Purifying Naphthalene Using Recrystallization

Recrystallization

Objective:

 This lab objective is to purify naphthalene using the method recrystallization. By doing so choosing a good solvent system for crystallizing a solid. As well to evaluate the trades off of recrystallization and then evaluate the purity of a crystals through its melting points.

Introduction:

 Recrystallization is important because it is one of the most common way to purify solid organic compounds. One of the important solubility characteristics for a recrystallization solvent is how it reaction with each test. One is that with just plain distilled water, another is how it reacts with hot and cold temperatures. With hot temperatures the goal is that the solvent becomes soluble (melts) while in cold temperature it becomes a insoluble (becomes a solid). Finally, one of the last test and most important, is how the solvent boiling point, compare to naphthalene melting point. Throughout this process its learned that different solvent have different melting/boiling point which then present different properties when presented with different test of solubility.

Overall it is expected that water is the most polar, while hexane and toluene. (organic molecules) can be polar as well as long as it’s with other organic molecules. Though, as the table tells that water can bot be the solvent for recrystallization due to having such a high boiling temperature this is the same for toluene. By this point the way to see which solvent should be chosen is by doing the solubility test (with hot and cold temperature and observe how each solvent reacts).

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Once the crystals are recovered percent recovery equation will be used to find out how much of the actual crystal was manage to get. The equation is mass recovered (taken after the crystals is receive and weighed out) divide by the original mass (calculate it before the experiment began) and multiplied by 100 to achieve the percent. Below is the equation:

$\%\mathit{ Recovery}=\frac{\mathit{mass\; of\; Recovered}}{\mathit{Original\; mass}}\mathit{X }100$

The instrument that will be used to collect the temperature range is called the Mel-Temp apparatus. The Mel-Temp³ apparatus has a knob where you must move it for the temperature can increase 2 °C per minute.

Experimental Procedure:

 Materials:

Beaker

Small test tubes

Small Erlenmeyer flask

Boling chip

Coffee filters

250 mL side arm flask

Buchner funnel

Hot plate

Glass stir rod

Wire mesh

Watch glass

Drying oven

Melting point determination apparatus

Capillary tubes

Thermometer

Water, methanol, acetone, hexane, toluene

Decolorizing carbon

 A small beaker is obtained and heated, while a large beaker is filled with cold water. With the small test tubes (5) label each and add 3 mL of water, methanol, acetone, hexane, toluene respectively. Place them in cold-water bath for 2 min and add 0.10g of C₁₀H₈. The mixture that were insoluble at cool temperature heat in hot water to a gentle boil. Then see the melting points and boiling points, the boiling point of solvent should be lower than the melting point.

 Add 2.0g of C₁₀H₈ in solution flaks and 30mL of chosen solvent in solvent flask. Place them in hot plate until boil. After this the solvent was transfer to solution and then decolorizing carbon was added(pea-sized) to get the impurities out. Then filtrate it and put it in the oven to dry for 5 minutes. With the capillary tubes in one put the crystals that were made the other put impure crystals and then place them in the Mel. Point apparatus. Give the number as a range.

Results:

Table 1: Observation of Solubility

The observation in table 1 after distilled water was added to each solvent. This helps to tell what type of reaction they have and one of the test to see which solvent will be best to use for the recrystallization process.

Table 2: Observation with 0.10g C₁₀H₈ in Hot solvents

 In this part the hot solvent had to be soluble (melt the solids). Going through at table 2 the only ones that soluble were water, methanol and acetone. Though it should be noted that for methanol and acetone once shaken greasy particles were floating.

Table 3: Observation of 0.10g in Cold bath

 In this part the want is that the solvents are insoluble, they form the solid, when placed in a cold bath. The solvent that presented these characteristics is water, methanol, and acetone. So far, form what table 2 and 3 observation, hexane and toluene are not suitable for recrystallization.

Table 4: Boling points and melting points

 The solvent that was chose was Methanol due to passing the test for solubility it was the one that was insoluble in a cold bath and soluble in hot temperatures. Also, the main test, was that the boiling point must be lower than the melting point of naphthalene. Which is where water was tossed out due to being 100 (too much). The boiling point, as seen in table 4, for methanol is 64.7 while the melting point for naphthalene is 80.26, methanol is significantly lower than naphthalene.

Table 5: Starting Mass and Volume

 The naphthalene was added into the solution flask while the solvent was added into the solvent flask. They were both in the hot plate and once they were boiling small portions of solutions were transferred into the solvent.

Table 6: Impurities

 Impurities were still in the crystals at this point so decolorizing carbon was used to get some out. Only a bit is used and once it was added the color turned black/orange. Minutes later it turned black but clear around the edges.

Table 7: Physical property of crystals

 The observation of the crystals once they were made. They were overall hard and shiny when the light hits. The color was orange but overall white/clear, this can mean that some impurities are still present.

Table 8: final mass of Naphthalene

 It should be noted that only 25-28 mL of the solvent was added from the starting due to some of it was lost from evaporation.

Table 9: % recovery

 Not much of the crystals were recovered, less than 5 %.

Table 10: melting point ranges for recrystallized and impure C₁₀H₈

 In table 10, shows the range where which crystals started to melt and when it all melted. It should be noted that they both started to melt at the same time but overall the impure crystals melted completely first.

Discussion:

 In the process of solvating naphthalene, first the solvent that was chose was methanol. The solvent methanol was chosen because of its solubility, view table 1-4, overall it passed all solubility test and even the last test which was that its boiling point was lower than naphthalene melting point. In a molecular level¹, the goal was that for the Hot solubility was for the bonds to be broken (which is why a soluble is wanted) and for the cold bath a bond needs to form (wanting a solid to form). Methanol checked all these characteristics. This solvent was boiled in the hot plate and later was added a bit at a time into the solution (naphthalene). Once it was added the color was changing to an orange tint, see table 5 for what is the starting of each component. There were still impurities present so the process decolorizing carbon was sued to get rid of these impurities, view table 6 for observation. The decolorizing carbon was added (a small amount), this decolrized² solutions and also provided a large surface area where large colored molecules may be absorbed. This relates to the observation because once it was added a black tint came around but later it as black in the center but outside it as becoming clear.

 The melting point can be used to verify the purity because the more impure the solution is the higher the melting point, stronger bonds. Rather with pure solutions, there boiling points start earlier, it mostly that the comparison of the two where the boiling point of the solvent is compared to the naphthalene melting point. A range was given for this because it needs to be noted when it started to melt then when it all melted, one because one thing started to melt first does not necessary mean that it will be the first to melt all the way. The crystals that was made started to melt around the same time as the impure, view table 10, though it should be noted that it took a longer time to be fully melted. This can tell us that the crystals that were made had many impurities still in it due to it staring to melt around the same temperate of the impure crystals and also having an orange color similar to the impure crystals too. As well, the target amount of naphthalene that was made was less that wanted, only 3 % was made. This concluded that recrystallization wasn’t as successful due to errors.

 Some errors can be where when solvent was added, too much of the solvent can give off bad data. Another is that when filtrating the steps need to be carry out precise and accurate, a step may have been not in placed correctly, for example cooling of the solution before the filtrate for some minutes. At this step the solution may have been there less than a minute due to time constriction. Other errors were when measuring the melting point (to find the range), the knob needs to be tuned SLOWLY, if ignored then it will be carried out wrong. Due to the both having the same starting point but different melting point then this mean that turning of the knob was done too quickly.

Conclusion:

 In conclusion, the solvent that was chosen was methanol due to passing the characteristics for the best solvent. A low recovery was present this is due to recrystallization where it is most likely to lose crystals (especially when adding decolorizing carbon which eats away the solvent). Due to this only a small percentage was made and even then, it was found to be impure to its color (orange tint) and the melting range. Though all these could also be because of the errors that were made, not being precise in letting the solution to sit and wait for it to cool down. Also, for the melting point stage it is necessary no to turn the knob fast. In all the experiment was meant a solvent was chosen that meant the characteristic for this experiment, though a pure crystal was not made but it was know what errors caused it from not being made.

References:

1. Murmson, S. (2016, September 29) What Happens on the Molecular Level During an Endothermic Reaction? What Students Learn From Dissecting a Cow’s Eye | Education – Seattle PI.
2. Firewalking Myth vs Physics.
3. Gas Chromatography Technique Quiz.

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