. Separate the layers. Which layer, top or bottom, should you keep? Give reasons.

d. Wash the ether layer with 2.0ml of 5% sodium bicarbonate solution. Remove the lower aqueous phase. What does the sodium bicarbonate do? What observation tells you the sodium bicarbonate is working?

e. Add anhydrous calcium choloride pellets to the reminaing ether solution, and allow to dry for 10 minutes. What does the calcium chloride do?

f. If peroxides are present, will the same product as in part (a) form? Give reasons

Answer 1

a) The reaction that is discussed is a C=C addition reaction with HBr. The products are Markonikov addition product the entire reaction is shown here.

As per the volume of material taken the weight of 1hexene is 0.23 g (0.35 ml x 0.67 density). That will be 0.0028 moles MW =84g/mol

HBr in acetic acid 30% density is about 1.3 g/ml so 1ml is 1.3 g but only 30% is HBr so we have 0.39 g of HBr that is MW of 81 gives 0.0048 moles.

HBr is a gas and can be added to a reaction either in gas form or as a solution in some liquid in this case Acetic acid is the sovent in which HBr has been dissolved also since the other reactant is an organic compound it helps in miscibility.

b)The density of water is 1g/cm³ and the density of acetic acid is 1.05 g/cm³ on the basis of this acetic acid should be in the bottom layer but as per the mole calculations above the HBr has been taken in excess and after the reaction the excess HBr will dissolve in water increasing the density of water so water will be in the lower layer. The acetic acid with the product will be in the top layer.

c) After addition of water and ether to this organic layer any unreacted HBr will go to the water layer along with some acetic acid and the ether layer will have the product. Ether or diethyl ether has a density of 0.713 g/cm³ so it will be the top layer and the water part will be the bottom layer. We should take the top ether layer which has the product.

d) Washing with sodim bicarbonate neutralises any acid present in the ether layer. Sodium bicarbonate is a base wand when it reacts with acid it releases carbon dioxide gas. So gas evolution will tell us that the acid is being neutralised.

e) Anhydrous calcium chloride is added to remove any moisture or water present in the ether layer and it is required to get a clear organic product as the final material. The calcium chloride will take up the water as it is in anhydrous or absolutely dry state.