Preparation of 1

Preparation of 1-Bromobutane from 1-Butanol by Sn2 Reaction Essay Name: Tonny, Chan Kar Yu, Student ID: 10297729 Date of Experiment:19th March 2005, Group: B1 Title:Preparation of 1-bromobutane from 1-butanol by SN2 reaction Objective: 1. To study the preparation of 1-bromobutane from 1-butanol by an SN2 reaction 2. To study the method of purification of an organic compound by simple extraction 3. To study the test of identification of alkyl halide Chemicals and Apparatus: ~18. 0 g 1-butanol, ~20. g sodium bromide, 15 ml of concentrated sulfuric acid, anhydrous magnesium sulfate, ~10 ml of 5% aqueous sodium bicarbonate, ~1 ml of sodium iodide – acetone reagent, 1 ml of bromine in chloroform, 1 piece of 100 cm3 round-bottomed flask, 1 piece of 50 cm3 of beaker, 1 piece of 50 cm3 of conical flask, 1 piece of 250 cm3 of separating funnel, 1 piece of 10 cm3 of measuring cylinder, 1 piece of electronic hot plate, oil bath, several pieces of anti-bumping granules, ice, filter paper and filter funnel. Background: Nucleophilic substitution is an imp ortant class of organic reaction. In the experiment, 1-bromobutane was synthesized through the second order mechanism in the present of concentrated sulphuric acid and sodium bromide. The nucleophile in the experiment is bromide ion (Br-) while the leaving group is water. Purification of the product was then obtained by extraction. The principle of extraction as a purification method was based on the difference in solubility between impurities and product. Identify of the product (1-bromobutane) can be confirmed by carrying out sodium iodide in acetone test. The test involves displacement of bromide by iodide. Since iodide is strong nucleophile which can displace the bromide ion from attached carbon. The reaction was carried out in acetone since acetone dissolved NaI, but not the products NaBr. So the appearance of precipitate and the time it takes to form is the basis for a qualitative test for alkyl halides. Procedures: The experiment was divided into three parts and worked in pairs. A) Preparation of 1-bromobutane from 1-butanol 1. 18. 0 mL of 1-butanol was weighed into a 50 cm3 beaker. The exact amount of 1-butanol used was being marked. 2. The 1-butanol was transferred into a 100 cm3 round-bottomed flask. 3. The beaker was rinsed with 15 mL deioned water and the rinse was put into the round-bottomed flask. 4. 20 gram of sodium bromide was added into the round-bottomed flask. 5. The mixture was cooled in a ice water bath. 6. To the flask in the ice bath, 15 mL of concentrated sulphuric acid was added gently. The flask was swirled to mix the content and a few anti-bumping granules were added. 7. The flask was fitted with the apparatus for reflux on a hot plat with an oil bath; a condenser was placed with tubing connected on top of the round-bottomed flask as shown in figure 1 of the laboratory menu and the join was wrapped with grease. . Checked that water was flowing through the condenser, and then the mixture was heated to reflux. 9. The mixture was allowed to reflux for 45 minutes, during which time mark the observation of the reaction mixture. 10. After the reflux period, the heat was removed and the mixture was allowed to cool at room temperature. B) Purification of 1-bromobutane by e xtraction 1. The cool mixture was decanted into a 250-mL separatory funnel. 2. The round-bottomed flask was rinsed with small amount of water and the rinse was put into the separatory funnel. 3. The two layers were allowed to separate and the aqueous later was drained off. . The organic later was washed in the separatory funnel successively with 10 mL water, 10 mL of 5 % aqueous sodium bicarbonate and 10 mL of water. 5. The funnel was shaken well in each case. The gas pressure was released regularly during the shaking. 6. The aqueous layer was discarded down the drain. 7. The organic later was collected into a 50 mL conical flask. 8. To the organic later, about 1 gram of anhydrous magnesium sulphate was added. The conical flask was swirled thoroughly. 9. The organic layer was filtered using the filter paper and funnel to remove the used magnesium sulphate. 0. The dry organic extract (1-bromobutane) was collected in a pre-weighed beaker and the exact mass of product was obtained. C) Test for the product i) Sodium iodide in acetone test 1. 5 drops of the product (1-bromobutane) was placed to a test tube 2. A few drops of sodium iodide-acetone reagent were added to the test tube 3. The mixture was shaken and let it stand for 3 minutes. 4. Changes were observed. ii) Bromine test 1. 5 drops of the product (1-bromobutane) was placed to a test tube 2. A few drops of bromine-chloroform reagent were added to the test tube. . The mixture was shaken and let it stand for 3 minutes. 4. Changes were observed. Results: Preparation of 1-bromobutane from 1-butanol Mass of 1-butanol used = 18. 0 g Mass of 1-bromobutane (dried) = 9. 207 g Percentage yield Observation during the course of reaction: two immiscible liquid formed, the one on the top is yellow colour, and the one on the bottom is colourless. Purification of 1-bromobutane by extraction Observation during the exaction: Two colourless layers formed, the yellow layer was decolourized, and the organic layer has glue-like smell. Chaucers the House Of Fame: The Cultural Nature EssayIf two immiscible layers form in the test tube, the lower layer in the funnel is organic (in this case return the contents of the test tube to the funnel). 7. In the experiment, especially the extraction process, it was a good practice to save all layers until the product was surely in hand. 8. In the part C of the experiment, sodium iodide in acetone provides reaction conditions favourable to SN2 reactions. The test tubes used must be totally dry because the anhydrous acetone reaction made the reaction forward, from alkyl bromide to alkyl iodide. Sodium iodide was soluble in this solvent, whereas sodium chloride and sodium bromide were not. When the reaction occurred, a precipitate of sodium bromide would form. Conclusion: 1-butanol went through an SN2 reaction by using sodium bromide in water to create hydrobromic acid in the presence of excess sulfuric acid. Thus, water was the leaving group and bromide ion was the nucleophile in this in situ reaction. This displaced the alcohols -OH group with a bromine creating 1-bromobutane. The excess sulfuric acid pushed the reaction far to the right corresponding with LeChatlier’s principle which states that more reactants yield more products. The extractions further isolated the product, the substance which has higher density was always the bottom layer and the lighter was on top. It turned out that the organic layer was always on the bottom of each extraction. Anhydrous magnesium sulphate was a very good drying solution. This allowed it to soak up the remaining water to a great extent. The amount collected was 9. 207 gram of 1-bromobutane. The percent yield of 1-bromobutane obtained was 31%. The sodium iodide/acetone classification tests proved positive. This reaction must be a SN2 reaction as the product was almost entirely 1-bromobutane, if by an SN1 mechanism, the product should consist of mostly 2-bromobutane derived from the sec-butyl cation formed via a 1,2-hydride shift. Such an interpretation of the product distribution must be made with caution. The presence of small amounts of 2-bromobutane in the reaction product did not necessarily mean that it was produced from 1-butanol via an SN1 mechanism because an alternative sequence of an E2 elimination of the protonated 1-butanol to give 1-butene followed by HBr addition to the 1-butene would also produce 2-bromobutane. pic] Hazard and precaution: Precautions were taken to prevent the highly inflammable and volatile alcohol from catching fire or lost through evaporation. The pressure of the separatory funnel must be periodically released to avoid the stopper being pushed out and product being lost and sprayed. The pressure is due to liberated carbon dioxide. All substances must be handled in the hood, wearing gloves and eye protection. Avoid contact with eyes and inhalation. Skin contact procedures for these substances: Wash affected area with plenty of soap and water. Bromobutane is a lachrymator. Avoid breathing vapors and skin contact. Wear gloves when handling sulfuric acid or pour very carefully! Be wary of drips on the outside of containers. It is too viscous for a Pasteur pipet, so pour a small amount into a small beaker. A graduated cylinder wasn’t necessary since it’s used in excess Eye contact with sulphuric acid required immediate attention. Wash with plenty of waster and see a physician as soon as possible.