Synthesis of Butyl Bromide - Introduction to Organic Chemistry - Lecture Notes, Study notes of Organic Chemistry

Download Synthesis of Butyl Bromide - Introduction to Organic Chemistry - Lecture Notes and more Study notes Organic Chemistry in PDF only on Docsity! 42 EXPERIMENT SIX Synthesis of 1-Bromobutane from 1-Butanol DISCUSSION The treatment of a primary alcohol with a hydrogen halide yields a primary alkyl halide. The reaction proceeds by an SN2 mechanism, and competing dehydration is minimal. The reaction requires a strong acid to protonate the hydroxyl group. Aqueous HBr, gaseous HBr, and "constant boiling" HI (57% aqueous solution) can all be employed, without additional catalyst, to prepare the alkyl halide. In this experiment, HBr is generated in the reaction mixture by treatment of NaBr with H2SO4. NaBr + H2SO4 HBr + NaHSO4 When the mixture of alcohol, H2SO4, and NaBr is heated, gaseous HBr is given off; therefore, if the reaction is not carried out in a fume hood, a trap for the HBr must be arranged (See Figure). In the trap, the HBr emitted from the reflux condenser is passed over aqueous sodium hydroxide and thus converted by an acid-base reaction to sodium bromide and water. Docsity.com 43 An excess of sulfuric acid is used in the experiment to provide a strongly acidic medium for the protonation of the alcohol. Sulfuric acid, which is a dehydrating agent, also combines with the water that is formed as a product of the substitution reaction. A number of side reactions occur in this reaction. 1-Butanol can react with HSO4 - ions present in solution to yield a hydrogen sulfate ester (ROSO3H). This inorganic ester, in turn, can undergo elimination to yield 1-butene (a gas that is lost during the reflux and work-up) or substitution with 1-butanol to yield di-n-butyl ether (which must be removed during work-up). Another side reaction that occurs is oxidation of the 1 -butanol by either H2SO4 or Br + (formed by oxidation of Br- by H2SO4). At the end of the reaction, the mixture consists of two phases. The upper layer contains the desired 1-bromobutane plus organic by-products, and the lower layer contains the inorganic components. The work-up techniques in this experiment consist of four steps: (1) an initial steam distillation; (2) extraction ; (3) drying ; and (4) a final distillation to purify the product. In the steam distillation, the water and 1-bromobutane co-distil, leaving the inorganic compounds behind in the distillation residue. Unfortunately, di-n-butyl ether, butanoic acid, and unreacted 1 -butanol also co-distil with water and must be removed from the distillate by extraction. The first extraction, a water wash, removes some of the 1-butanol, which is slightly soluble in water. The second extraction is with cold, concentrated sulfuric acid. (If the acid is not cold, extensive charring of the organic material will occur.) Each of the two major impurities (1- butanol as di-n-butyl ether) and the minor impurity (butanoic acid) contains an oxygen atom. In strong acid, each of these compounds is protonated to yield a sulfuric acid-soluble salt. 1- Bromobutane does not form a salt with sulfuric acid; consequently, it remains in the separatory funnel as a separate layer. This extraction is thus an example of a chemically active extraction. A subsequent extraction with aqueous sodium hydroxide solution removes any sulfuric acid clinging to the sides of the separatory funnel. The wet alkyl halide is then dried with anhydrous calcium chloride. Calcium chloride is the drying agent of choice in this reaction because it forms complexes with any residual alcohol, as well as with water. After drying, the 1-bromobutane is purified by distillation. EQUIPMENT condenser for reflux distillation assembly dropper or disposable pipet three 50-mL and one 125-mL Erlenmeyer flasks funnel, glass tubing, and rubber tubing (or fume hood) 1 00-mL graduated cylinder heating mantle and rheostat ice bath 250-mL round-bottom flask 50-mL round-bottom flask with glass stopper 125-mL separatory funnel thermometer Docsity.com 46 EXPERIMENTAL NOTES 1) The arrangement of an HBr trap is shown in the figure. The liquid in the beaker is approximately 5% aqueous NaOH. (CAUTION caustic!) 2) To verify that no oil is co-distilling with the water toward the end of the distillation, collect a few milliliters of the distillate in a test tube. Shake or flick the test tube with your finger. If any oil droplets a present, they will become visible when you hold the test tube up to the light. 3) Pay careful attention to the identification of the layers in the separatory funnel during this experiment. During one extraction, the product is in the lower layer; during another, it is in the upper layer. It is prudent to save all layers in labeled flasks until the completion of the experiment to avoid inadvertently throwing away the wrong layer. 4) Emulsions are common in the extraction, and standing time will be necessary to allow the phases to separate. Even so, the interface between the layers may be indistinct, and judgment may be necessary when the layers are separated. Docsity.com 47 Name Date BROMOBUTANE WORKSHEET Grams % Yield Refractive Index Bromobutane (expt'l) Bromobutane CRC or calc. 1) Please give the mechanism of this reaction. 2a) Why was sulfuric acid used in the reaction of 1-butanol with sodium bromide instead of HNO3 or HCl? 2b) Could concentrated HI be substituted for sulfuric acid in this experiment? Explain. 3) Which of the following types of boiling chips would be suitable for the reflux of reactants in this experiment? Explain. (a) marble (CaCO3) (b) alumina (Al2O3) (C) silicon carbide (SiC) 4) To prevent gaseous HBr from contaminating the laboratory, one student corks the top of his reflux condenser. Why is this a bad idea? Docsity.com 48 5) Suggest a reason why the product 1-bromobutane does not react with water in the reaction mixture and revert to 1-butanol. 6) Suppose that you have only 15 g of 1-butanol to use in this experiment. What is your theoretical yield of 1-bromobutane? 7) If this experiment were carried out using the following alcohols in place of 1-butanol, what organic products and by-products would you expect in each case? (a) 2-hexanol (b) benzyl alcohol (C6H5CH2OH) (C) methanol Docsity.com