CHEM 232 final study guide PDF

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CHEM232 Organic Chemistry Lab I Final Notes...

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Chem 232 Final Study Guide Stuff to know

Experiment 1 - Recrystallization of an Impure Solid -

The main point of this experiment is to use a solvent to turn a crude solid into a pure solid. This is done with heating and cooling. Recrystallization is the technique used. This is done using polarity.

Experiment 1 Background Recrystallization - purification technique where a crude solid becomes more pure using minimum amount of hot solvent, and cooling. At the end, a purer solid precipitates. Keep in mind that product recovery is not 100%

- Needs a good solvent to get good recrystallization. Criteria are… 1. Solvent Polarity 2. Differential solubility of solid in solvent 3. Solvent Boiling Point 4. Chemical Interaction between solid and solvent 5. Solubility of impurity in solvent

- It is important that the solvent and crude do not have the exact same polarity. If they do, the crude solid will dissolve completely into the solvent, which is not wanted. Ideally, the solvent should be insoluble at low temperatures and soluble and high temperatures. Allows for solvent to be dissolved at high temp, precipitate forms when cool. Use a solvent with similar polarity, just not TOO similar.

- Water is a very polar solvent. It is preferred universal solvent because it can hydrogen bond and has a high boiling point. - Compounds that have 6 or more carbons are not soluble in water at room temperature - most organic compounds are not water soluble - we can manipulate a solid’s solubility in a solvent by increasing the temperature of it - At higher temperatures, compounds with high polar regions can be dissolved - As temperature cools, the pure solid precipitates back - If you want to use water as a solvent, the compound needs a polar region

- The boiling point of the solvent cannot be too low or it will evaporate before it dissolves the solvent - The boiling point cannot be too high or it will take a long time and high temperatures would be needed, making the purification technique very difficult - solvent must be chemically inert so that it would not interact with the solid chemically Hot Gravity Filtration - a method used in case there are still impurities in the pure substance. Essentially placing the solid on a filter paper, and pouring more solvent on it until its ready. - If no precipitate forms, scratch the bottom of the flask, this is known as elongation. Or, a seed crystal could be added to aid crystal growth. - Melting Point Analysis - a method to determine an unknown solids identity, by comparing the melting point of the unknown to the known. Compare the melting point of the sample to the melting point of a known compound. - impurities lower the melting point of a solid because it prevents the crystal lattice from forming intermolecular forces, lower energy required to break, lower melting point observed Experiment 1 Procedure - To determine the best solvent, place a small amount of crude solid in each of the solvents (Hexane, Acetone, Ethanol, and water) and place in a hot water bath. Take out of the bath, cool to room temperature, then ice bath. Most precipitate formed is the right solvent.

- Weigh 0.40 grams of the crude sample and place in a 25ml flask - Add a minimum amount of solvent - Add a magnetic stirring rod and place it on a heat plate near boiling until the sample is dissolved completely. - Gravity filtration if needed, do it while hot - Collect sample using vacuum filtration, this is done to take the liquid out and keep the sample in

Experiment 1 Understanding -

You will never expect 100% recovery because there are many factors that prevent it. Some crystals may not have been properly elongated, pure substance could have boiled out with the impure substance, etc

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% recovery is found by… actual/theoretical x 100

Experiment 2 - Selective Recrystallization and Melting Point

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The main idea is that we can selectively recrystallize a compound with the solvent we choose. We used a solution of 50:50 Benzoic Acid:Benzil and we can crystallize the compound we want by using certain solvents. Very similar to experiment 1

Experiment 2 background -

Melting point is a physical property of solids

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A high crystalline compound will have a sharp melting point, usually high as well

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A compound with impurities will have a high range melting point and usually lower

Mixed melting point analysis - Used to determine if two solids are the same compound or different. Then compare it to known compound. -

Collect a sample of solids A and B, the solution will have mostly solid A and a little of solid B. If they are the same, the melting point would be the same and sharp. If not, the melting point would be lower and solid B would be considered an impurity.

Experiment 2 Procedure -

Place small amount of pure solvent in ethanol and water, then place in a hot-bath. Record findings, the proper solvent is the one that forms more precipitate

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Weigh 0.50 grams of crude solid and transfer to 25 ml beaker, add magnetic bar

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Place in heat bath and let it dissolve completely, remove from hot plate and let it cool to room temperature. Use gravity filtration if needed.

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Collect sample back using gravity filtration and filter flask

Experiment 2 Understanding -

Considering that the compound is 50:50 of a solution, the theoretical yield is half of the initial mass.

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The purpose of hot gravity filtration is to remove liquid impurities from the pure solid

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The purpose of vacuum filtration is to remove excess solvent

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Mixed melting point analysis is basically collecting the melting points of several compounds, then finding the melting point of an unknown. Then we know that the compound is by comparing it to the known melting points.

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The solvent is what creates the most precipitate

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The speed and effectiveness is dependent on the strength of the polarity and the compound.

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Just because 2 compounds have the same melting point doesn’t mean they are the same, a mixed melting point analysis must be done to assess. Sharp and high if they are the same, broader range if they are not

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If you wanted to do selective recrystallization, the compounds in the solution must be different enough. This means that they cannot have the same polarity, they must be different enough so that a certain solvent can target it.

Experiment 3 - Extraction -

In this experiment, we have a solution with 3 compounds in it. We will use extraction to separate these compounds with the power of immiscible solvents with different densities, and different refractive indices

Experiment 3 Background Extraction - a technique defined as transferring a solute from one solvent to another -

An example of this would be a tea bag in hot water, the water extracts the flavor from the bag

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For this lab, the solvent must have most similar polarity as possible, must be so similar that it could occur at room temperature

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The solvent’s boiling point cannot be too high so that we can isolate the solid

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The solvent must ONLY extract the compound of interest, this is known as selectivity

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To isolate the most amount of solid possible, a little heat is used

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Grinding solid provides more surface area and enhances extraction

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Solvent must be inert, it cannot interact with the solid chemically

Liquid-Liquid Extraction - a common type of extraction. Uses 2 immiscible solvents to separate a mixture of solids. The solvents must be immiscible, have two different densities, and have different refractive indices. -

The solvents must be immiscible so that there are 2 layers of liquid

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They must have different densities so that the lighter liquid will be on top while the heavier one is on the bottom.

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Refractive indices is a measurement of polarity, but they do have different colors which allow us to notice the layers.

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When the 2 solvents are mixed with the solute, the solute partitions itself between the 2 phases. We take advantage of acidity and basicity to create an aqueous phase which is removed in extraction.

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For example, if a solution has a carboxylic acid, we can introduce a weak base to deprotonate it and create a negatively charged conjugated base. Considering that the species is now a salt, which is highly polar, it will favor the aqueous phase and then be extracted. Sodium Bicarbonate could work as a weak base

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Phenol is a weak acid, requires a strong base like NaOH to be extracted

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Amines are protonated with HCL to be extracted

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It is very important to know which layer is which, the heavier layer will be on bottom

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Use the water drop test to determine which layer is which. Essentially drop a drop of water into the solution and see if it stays above or below the solvent.

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Emulsions may form if you shake too vigorously, may look like a third cloudy layer. Separate this by gently stirring with a glass rod.

Experiment 3 Procedure -

Weigh 0.4 grams of the mixture (containing 3-nitrobenzoic acid, 2-naphthol, and 1,4dimethoxybenzene) and add 6 ml of diethyl ether to the mixture to dissolve it, transfer to separatory funnel

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Add 5 ml of sodium bicarbonate to the separatory funnel

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Stopper it and shake gently, do this until the layers form

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Drain the sodium bicarbonate fraction into a beaker and set aside

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Add 5 ml of cold NaOH to the organic layer and shake gently, then drain into a beaker and set aside

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Transfer remaining material to Erlenmeyer flask and add anhydrous sodium sulfate, loosely cork and occasionally swill

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Add HCL to both the sodium bicarbonate and NaOH fractions until it is acidic, place both on ice for 15 minutes

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Use vacuum filtration to separate the last solid

Experiment 3 understanding

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In the first extraction, we use sodium bicarbonate as a solvent. This is a weak base, meaning it will only deprotonate strong acids, 3-nitrobenzoic acid has the strongest acid group so this is extracted first.

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In the second extraction, NaOH is used as a solvent. It is a strong base so it will deprotonate weak acids. 2-napthol is a weak acid so it will be extracted in the second step

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Add HCL to both the first 2 extractions to re-protonate the compounds. Remove excess solvent from 1,4-demethoxybenzene under reduced pressure to extract.

Experiment 4 - Thin Layer Chromatography of Spinach

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The general idea is that we are using polarity and solvents to partition a compound and separate the polar and nonpolar components. This is TLC.

Experiment 4 background -

Chromatography - Separation and purification technique, the idea is that the sample will partition between the two phases.

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There are two classifications involved, column and planar, they are responsible for bringing two phases together

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In both of the classifications, stationary and mobile phase -

In planar chromatography, the stationary phase is what does not move (paper or coating), while the mobile phase moves with capillary action. TLC is an example of planar chromatography

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In column chromatography, the stationary phase is packed in a tube (column), and the mobile phase moves via gravity. Liquid chromatography, gas chromatography, and capillary electrophoresis are all examples.

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TLC is the chromatography that is being used in this lab

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Use silica gel (or alumina) as the stationary phase, different solvents as mobile phase

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Separation is caused by difference in polarity between the stationary phase and mobile phase. Silica can hydrogen bond and is very polar, mobile phase is usually nonpolar

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For example, polar substances will stick to the stationary phase and thus elute very slowly. Nonpolar substances will be attracted to the mobile phase and elute farther

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Usually the substances are colorless so make sure to use a UV light so that it would show

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RF - retention factor, this is a measurement of polarity between the compounds -

RF: Distance the sample traveled/ Distance the solvent traveled

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Basically, distance sample travelled / total

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Lower numbers means polar, higher numbers mean nonpolar

If the compound has a low RF, it has a high affinity to the stationary phase -

Methanol has high affinity, Hexane has low affinity

Experiment 4 Procedure -

Place some spinach into a mortar and pestle. Grind with sand, anhydrous MgSO4

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Transfer into 50 mL beaker and add 2 mL of Acetone. Mix and stir

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Obtain a TLC plate, draw a line 2mm from the bottom, this is where the solution will be applied

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Place in chamber with solvent of hexane and acetone that is 0.5 cm deep. Capillary action will elute the compound

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Evaluate the movement and use the UV light to analyze

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Calculate RF value

Experiment 4 Understanding

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The image above are the compounds we are separating

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Xanthophyll should have lowest RF, then Chlorophyll B, then Chlorophyll A, then Carotene should have the highest RF

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Keep in mind that the mobile phase solvent have different interactions with the compounds. -

A mobile phase with a weak base (like triethylamine) can deprotonate a strong acid. This would deprotonate a carboxyl, creating a salt which is very polar. This would make the compound elute less.

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Mobile phase with strong acid (acetic acid) with protonate a weak base (amine group).

Experiment 5 - Column Chromatography of Spinach -

Very similar to Experiment 4. Instead, column chromatography is being used, thus we are using a different method to get similar results.

Experiment 5 background -

Column chromatography is a common technique to separate liquids

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The difference between this and TLC is that TLC is more qualitative and 2 dimensional, while column is more 3D and quantitative

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This is done by packing the column and the mobile phase is applied to the top and is pushed down by gravity

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The stationary phase and mobile phase are similar to the TLC, so silica or alumina as stationary phase and organic (nonpolar) solvents as mobile phase

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The basis of this is difference in polarity

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More polar compounds stick to stationary phase and are very slow, the nonpolar substances interact with the mobile phase and more faster down the column.

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In this experiment, we use TR instead of RF. TR stands for Retention time. -

TR and RF are inversely related. A compound with low RF values would take a long time to leave the column, thus it would have a high TR.

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Essentially, nonpolar substances leave first

To achieve better resolution and complete separation of the components, a polarity gradient is applied to the mobile phase. -

The gradient must have a wide difference in polarity

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We could apply a nonpolar substance to remove the nonpolar component, then a polar substance to remove the polar component. Or this could be done in 1 mobile phase

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We can only use substances with visible difference in color

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Stationary phase is polar, mobile phase is nonpolar

Experiment 5 procedure -

Get spinach and grind it in a solution of sand and anhydrous MgSO4.

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Get solution and place it into a 50 mL beaker and add 2 mL acetone

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Place cotton into the column (pasteur pipette) with some sand. Add silica gel as well.

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Mix spinach solution with 100 mg of silica gel until it becomes slurry

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Get the solvent of acetone/ hexane solution and place it into the top of the column. Let Bands form and make sure that you collect the separate bands in separate beakers.

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For the final mixture, use vacuum filtration to remove the excess solvent

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Analysis is done via TLC -

Label the plate and place a small sample of each band, and the original mixture, to the TLC plate. Record the results, determine the RF values

Experiment 5 Understanding -

A 50/50 mixture of acetone/hexane is ideal, provides the best results.

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Higher RF values go to the compounds that are more nonpolar

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If you want to switch the solvents, make sure that it is more nonpolar. If it is LESS, the separation would not work as well. LESS nonpolar solvents would not be able to seperate bands that have very close RF values.

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Remember that if the compound has an amide, and the base is weak. The amide will most likely become a salt and thus become more polar.

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If there are multiple splotches in a line, it indicates impurities. Which would mean that the data isn’t very reliable.

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Evaluate the plates using logic. Certain compounds shouldn’t elute as much as others, so if they do elute the same, then there is an issue.

Experiment 6 - Distillation -

Use of simple and fractional distillation to separate compounds using boiling point.

Experiment 6 Background -

Distillation is a process where liquid is heated to its boiling point and the resulting vapor phase is condensed into a different container.

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This is a separation and purification technique

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The mixture of liquids must have different boiling points

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In this experiment, we are focusing on simple and fractional distillation

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Basically, the solution is boiled and becomes a vapor. External Pressure has a part to play in this, basically boiling occurs when the vapor pressure is equal to the external pressure, so if there is lower external pressure, there is lower boiling point. On Mt. Everest, boiling point is -76 degrees for water.

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Compounds with lower boiling points elute first, compounds with higher boiling points elute later

Simple Distillation - Useful when there is a 50 degree temperature difference between the liquids boiling point. Very fast. Essentially, the solution is boiled, then the vapor gets to an area of cooler heat where it condenses into a purer form.

Fractional Distillation - addition of a fractionating column to get better results. This is slower. Very similar to simple distilla...