Experiment 3

Experiment 3 - Experiment 3: Acid, Base, Neutral Separation...

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4 Experiment 3: Acid, Base, Neutral Separation (Liquid-Liquid Ex- traction, Drying, Recrystallization) Mohrig, Chapter 8-9 This experiment is designed to demonstrate that 1) immiscible solvents, such as water and ether, form separate layers on the basis of density (top = lighter, bottom = heavier) 2) extraction of a solute (liquid or solid) from one solvent to another can be done on the basis of its partition coefficient, 3) pK a ’s of acids or bases can change the partition coefficient of an acidic or basic substrate, 4) using a flow-chart to rapidly summarize a laboratory procedure is beneficial, and 5) a drying agent removes traces of water from non-aqueous solutions, thereby drying the organic layer. One central method for purification of an organic solid or liquid is a process called ex- traction. Liquid-liquid extractions take advantage of the difference in solubility of a substance in two immiscible liquids. The two immiscible liquids used in an extraction process are called the solvent and the extracting solvent. The two immiscible liquids are usually separated in a separatory funnel or a centrifuge tube. The following is an example of a flow chart. It shows the fate of each compound through different operations. Interesting Questions: Write the balanced equation for the aniline + HCl gas. Where will the equilibrium lie? By how much? What about benzoic acid and triethylamine? If a mixture of benzoic acid and triethylamine were suspended in a mixture of water and methylene chloride, what would be in the aqueous layer? What is the purpose of the brine wash? What would Raoult say about azetropes? The following questions will be answered in the next few lectures we have: Define the following stationary phase, mobile phase, elution, eluent, normal phase, reverse phase. List several common adsorbents for chromatography. Write the equilibrium expression for the adsorption process observed on a TLC plate. List several precautions for spotting a TLC plate. List several visualization methods for TLC and classify them as destructive or nondestructive. Does a single spot indicate a single compound? List any additional experiments to come to a conclusion. Arrange the following in order of increasing ease of elution from a TLC plate using SiO2 as the adsor- bent: cyclohexane, butyl acetate, benzoic acid, n- decyl alcohol, (e) 1,3-dimethylbenzene Which of the following compounds would elute from reverse phase first? a phenol, an ester, hydrocarbon. Why are hexanes not easy to visualize in TLC? Now you Know… pKa Acid-base extraction with hydroxide only works for compounds with pKa < 15, because the pKa of H 2 O = 16. This would be a good time to re- view the calculation of the equilibrium constant from gen chem. Hydrates can be used to dry solvents and solute.
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Experiment 3 - Experiment 3: Acid, Base, Neutral Separation...

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