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Sunday, March 17, 2013

Synthesis of 1-Bromobutane from 1-Butanol - Chemistry of Alcohol - Lu Le Laboratory




1-Bromobutane is a colorless liquid that is insoluble in water, but soluble in ethanol and diethyl ether. As a primary alkyl halide, it is especially prone to SN2 type reactions. It is commonly used as an alkylating agent, or in combination with magnesium metal in dry ether (Grignard reagent) to form carbon-carbon bonds. (Wikipedia)
 

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


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.


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 Br2 (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.

Chemicals

1.      anhydrous calcium chloride: ~0.5g
2.      10% aqueous sodium hydroxide: 6.25mL
3.      1-butanol: 4.63 g
4.      concentrated sulfuric acid: 12.5 mL
5.      sodium bromide: 7.5 g


Procedure

1.      Dissolve 7.5g NaBr with 7.5mL water in a 50mL round-bottomed bottle.
2.      Add 4.625g n-butanol in the bottle and cool it down to 5~10.


3.      Drop 6.25mL concentrated sulfuric acid into the bottle slowly, and stir it thoroughly.

When I was adding sulfuric acid into the mixture,
 HBr gas formed immediately.

4.      Reflux the mixture for 30 minutes.

The gas trap is used to absorb HBr gas which may 
cause serious corrosive for people and apparatus


5.      Cool the reacting mixture down and distill it.

Some 1-bromobutane was oxidized into bromine

6.      Collect all the distillate before the temperature arrived 110~115


7.      Pour the distillate into a separatory funnel and wash is with 6.25mL water. Collect the lower layer.


8.      Pour 6.25mL cold sulfuric acid in to the separatory funnel and mix with 1-bromobutane thoroughly. Let the mixture stand for a while until it separate into two layers. And then collect the upper layer.


9.      Mix the upper layer with 6.25 mL water in a separatory funnel, and collect the lower layer.
10.  Use 6.25 mL NaOH(aq) to wash the 1-bromobutane in a separatory funnel. Collect the lower layer.


11.  Add some CaCl2 anhydrous as a desiccant to remove water from 1-bromobutane.


12.  Measure the index of refraction and yield.

Experimental Record

Weight of 1-bromobutane
3.135g
Theory weight of 1-bromobutane
8.550g
Yield
36.7%
nD22.4
1.4382
nD20.0
1.4490


5 comments:

  1. Really helped me with my n-butyl bromide preparation organic chemistry lab!Thanks!

    ReplyDelete
  2. Thank you! Did you notice a red gas that appeared when sulfuric acid was added in the beginning of the experiment?

    ReplyDelete
    Replies
    1. The red/yellow gas that appears is the formation of HBr.

      Delete