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Expt 06 - Synthesis of Bromobutane from Butanol

Expt 06 - Synthesis of Bromobutane from Butanol -...

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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 S N 2 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 H 2 SO 4 . NaBr + H 2 SO 4 ---> HBr + NaHSO 4 When the mixture of alcohol, H 2 SO 4 , 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.
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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 HSO 4 - ions present in solution to yield a hydrogen sulfate ester (ROSO 3 H). 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 H 2 SO 4 or Br + (formed by oxidation of Br - by H 2 SO 4 ). 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.
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Expt 06 - Synthesis of Bromobutane from Butanol -...

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