- Leave the distillate to settle into two layers, an upper aqueous layer and a lower alkyl bromide layer.
- Separate the layers with a separating funnel, discarding the top layer.
- This leaves the alkyl bromide layer, which is still full of impurities. These include unchanged butan – 1 – ol, water, hydrogen bromide, bromine and sulphur dioxide.
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To purify the alkyl bromide layer transfer the liquid into a separating funnel. Mix the compound with 10 cm3 of pure water, again leave the solution to settle and separate the two layers.
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Now mix the bottom layer with 10 cm3 of 2 molar sodium hydroxide. This is to remove the acidic impurities and bromine.
- Once the solution has settled run the bottom layer of the solution off into a dry conical flask, and add several pieces of anhydrous calcium chloride.
- The liquid will turn cloudy due to the suspended droplets of water. To get rid of these allow the liquid to ‘dry’ for about ten minutes, swirling the flask occasionally until the liquid becomes clear.
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When clear filter the liquid through cotton wool into another clear dry 50 cm3 pear-shaped flask. Add a small amount of powdered pumice and distil, collecting the 1 – bromobutane that will boil of between 99°C and 103°C. This will produce a yield of about 7g or 5.5 cm3, which is 65% of the theoretical yield.
Equation
C4H9OH + NaBr + H2SO4 = C4H9Br + NaHSO4 + H2O
Mechanism
The first stage of the reaction the sodium bromide reacts with sulphuric acid and forms hydrogen bromide and sodium hydrogen sulphate:
NaBr + H2SO4 = HBr + NaHSO4
The hydrogen bromide is then oxidised to bromine molecules due to the fact that concentrated sulphuric acid is a very good oxidising agent. The sulphuric acid reacts to form sulphur dioxide gas:
HBr + H2SO4 > Br2 + 2SO2 (g)
During the next stage of the reaction the hydrogen bromide dissociates and the bromide ion from it attacks the Carbon atom with the -OH function group in Butan-1-ol and displaces the -OH function group forming a bromo function group and a hydroxide ion. This bromo group then associates itself with another H+ ion to form water:
CH3CH2CH2CH2OH + Br- > CH3CH2CH2CH2Br + OH-
In the final stage a molecule of sulphuric acid attacks the lone pair on an -OH function group. This releases a molecule of water, and a mixture of Butoxybutane and But-1-ene is formed, along with the regenerated Sulphuric Acid:
CH3CH2CH2CH2OH + H2SO4 > CH3CH2CH=CH2 + H2O + H2SO4
or
2 CH3CH2CH2CH2OH + H2SO4 > CH3(CH2)3O(CH2)3CH3 + H2O + H2SO4
Results
Mass of NaBr in pot = 11.47g
Mass of pot = 3.49g
Mass of Sodium Bromide = 7.98g
Mass of collecting beaker = 54.25g
Mass of distillation and beaker = 58.47g
Mass of 1 bromobutane collected = 4.22g
Yield Obtained
Theoretical yield = (moles of limiting reagent)(stoichiometric ratio; desired product/limiting reagent)(Mr of desired product)
= (0.0941 mole)(1 mole / 1 mole)(137.03g/ mole)
= 12.9g
Actual mass gained = 4.22g
Therefore actual yield = (Actual yield / Theoretical yield) x 100
= (4.22/12.9) x 100
= 32.7%
Conclusion
From my experiment I have found out that the practical yield of 1 bromobutane when made from butan – 1 – ol, is a lot lower than the theoretical yield, which was stated in a textbook. This difference in yields could be caused by several reasons most of which are down to human error. The first of these human errors could have occurred whilst the liquids were being decanted from one another, at this stage some of the 1 bromobutane may have been left in the beaker and discarded as to keep the amount of impurities in the flask to a minimum. Another human error, which may have caused a lower yield, would be during direct distillation where the temperature may not have been kept between 99 and 103°c. This would have reduced the yield as the chemicals collected would not be pure and therefore the yield would not be as large. A final human error may have occurred whilst waiting for the substance to “dry” after the anhydrous calcium chloride has been added, this would have caused a reduced yield as again the substance which was final distilled would have been less pure. Therefore the amount of 1 bromobutane boiled off between the fractions would have been lower than in the theoretical experiment.
Another reason that the yield may have been lower other than human error would have been any impurities within the solutions, if these impurities did occur then the boiling temperatures of the substances might have been altered. This would effect the amount of substance that is collected in a fraction temperature range and have an overall effect on the final yield. A final reason the practical yield may have been smaller than the theoretical yield is that the reactants may not have been mixed together at optimum conditions, which would reduce the rate of reaction and therefore reduce the final yield.
When running the experiment two changes were made, the first was that instead of using potassium bromide as first planned sodium bromide was used. This is because sodium bromide was easier to get a hold of and was more likely to give a decent yield of 1 bromobutane at the end of the experiment. Another change that was made was that at the first distillation, instead of waiting for all the oily drops to fall the only fraction collected was that which boiled of between 99 to 103°c, the boiling point of 1 bromobutane. This increased the accuracy of the experiment as instead of using qualitative information to decide when to stop distilling the liquid we were using quantitative, making the distil more accurate.
Overall however the experiment produced a decent yield of bromobutane even though it was not as high as that worked out in the theoretical experiment.
Evaluation
Overall I feel that the experiment went well as the final yield produced was at least half of the theoretical yield. However there were a few sources of error and these have been highlighted in the conclusion.
There are several ways that some of these problems could have been eradicated, and therefore the final yield would have been even greater. One of the easiest problems would have been to use sensors during distillation to record the temperature this would mean that fewer impurities would have been boiled off by accident. Also by setting up the experiment so it was easier to change between conical flasks would reduce that amount of distil which was missed during the change over. Another improvement to increase the yield would be to run the experiment at optimum conditions, which would promote a faster rate of reaction due to more collisions occurring and therefore more products being formed and a higher yield being collected. A final improvement to gain a higher yield would be to have allowed the solution to “dry” for longer this again would cut out some more impurities and therefore increase the yield. This was not done during the actual exam as time was running out and instead of letting the solution settle and then filter out the solid, the solution was allowed to settle and then the clear layers were removed and placed into the pear shaped flask using a pipette.
Even though there were some areas in which the experiment could have been improved I feel that the experiment went to a decent degree of accuracy and this is shown by the fact that I got quite a large percent of the theoretical yield in my practical yield.
