Organic Chemistry : How to Purify Compounds

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : How To Purify Compounds

A chemist carries out a reaction that generates several side products. Which of the following can she eliminate from the reaction mixture by washing with an aqueous solution of sodium hydroxide?


Q1

Possible Answers:

I, III, and IV

I, II, III, and IV

I and III

III and IV

I only

Correct answer:

I and III

Explanation:

In organic chemistry, chemists often exploit the acidic and basic properties of side products in order to remove them. Compounds I-IV are all organic-soluble, however, if they are converted to a sodium salt via deprotonation with sodium hydroxide, they will become fully soluble in the aqueous sodium hydroxide solution and be washed away with this layer. The question we must then address is which of these four compounds will be deprotonated by sodium hydroxide.

Sodium hydroxide is a relatively strong base. Given its conjugate acid is water, which has a pKa of 15.7, sodium hydroxide can deprotonate any compound with a pka lower than 15.7. Another way to say this is that sodium hydroxide has a pkaH (defined as the pKa of the conjugate acid) of 15.7. Thus, we see that I and III may be deprotonated and washed away, leaving II and IV behind in the reaction mixture.

Example Question #1 : How To Purify Compounds

Which of the following compounds has the highest boiling point?

Possible Answers:

2-methyl-2-propanol

2-methyl-2,4-pentanediol

2-methyl-2-butanol

1,2-ethanediol

Hexanol

Correct answer:

2-methyl-2,4-pentanediol

Explanation:

There are a few rules to follow when estimating the boiling point of certain compounds. One rule is that the more carbons there are, the higher the boiling point. However, hydrocarbon molecules only exhibit Van der Waals forces (the weakest intermolecular force).

The reason 2-methyl-2,4-pentanediol is correct is because it has two -OH groups that are capable of hydrogen bonding (the strongest intermolecular force). This compound also has a significant number of carbons compared to the other compounds. Combined, these factors will each raise the boining point of the compound.

Example Question #2 : How To Purify Compounds

Suppose that a chemistry student wants to separate a mixture consisting of benzoic acid, napthalene, and n-butylamine dissolved in ether. First, the student adds an aqueous solution of  to the mixture and then extracts the aqueous layer to give solution A. Next, the student adds an aqueous solution of  to the mixture and then extracts the aqueous layer to give solution B. The remaining organic phase of the mixture is assigned as solution C.

Which of the following gives the correct assignment of the contents of solutions A, B, and C?

Possible Answers:

Solution A contains napthalene, Solution B contains benzoic acid, and Solution C contains n-butylamine

Solution A contains n-butylamine, Solution B contains benzoic acid, and Solution C contains napthalene

Solution A contains napthalene, Solution B contains n-butylamine, and Solution C contains benzoic acid

Solution A contains benzoic acid, Solution B contains n-butylamine, and Solution C contains napthalene

Correct answer:

Solution A contains n-butylamine, Solution B contains benzoic acid, and Solution C contains napthalene

Explanation:

This question is giving us a scenario in which a mixture of organic compounds is being separated via extraction. When doing extractions, the chemist is able to take advantage of the fact that organic compounds can be found in one of two layers: either the aqueous layer or the organic layer. The aqueous layer will contain molecules that are hydrophilic or that possess a net charge, while the organic layer will contain molecules that are hydrophobic and do not possess a charge.

Initially, all three compounds are dissolved in a solution of ether. From this, we can infer that the three compounds are starting off as being relatively hydrophobic and with no net charges.

The first solution to be added is aqueous . Upon adding this, we will have two layers - one aqueous and one organic. Since we know that this is a strong acid, we can expect that the n-butylamine that is dissolved in the ether to become protonated (since it will act as a base). Furthermore, since the protonated n-butylamine now carries a net positive charge, it will be expected to transfer from the organic phase into the aqueous phase. Removal of the aqueous layer in this case gives us Solution A, which contains n-butylamine.

Next, we are told that an aqueous solution of  is added to the mixture. Just as before, adding this solution results in a scenario where we have an organic phase and an aqueous phase. Since we know that  is a strong base, we can expect the benzoic acid in the organic phase to become deprotonated. Upon deprotonation, benzoate now carries a net negative charge, making it more favorable for it to move over and dissolve within the aqueous layer. Therefore, Solution B will contain benzoic acid.

And lastly, the remaining organic phase, which we call Solution C, will still contain napthalene because it was never able to cross over to any of the aqueous layers, owing to its hydrophobic nature.

Example Question #4 : How To Purify Compounds

During fractional distillation, substances are separated according to which of the following characteristics?

Possible Answers:

Molecular weight

Boiling points

Molarities

Melting points

Correct answer:

Boiling points

Explanation:

The process of fractional distillation separates an organic mixture containing different hydrocarbons by heating them in long, insulated copper columns until they vaporize. Therefore, the hydrocarbons are separated according to their boiling points-compounds with lower boiling points will be separated first and compounds with higher boiling points will be separated last.

Example Question #4 : How To Purify Compounds

Which of the following laboratory procedures could best be used to separate benzoic acid from phenol if they are both solvated in hexanes? 

Possible Answers:

Extraction using sodium bicarbonate

Fractional distillation

Extraction using sodium hydroxide

Recrystallization

Distillation

Correct answer:

Extraction using sodium bicarbonate

Explanation:

Extraction with sodium bicarbonate is correct because it will deprotonate benzoic acid and not phenol, which will allow benzoic acid to enter the aqueous layer while leaving most of the phenol in the organic layer. Once extracted, benzoic acid can be further isolated by other laboratory techniques. Sodium hydroxide would deprotonate both benzoic acid and phenol, and thus would not effectively seperate them. Since both phenol and benzoic acid have melting points above hexane's boiling point, any form of distillation will not work for isolating benzoic acid. Recrystallization will also not work, as the solubilities of benzoic acid and phenol are similar. 

Example Question #5 : How To Purify Compounds

When is fractional distillation used?

Possible Answers:

When attempting to separate two mixtures whose boiling points are similar

When attempting to separate two mixtures whose boiling points are very high

When attempting to separate two mixtures whose boiling points are very different

When attempting to separate two mixtures whose boiling points are very low

Correct answer:

When attempting to separate two mixtures whose boiling points are similar

Explanation:

Distillation is used to identify and purify organic compounds. In the process of purification, we separate a compound from another material by exploiting their boiling points. When different compounds in a mixture have different boiling points, the mixture separates into its component parts when it is distilled. 

A simple distillation apparatus contains a round-bottom flask attached to an adapter holding a thermometer (to determine the boiling point of the liquid). The adapter connects to a condenser into which cold water passes through. The condenser leads to a collection flask for the pure liquid. Simple distillations are used when the liquid is already relatively pure, when the liquid has a non-volatile component (like a solid contaminant), and when the liquid is contaminated by a liquid with a boiling point that differs by at least ° Celsius (when the boiling points are very different).

A fractional distillation apparatus is basically the same as the simple distillation apparatus, but in the former, a fractionating column is placed between the boiling flask and the condenser. Fractional distillation is used when trying to separate liquids whose boiling points differ by less than ° Celsius--when attempting to separate liquids whose boiling points are similar.

Vacuum distillation is distillation at a lower pressure. As the boiling point of a compound is lower at a lower external pressure, the compound will not have to be heated as much as it normally would in order for it to begin boiling. Vacuum distillation is used to distill compounds that have a high boiling point (over ° Celsius), or any other compound which may decompose upon heating at atmospheric pressure.

Example Question #3 : How To Purify Compounds

The following oxidation of benzyl alcohol to benzoic acid was performed. It was determined that 13% of the alcohol was unreacted after the reaction. What purification method would you use to best separate the two compounds?

1q2

Possible Answers:

Infrared spectroscopy

Fractional distillation

Column chromtagraphy

Filtration

Acid-base extraction

Correct answer:

Acid-base extraction

Explanation:

The two compounds would be separated very quickly and easily by acid-base extraction, using minimal solvent. Column chromatography uses large quantities of solvent and is not very suitable for very polar compounds, such as carboxylic acids. Fractional distillation does not work very well for compounds with high boiling points such as benzoic acid . Filtration only works when one compound is soluble in a solvent that the other is insoluble. These two compounds have almost identical solubility. IR spectroscopy is an identification technique, not a separation technique.

The acid-base extraction uses a base such as aqueous bicarbonate to convert the carboxylic acid into a water soluble salt (carboxylate). The alcohol is unaffected by this weak base  and stays in the organic solvent layer. The water layer is separated and then acidified to recover the pure carboxylic acid.

Example Question #7 : How To Purify Compounds

A reaction is run to form a compound that has a boiling point of . Assume there is 13% starting material unreacted that has a boiling point of , what is the best method to purify the product?

Possible Answers:

Micro distillation

Vacuum distillation

Simple distillation

Fractional distillation

None of these

Correct answer:

Fractional distillation

Explanation:

The rule of thumb is if the compounds have a boiling point difference of less than  you use fractional distillation. Simple distillation is only used to separate solids from liquids or liquids from liquids that have a large boiling point difference . Vacuum distillation is only used for compounds with a very high boiling points . Vacuum is applied that will reduce the pressure closer to the compounds vapor pressure (and is not considered very efficient separating compounds with close boiling points). Micro distillation is a special technique used to separate compound mixtures .

Example Question #8 : How To Purify Compounds

Recrystallization is a technique used to purify solid compounds, which involves growing crystals. The compound you are recrystallizing should have which of the following solubilities in the recrystallization solvent:?

Possible Answers:

Insoluble hot; soluble cold.

Soluble hot; insoluble cold.

Polar soluble; nonpolar insoluble.

Nonpolar soluble; polar insoluble.

Acid soluble; base insoluble.

Correct answer:

Soluble hot; insoluble cold.

Explanation:

There is only one ideal characteristic of a recrystallization solvent: to dissolve the compound when hot and the compound should be insoluble cold. The technique uses a minimum amount of solvent to dissolve the compound being purified at or near the boiling point. The resulting solution is allowed to cool, which decreases the solubility. The compound begins to grow crystals, as the lattice excludes impurities resulting in a pure compound.

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