Question

Propose a method for separating a mixture of cyclohexane- carboxylic acid, tri-t-butylamine, and decane.

Short answer

To separate a mixture of cyclohexane-carboxylic acid, tri-t-butylamine, and decane, perform the following steps: 1. Perform an acid-base extraction using hydrochloric acid (HCl) to selectively solubilize cyclohexanecarboxylic acid and tri-t-butylamine in the aqueous phase, leaving decane in the organic phase. 2. Treat the aqueous phase from Step 1 with sodium bicarbonate (NaHCO3) to selectively deprotonate cyclohexanecarboxylic acid, forming its water-soluble carboxylate anion. 3. Evaporate the aqueous layer from Step 2 and treat with a 6M HCl solution to isolate cyclohexanecarboxylic acid. 4. Add sodium hydroxide (NaOH) to the organic phase from Step 1, deprotonating tri-t-butylamine and forming its water-soluble conjugate base. Isolate the free amine from the aqueous layer using a 6M HCl solution. 5. Isolate decane from the organic phase obtained in Step 1 through filtration or column chromatography if needed.

Step-by-step solution

Solvent extraction

First, we will perform an acid-base extraction to separate the acidic and basic compounds from the nonpolar compound (decane). Acid-base extraction utilizes the difference in solubility of acidic or basic compounds in an organic solvent and their corresponding conjugate bases or acids in an aqueous phase. Add the mixture to a separatory funnel. To this separatory funnel, add an aqueous solution of hydrochloric acid (HCl) and shake gently. This will protonate the cyclohexanecarboxylic acid and tri-t-butylamine, making them soluble in the aqueous phase. Decane, being nonpolar, will remain in the organic phase. Separate the aqueous (bottom) and organic (top) layers.

Basic extraction

Add the aqueous phase obtained in Step 1 to a new separatory funnel, and then add an aqueous solution of sodium bicarbonate (NaHCO3). Sodium bicarbonate will selectively deprotonate the cyclohexanecarboxylic acid to form its water-soluble carboxylate anion, while tri-t-butylamine will remain protonated due to its higher pKa. Separate the aqueous (bottom) and organic (top) layers.

Acidic extraction

Now, to isolate the cyclohexanecarboxylic acid, evaporate the aqueous layer obtained in step 2. This will leave behind the sodium salt of cyclohexanecarboxylic acid. To convert it back to its acidic form, dissolve the residue in water and add an excess of a 6M HCl solution. This will protonate the sodium salt to form free cyclohexanecarboxylic acid, which can be isolated through filtration.

Isolate tri-t-butylamine

To separate the tri-t-butylamine from the organic phase obtained in Step 1, add an aqueous solution of sodium hydroxide (NaOH) to the organic phase. This will deprotonate the tri-t-butylamine, forming its water-soluble conjugate base. Separate the aqueous (bottom) and organic (top) layers. Evaporate the aqueous layer to obtain the sodium salt of tri-t-butylamine. To convert it back to its free amine form, dissolve the residue in water and add an excess of a 6M HCl solution. This will protonate the sodium salt to form free tri-t-butylamine, which can be isolated through filtration.

Isolate decane

Decane, being the only nonpolar compound in the mixture, can be easily isolated from the organic phase obtained in step 2 by simple filtration or, if necessary, by purification using column chromatography. In conclusion, the mixture containing cyclohexanecarboxylic acid, tri-t-butylamine, and decane can be separated using a combination of acid-base extraction techniques based on the differences in solubility of these compounds in the organic and aqueous phases, followed by column chromatography if required to further purify decane.

Key concepts

Organic Chemistry Separation Techniques

In organic chemistry, separation techniques are crucial for purifying compounds, recovering desired products, and conducting analyses. An effective separation exploits distinct physical or chemical characteristics of the components in a mixture. The goal is always to isolate target molecules with high purity and yield while minimizing the loss of product. Methods include simple approaches like filtration and advanced ones like chromatography.

• Filtration: employed for separating solids from liquids or gases.
• Distillation: based on boiling point differences, used for separating liquids.
• Chromatography: exploits differences in compound affinity towards a stationary phase and a mobile phase.
• Crystallization: separates components based on solubility in solvent at different temperatures.
• Acid-base extraction: a form of liquid-liquid extraction that separates compounds based on their solubility in acid or basic environments.

By understanding the principles of these techniques and how they apply to each scenario, chemists can effectively plan separation strategies tailored to their specific needs, just as demonstrated in the given exercise where an acid-base extraction sequence is smartly used.

Solvent Extraction

Solvent extraction, also known as liquid-liquid extraction, is a process by which compounds are separated based on their relative solubilities in two different immiscible liquids, typically water and an organic solvent. This technique is often leveraged in organic chemistry to isolate particular components from complex mixtures.

• Step 1: Two immiscible liquids are mixed, one of which contains the compounds of interest.
• Step 2: The compound of interest preferentially dissolves in the solvent where it is more soluble.
• Step 3: The two phases are then separated usually utilizing a separatory funnel, allowing for separation of the target compound.

It's important to choose a solvent that not only forms a separate layer with the other liquid but also displays a high affinity for the compound to be extracted. The extraction can be further refined by adjusting variables such as pH, which was elegantly used in the textbook solution to drive the extraction of acidic and basic compounds.

Liquid-Liquid Extraction

In the context of the exercise, liquid-liquid extraction's pivotal role can't be overstated. This technique is a subset of solvent extraction methods that specifically involves two liquids. It separates compounds based on their preference to reside in either the polar aqueous layer or the nonpolar organic layer.

Importance of Polarity

Chemical polarity dictates where a compound prefers to reside; nonpolar substances are oil-soluble and polar substances are water-soluble. Through the strategic use of solvents and pH manipulation, liquid-liquid extraction achieves what might seem like an alchemical sorting of various compounds.

• Polar, ionic, and salt forms of compounds generally localize in the aqueous phase.
• Nonpolar compounds tend to remain in the organic phase.

By layering these basic principles with an understanding of how a compound's solubility changes with pH, chemists perform extractions with precision, as shown through the isolation of cyclohexanecarboxylic acid and tri-t-butylamine from decane in the provided solution.

Chemical Solubility

Chemical solubility is the ability of a substance (solute) to dissolve in a solvent, creating a homogeneous mixture at a molecular level. Solubility is influenced by factors such as temperature, pressure, and the chemical nature of the solute and solvent. In the field of organic chemistry, solubility is particularly important for separating components of mixtures.

Role of pH in Solubility

pH plays a key role in solubility, especially when dealing with acid-base extractions. Many organic compounds have acidic (carboxylic acids) or basic (amines) functional groups that can be modified by pH changes. By adding acids or bases, these functional groups can be converted into their ionic forms, making them more soluble in water.

• Increasing the pH can deprotonate acids, enhancing water solubility.
• Decreasing the pH can protonate bases, increasing their water solubility.

In the exercise solution, the clever alteration of pH resulted in the selective solubilization of both cyclohexanecarboxylic acid and tri-t-butylamine from a mixture, demonstrating chemical solubility's integral part in separation techniques.