Question

a.       Discuss which technique, simple distillation or fractional distillation, shall be the most efficient on purifying the mixture of solvents? Elaborate     (4 points)

b.      Which solvent should be distillated first, cyclohexane or Toluene? Elaborate.(3 point)

c.      How the RI measurements support (or not?) your expectations in the item b? (2.5 points)

d.      Making use of the RI measurements, explain if your distillation was successful. (3 points)

RI values:

Fraction	Observed Refractive Index (n20D)	Corrected Refractive Index (n20D)
Fraction 1	1.3975	1.3989
Fraction 2	1.4260	1.4274
Fraction 3	1.4865	1.4879

e.       Would re-distillation of individual fractions improve the purification of individual solvents? Elaborate.                                                        (2.5 points)

Answer 1

a) First to all, Distillation is a laboratory technique used for separating and purifying liquids; it  works by exploiting the different boiling temperatures of liquids. To separate two or more liquids by distillation, you first heat them in a flask. The more volatile liquid (the liquid with the lower boiling point) will typically evaporate first and the vapor will pass into a condensing column, where it can revert into a liquid (condense) on the cool glass where it trickles into a collection flask. Heating further will cause the less volatile liquids to evaporate and distill at higher temperatures. The two main kinds of distillation are simple distillation and fractional distillation, and both are used widely.

A simple distillation apparatus consists of a boiling flask (round-bottom flask) attached to an adapter holding a thermometer (to determine the boiling temperature of the liquid). The adapter connects to a condenser into which cold water is constantly passed through. The condenser leads into a collection flask for the purified liquid.

Fractional distillation is essentially the same as simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with glass or plastic beads. These beads improve the separation between the liquids being distilled. The reason that fractional distillation gives better separation between the liquids is because the glass beads in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compound over and over. The more volatile liquids will tend to push towards the top of the fractionating column, while lower boiling liquids will stay towards the bottom, giving a better separation between the liquids. Of course, the more theoretical plates that you add to a column (the more surfaces or beads), the longer the distillation will take (typically), and the more energy required to keep reevaporating liquid in the fractionating column.

The choice of whether to use fractional distillation or simple distillation depends on the two liquids being separated. Typically, using simple distillation is preferrable because the apparatus is, well, simpler, and a simple distillation typically goes faster than a fractional distillation (and requires less energy). On the other hand, fractional distillation gives better separation between the liquids. The choice of whether to use simple or fractional distillation, then, depends usually on the difference in boiling temperatures between the two liquids. If there is a large difference in the boiling points (>40°- 60°C)between the two liquids then simple distillation is probably the best option. On the other hand, if there is only a small temperature difference between the two liquids a fractional distillation is the preferrable option.

So in the case of Toluene (BP: 110.6 °C) and Ciclohexane (BP: 80.6 °C) there's a difference of 30°C between it's boiling points, so it's better use a Fractional distillation to separate these two liquids, according to the the stated above.

b) According to the stated above in a), the ciclohexane has a lower boiling point, therefore, it will be distillated first. Because it has the lower boiling point, this component will evaporate first and will pass through all the plates of the column, having a better separation. Of course, this takes more time than in simply distillation but due to the closeness of their boiling point, it's better use fractional distillation as stated before.

c) and d) I don't have much RI knowledge at this point, but, looking at the data provided and the reported RI of both compounds (Ciclohexane: 1.4266; Toluene: 1.496), we can actually that the RI support the choice in b) because the first fraction would be the from the first compound distillated (the one with the lower bp) and it's RI should be closer to the reported of the first compound. So in this way we can support that. That's my assumption.

In part d) Yes, we can say that distillation was successful mainly because the RI obtained are really close to the reported. The first fraction is a little far from the reported (3 units below the reported). This actually means that the first compound (Ciclohexane) it's getting distillated and it still it's not purified at all (with some traces of toluene). But the second fraction it's more close to the reported (some decimals above) this mean that the first compound is practically distillated and separated from compound B (Toluene). Finally with the third fraction, we are only seeing the compound B because compound A was already distillated in the second fraction, so it only takes to distillate (purify) compound B. The RI reported is closer to the obtained.

e) Of course it would improve the purification of individual solvents, however, it's not neccesary, because according to the RI measurements, they are too close to the reported RI of the compound, so we can actually say that the solvent is mostly pure from distillation, so it's not neccesary to distillate again.

I also need you to tell me if part a) and d) has to do with the mix of ciclohexane and toluene. I answered both questions, assuming that. But tell me if it's that or not to fix my answer.