Question

Lab 8:  Synthesis of n-butylbromide

Draw the complete mechanism for the main reaction and indicate the expected mechanism type. Explain your reasoning.

Discuss the differences of the of n-butyl alcohol and n-butyl bromide based to their physical properties (think intermolecular forces), and why they are important to this experiment.

Discuss the general purpose of the initial heating.

Discuss the general purpose of the extraction steps.

Discuss the general purpose of the distillation.

Answer 1

Introduction

In this experiment, here we are going to convert n-butanol to n-butyl bromide by reacting with sulfuric acid and sodium bromide.Primary alkyl bromides could be synthesized from the analogous alcohols by reacting them with sodium bromide and sulfuric acid. Hydrogen bromide is generated in situ based on the equation.

NaBr + H₂SO₄                        HBr + NaHSO₄

Glut of sulfuric acid is utilized, and the mechanism engages the SN2 displacement of the protonated hydroxyl group (water) by bromide ion. Water is the leaving group.

Materials for the synthesis of n-Butyl bromide:

Chemicals:

n-butil alcohol, sodium bromide, concentrated sulfuric acid, 5% aqueous sodium hydroxide and calcium chloride.

Apparatus:

100mL round bottom flask, cork ring, 1 stirr bar, stirbar retriever, reflux condenser, small metal clamp, wire clips, separatory funnel with stopcock and glass stopper. .

Procedure:

To the 13.6g of sodium bromide add 12mL of water and 10mL of n­- butyl alcohol in a 0.100L round bottom flask. Place the round bottom in ice water bath about 10-15mins for cooling the reaction mixture under stirring condition. Under these conditions add 15mL of concentrated sulfuric acid in 1-2 mL portions. Affix a condenser to the round bottom flask and connect the condenser to the tap water, heat the reaction mixture under refluxing for 30 min. During refluxing time, if the white fumes are appear , then reduce the temperature. After 30mins of time, stop the heating and cool the mixture by submerging in ice bath. Transfer the reaction mixture to a separating funnel. If there is any solid left at the round bottom flask, add 24mL of H₂O to solubilise these solids and shift this solution into the separating funnel. Shake the funnel thoroughly and allow the layers to divide. Throw away the aqueous part into a separate beaker and keep the organic layer. Look at Table 1.Wash the organic layer thoroughly by adding adding 12 mL fractions of concentrated sulfuric acid , water , 5% aqueous sodium hydroxide and water again. Save the concentrated sulfuric acid and aqueous sodium hydroxide washings in separate beakers. Be sure that in each and every operation save the organic layer and do not discard your product accidentally. When the product is in bottom layer, drain it out into a beaker, discard the wash layer and then transfer the product back into the separating funnel to carry on.

                               Shift the crude product to a dry Erlenmeyer flask (5o mL) having sufficient anhydrous calcium chloride, to occupy the flask bottom. Spin the reaction mixture till the solution is plain, it shouldn’t be cloudy. Filter the clear solution using a small plug of cotton into a earlier weighed container or vial. Again weigh the vial or container with n- butyl bromide in it.

Table1: Aqueous washings of n-butyl bromide product

Wash with:	1st water	Conc. Sulfuric acid	2nd water	5% NaOH(aq)	3rd water
n-butyl bromide layer:	Top	Top	bottom	Bottom	Bottom

The general purpose of initial heating:

The purpose of heating is to make the raction to yield product in faster, i.e. without heating also reaction can be processed and we can get the product partially. But if you are maintain the heating condition it will make the reactants to react each other in very less time because the heating procedure creates lot of energy to each and evary molecule so that the move rapidly with high speed and react with each other.

The general purpose of extraction steps:

The purpose of extraction is to extract the required product from the mixture of components.

The general purpose of distillation:

The purpose of distillation is to separate out the final yield from the mixture of solvents. We give heat energy to the mixture above the boiling points of the solvents to separate the product.