Chem Lab Final

Here is a photo of a TLC plate from a laboratory experiment, using a 1:1 hexanes:ethyl acetate solvent system(eluent). Which lane had the least polar compound? How do you know? Which lane had the most impure compound? How do you know? What is the Rf of compound F? Draw a new TLC plate after development, using the same compounds, but with 4:1 hexanes:ethyl acetate solvent system. Explain any changes to your new TLC plate.

- Lane D had the least polar compound because it traveled up the least on the plate, and polar substances attract with the solvent and move slower than non - polar substances. - Lane E had more impurities because there are two spots on the TLC plate. - More hexane should lower all the spots - If you increase the mobile phase's polarity, that interaction will increase, decreasing the interaction of the solute with the stationary phase. Therefore, a higher ethyl acetate content in the mobile phase will increase Rf values.

Will your compound be more soluble in hot or cold solvent?

- Most solids, including sugar and salt, become more soluble with increasing temperature. - This is because heat increases molecular movement, causing more collisions between the water molecules and the solid

For the following equation, select the best explanation for the meaning of '[conjugate base]' pH = pkA + log10 = (conjugate base)/(acid)

- The concentration of the deprotonated acid in solution - a compound formed when an acid loses a proton

Buffers/Red Cabbage Lab

- The goal of this experiment was to determine the pH of the three 'unknown' compounds by using the Anthocyanin compounds in red cabbage. - If a strong acid is added to a buffer solution, the weak base will react with the hydrogen atom of the strong base to form a weak acid. - If a base is added to a buffer solution, the hydrogen atoms will not be fully consumed as much due to the acid and conjugate base consuming it, causing the pH to not rise as expected. - The unknown A compound that was in test tube 7 had a pH of 8-9. This was determined by comparing the color of the test tube to the same colored test tube, which was test tube 4. -Red cabbage contains a water-soluble pigment called anthocyanin that changes color when it is mixed with an acid or a base. The pigment turns red in acidic environments with a pH less than 7 and the pigment turns bluish-green in alkaline (basic) environments with a pH greater than 7.

The melting range of acetylsalicylic acid is 138-140 °C and the melting range of adipic acid is 151-154 °C. What effect would the presence of a small amount of adipic acid have on the melting range of acetylsalicylic acid?

- The melting range of acetylsalicylic acid would be lower and more broad - This is true even if the impurity is higher melting (when pure) than the major component. For example, if a chemical that normally melts at 130º is contaminated by a small amount of material that when pure melts at 200º, the resulting mixture will not melt between 130º and 200º. Rather, the melting point of the major component will be depressed, and the observed melting range will begin lower than 130º

How do you know which compound is in which layer?

- The solution with the lower density will rest on top, and the denser solution will rest on the bottom. Aqueous layer 1. Add a bit of water from a squirt bottle to the separatory funnel and watch where the water droplets go. If the top layer is aqueous, the water droplets should mix with the top layer, and they will look as if they disappear. If the bottom layer is aqueous, the water droplets will fall through the top layer to mix with the bottom layer 2. Consider relative volumes of aqueous and organic solvents, based on quantities used in the experiment. 3. The more dense liquid, typically the aqueous phase unless the organic phase is halogenated, sinks Diethyl ether 0.713 Ethyl acetate 0.902 Water 0.998

Be able to explain which solvent is the top layer, and which solvent is the bottom layer, and why, in a liquid-liquid extraction using a separatory funnel. Given a basic compound, like caffeine, what aqueous solution would you use to extract it from ethyl acetate? Why? Draw a picture of a separatory funnel filled with: ethyl acetate, 1M HCl, Caffeine, Salicylic acid. Identify the location of each component.

- The solvent with the lowest density is the top layer in the separatory funnel, and the bottom layer is the solvent with the highest density. - HCl would be used because it is denser than caffeine but less dense than ethyl acetate - The caffeine would be the organic layer mixed with 1m HCl and the bottom layer would be salicylic acid mixed with ethyl acetate.

How to conduct and interpret a TLC experiment.

- Thin-layer chromatography (TLC) is used for identifying compounds and determining their purity. You can even follow the course of a reaction by using the technique. - In short, you place a spot of the substance on the adsorbent surface of a TLC plate, let a solvent (or solvents) run up through the adsorbent, and visualize the plate to see what, if anything, happened to your compound - Don't touch the active surface with your fingers— handle them only by the edge. The layer on the plate is pretty thin, and bits can flake off at the edge if you're not careful. The thinness of the layer also means that you have to use very small amounts of your compounds in order not to overload the adsorbent. Spotting the TLC Plate 1. lightly mark the place where you're going to spot a plate with a pencil 2. Dissolve a small portion (1-3 mg) of the substance you want to chromatograph in any solvent that dissolves it and evaporates rapidly. Dichloromethane or diethyl ether often works best. 3. Put the capillary into the solution. 4. Touch the capillary to the plate briefly! The compound will run out and form a small spot. Try to keep the spot as small as possible—not larger than 1 4 in. in diameter. Touch the capillary to the same place. Let this new spot grow to be almost the same size as the one already there. 5. lightly draw in pencil a line all the way across the plate about 1 cm from the end and stop the development when the solvent gets up to it Developing A Plate 1. Take a 150-mL beaker and cover with a watch glass 2. Choose a solvent to develop the plate. You let this solvent (eluent) pass through the adsorbent by capillary action. Nonpolar eluents (solvents) force nonpolar compounds to the top of the plate, whereas polar eluents force both polar and nonpolar materials up the plate. 3. Pour some of the eluent (solvent) into the beaker, put no more than 1 4 in. of eluent in the bottom of the beaker! 4. Place the slide in the developing chamber Don't let the solvent in the beaker touch the spot on the plate, or the spot will dissolve away into the solvent! If this happens, you'll need a new plate, and you'll have to clean the developing chamber as well. 5. Cover the beaker with a watch glass. The solvent (eluent) will travel up the plate. When the solvent reaches the line, immediately remove the plate. Drain the solvent from it, and immediately mark—in pencil—the solvent front. 6. For visualization, put the plate under UV light Interpretation 1. Measure the distance from that solvent line drawn across the plate to where the spot started. 2. Measure the distance from where the spot stopped to where the spot began. Measure to the center of the spot rather than to one edge. If you have more than one spot, get a distance for each. 3. Divide the distance the solvent moved into the distance the spot(s) moved. The resulting ratio is called the Rf value. Mathematically, the ratio for any spot should be between 0.0 and 1.0, or you goofed. 4. Check out the Rf value—it may be helpful. In identical circumstances, this value would always be the same for a single compound. If this were true, you could identify unknowns by running a plate and looking up the Rf value. Unfortunately, the technique is not that good. 5. If the spot moves somewhere between the two limits and remains a single spot, the compound is probably pure. If more than one spot shows, the compound is definitely impure, and it is a mixture.

Discuss when you would use fractional distillation and when you would use simple distillation.

- You use a simple distillation when substances in a mixture have boiling points that differ largely. - Fractional distillation is used to separate mixtures whose components have boiling points that are too close to each other.

Use knowledge of the mechanism and theory of electrophilic aromatic substitution to explain the formation of the diacetylferrocene isomer:

1,1'-diacetylferrocene would be the isomer present. If 1,2 and 1,3-diacetylferrocene were to be formed, both C2 and C3 would have to be able to accept the acetyl group which means those position has to be nucleophilic. However if looked at the resonance structure of those positions, those carbon has the tendency to be positively charged which means they are electrophilic and would not be able to accept acetyl group. Therefore acetyl group at C1 an C1' is the best option

If placed in a nonpolar solvent, the compound that is the most ___ will move the farthest up the plate.

nonpolar

Calculate experimental yield

1. Ensure you have a correctly balanced equation for the reaction performed. 2. Determine how many moles of each species were used in the reaction. 3. Determine which species is the limiting reagent, remembering to use the reaction stoichiometry. 4. From the weight of product obtained, determine how many moles of product this corresponds to 5. Taking into account the stoichiometry, determine what % this is compared to what you could have obtained by 100 * [moles product obtained]/[ maximum moles product possible] For example: 8.21g of cyclohexene was reacted with 17.5g of bromine in chloroform, giving 20g of trans-1,2-dibromocyclohexane. Stoichiometry of this reaction is 1:1 Moles of each species involved: cyclohexene = 0.10 mol, Br2= 0.11 mol, dibromocyclohexane = 0.083 mol. Limiting reagent is therefore the cyclohexene. Theoretical yield of dibromocyclohexane is 0.10 mol., therefore, experimental yield = 0.083./0.10 = 83 %

calculate limiting reagent

1. Find the moles of each reactant present. 2. Calculate the moles of a product formed from each mole of reactant. 3. Identify the reactant giving the smaller number of moles of product. This reactant is the Limiting Reagent: 4. Calculate the grams of product produced by the Limiting Reagent. This is the theoretical yield. Na = Cl (Na G) x (1 mol/ Na MW) x (1 mol Cl/1 mol NA) x (MW Cl/1 mol) = Cl g

Errors of taking melting points:

1. improper amount of sample in capillary tube 2) mistaking sagging or shrinking sample as melting 3) presence of solvent cause crystals to "sweat"

A small amount (0.20 g) of pure Acetanilide (MP 113-115 deg. C) was mixed with a large (5.0 g) sample of pure Acetanilide. What is the expected melting range of this mixture? Select the answer with the best explanation.

113-115 deg. C. The compound is still pure

A more polar compound would be closer or farther away from the origin point?

closer

IR ketones

1750-1680 --> sharp strong peak

If you mix 1mL of 10% CaOH2 and 1 mL of 6M HCl, what is the theoretical yield of CaCl2 in grams?

2HCl + Ca(OH)2 → CaCl2 + 2H2O Theoretical Yield: 1.50 g CaCl2 (1 mL CaOH2)(1 mol/74.09g)(1 mol CaCl2/1 mol Ca(OH)2)(110.98g CaCl2/1 mol) = 1.50 g (1 mL HCl)(1 mol/36.46)(1 mol CaCl2/2 mol HCl)(110.98g CaCl2/1 mol) = 1.52

IR secondary amine

3300 --> broad with 1 sharp spikes

IR primary amine

3300 --> broad with 2 sharp spikes

IR alcohol

3300 --> very broad with a rounded tip

A sample gives string peaks at both 3300 and 1750. What kind of compound might it be? Would it make a difference whether the peak at 3300 is described as very broad or just broad?

A peak at 1750 cm-1 is likely a carbonyl, and the broad peak at 3300 is O—H. A broad peak could indicate a molecule with a carbonyl and an alcohol. A very broad peak would indicate a carboxylic acid.

What effect do you expect to see for this TLC plate if it had been developed using a MORE polar solvent?

Both spots A and B will move farther (higher Rf).

Match the following common mistakes with the results you would expect.sample is spotted too low on TLC platetop is left off TLC chambereluent (solvent mixture) is too polareluent (solvent mixture) is not polar enoughsample is too concentrated (for example, an oil is spotted directly on TLC plate)

E. spots will dissolve into solvent reservoir, rather than move up plate B. solvent stops moving up plate (keeps evaporating away; calc'd Rf will be way too high) C. all spots travel at solvent front (Rf = 1) D .all spots remain at starting point (Rf = 0) A. silica gel will become overloaded, resulting in huge spots that can overlap

Why not remelt samples? Don't they just cool and solidify in exactly the reverse manner that they heated and melted?

It may undergo nasty chemical changes such as oxidation, rearrangement, or decomposition.

How to conduct and interpret a MP experiment.

Generally, melting points are taken for two reasons. 1. Determination of purity. 2. Identification of unknowns. Take your unknown, and mix it thoroughly with some chemical you think might be your unknown. This procedure is called "Taking a mixed melting point." Process 1. You usually take melting points in thin, closed-end tubes called capillary tubes. They are also called melting-point tubes or even melting-point capillaries. Take melting points only on dry, solid substances, never on liquids or solutions of solids in liquids or on wet or even damp solids. 2. Place a small amount of dry solid on a new filter paper. Thrust the open end of the capillary tube into the middle of the pile of material. Some solid should be trapped in the tube. Carefully turn the tube over, taking care not to let the solid fall back out of the tube, until the closed end is down. Remove any solid sticking to the outside. Now, the solid must be packed down. Drop the tube, closed-end down, through a length of glass tubing. Have 1-2 mm of packed solid. Melting Point Hints 1. Use only the smallest amount that you can see melt. Larger samples will heat unevenly. Recommend 1-2 mm of packed solid 2. Pack down the material as much as you can. Left loose, the stuff will heat unevenly. 3. Never remelt any sample. It may undergo nasty chemical changes such as oxidation, rearrangement, or decomposition. 4. Make up more than one sample. One is easy, two is easier. If something goes wrong with one, you have another. Duplicate and even triplicate runs are common. Mel-Temp Apparatus 1. Read the temperature, and wait for the heating block to cool if the temperature is less than 20°C below the melting point of your compound. You'll have to wait for the block to cool to room temperature if you have no idea of the approximate melting point of your compound. 2. Place the loaded mp tube in one of the channels in the opening at the top of the unit 3. Set the voltage control to zero if necessary. 4. Turn the on-off switch to ON. The light source should illuminate the sample. 5. Set the voltage control to the instructed setting. The point is to get up to within 20°C of the supposed melting point. If you have no idea what the melting point is, it may require several runs as you keep skipping past the points with a temperature rise of 5-10°C per minute. A convenient setting is 40 or 4. 6. After you've melted a sample, throw it away 7. Once you have an idea of the melting point, get a fresh sample and bring the temperature up quickly at about 5-10°C per minute to within 20°C of this approximate melting point. 8. When the first crystals just start to melt, record the temperature. When the last crystal just disappears, record the temperature. If both points appear to be the same, either the sample is extremely pure or the temperature rise was too fast. 9. Turn the on-off switch to OFF. You can set the voltage control to zero for the next person. 10. Remove all capillary tubes.

The TLC plate shown was developed in a 90:10 mixture of hexane:ether. How would you adjust the solvent mixture to raise the Rf of the spot?

I would add more ether, since ether is more polar than hexane

Would you consider ethyl acetate to be a polar or non-polar solvent?

I would consider ethyl acetate a polar solvent b/c the two oxygens pull the compound in different directions

What happen when hexane and water are mixed

Immiscible and top layer is hexane

What happens when methylene chloride and water mix?

Immiscible top layer is water

Ether and water

In miscible top layer is ether

When mixing a solid compound in a solvent, does heat generally increase or decrease the solubility of the compound?

Increase: Higher temperatures increase kinetic energy allowing the solute to overcome the intermolecular forces keeping it in solid form and dissolve in the solvent.

Liquid-liquid extractions are successful if the two solvents are ___.

immiscible (they are not soluble in each other...one solvent is usually nonpolar and the other is polar).

"If you can see it, you can see it melt." This should put the correct amount of a solid in a melting-point tube. If someone has put about one-quarter of an inch of solid in the tube, should they go for an eye exam?

Less sample is better because larger samples allow for uneven heating (and incorrect results). You just need a small bit in the bottom of the tube.

Imagine you were asked to assess the melting range of a product with a literature MP of 131-133 C. Which MP standard would you choose, and why?

MP Standard to the closest

Ethanol and water

Miscible they are both polar

How can you determine if the samples made through two synthetic routes are identical?

Mix together and take the melting point, if it is lower the two compounds are different

Phenols do not exhibit the same pka values as other alcohols; they are generally more acidic. Using the knowledge that hydrogen acidity is directly related to the stability of the anion formed, explain why phenol is more acidic than cyclohexane?

Phenols has a more stable conjugate bad because the double bond in the ring structure allow it to be resonance stabilized. When the proton is removed from the oh group the electrons in the sp2 orbital are hybridized and drawn into the anion. Does not occur in cyclohexane because it does not have double bonds in the ring. Therefore; it is not resonance stabilized

What is the equation for Rf value?

Rf = distance travelled by substance (dot) / distance travelled by solvent (line)

After attempting to purify the crude product via recrystallization, crystals never formed because your product was still soluble in the solvent, even when using the ice bath. Select the best option to recover the organic compound.

Rotovap: the compound must be removed from the organic solvent

A mixture of 20 mL of Ether (bp = 35°C) and 10 mL of Toluene (bp = 111°C) is separated. If these solvents are separated via simple distillation, select the expected results

Simple distillation can separate the majority of both compounds if the collection flasks are changed at 36 and 110 degrees.

How do you separate two compounds

Simple distillation. - Separating liquids boiling below 150°C at one atmosphere (1 atm) from a. Nonvolatile impurities. b. Another liquid that boils at least 25°C higher than the first. The liquids should dissolve in each other. Fractional distillation. - Separating liquid mixtures, soluble in each other, that boil at less than 25°C from each other at 1 atm. Extraction - Two insoluble liquids in a separatory funnel will form layers; one liquid will float on top of the other. You usually have compounds dissolved in these layers, and either the compound you want is extracted from one to the other, or junk you don't want is washed from one layer to the other. Vac Filtration Organic solvent is removed from liquids

Describe the practical reasons behind the direction of the flow of water through condenser:

Since the heated vapor from the distillation flask will travel from high to low, the water flows in the opposite direction in order to effectively cool the vapor which would liquify and flow down into the collection tube

Elute Chart

Some solvents arranged in order of increasing polarity (Least polar) Petroleum ether Cyclohexane Toluene Chloroform Acetone Ethanol (Most polar) Methanol

Caffeine is a basic compound (the nitrogen without a methyl group is the most basic), and citric acid is an acidic compound (the carboxylic acid in the middle of the molecule is the most acidic). One mole of HCl was added to a 1 mole:1 mole mixture of the two compounds.

The basic N atom of caffeine will deprotonate H-Cl by attacking the Hydrogen atom and forcing the bond between the H and Cl to move onto the Cl

You have a solution of NaCl and water. You heat the solution to boiling and observe bubbles forming in the solution, and vapor escaping the flask. What is the identity of the bubbles and the vapor?

The bubbles is the water and the NaCl is the vapor

What is present in the aqueous layer you drained? How do you neutralize it? How do you isolate it?

The carboxylate ion is present. In order to regenerate the carboxylic acid it must be added to HCl in an ice bath to induce crystallization. To isolate the crystals, vacuum filtration is performed.

What happens when you add sodium bicarbonate to organic solution with carboxylate acids and phenols?

The carboxylate will go into the aqueous layer at the bottom and the phenols will be in the organic layer at the top

The TLC plate has a shiny side and a dull side (as if it were coated with some kind of powder). Which should you use to spot your samples on?

The dull side.

Why is a pen not good enough to mark a TLC plate?

The ink from the pen is made up of organic compounds which will climb the TLC plate along with the solvent and go over any other samples.

What effects does insoluble impurity like sand and glass has on MP?

no effect

calculate theoretical yield

The theoretical yield is the amount of the product in g formed from the limiting reagent. From the moles of limiting reagent available, calculate the grams of product that is theoretically possible (Step 4)

Imagine you tried to recrystallize benzoic acid using water. After filtration and drying, your % recovery was 25%. What are some reasons for your low % recovery? How could you improve the recrystallization next time?

Using too much solvent will cause the compound you are trying to recrystallize to remain dissolved in the solution. Next time use less solvent and give compound more time to dissolve in hot solvent.

A compound dissolves in a heated solvent and it recrystallizes after cooling the solution with an ice bath. Select the best option (and explanation) to isolate the pure organic compound.

Vac Filtration: the crystals must be removed from the organic solvent and this allows the impurities to be removed with the organic solvent

How would the Rf values change if a more polar solvent (like ethyl acetate) were used?

When a more polar solvent is used instead of non polar one like ligroin, it will interact more with the polar molecules and less with the non polar molecules causing a less dramatic separation. Rf values would be higher.

Describe how to set up a MP experiment. How would you choose a MP standard? Given some MP data, come up with a correction factor and correct all MP data points in an MP table.

You would first fill a capillary tube with the compound you are trying to determine its melting point and if all the impurities were removed. Then you would fill another capillary tube with the standard, and you choose this by analyzing the literature melting point of the pure compound and choosing a compound/mixture that has a similar melting point.

Why does the presence of a soluble impurity in a sample has the effect of lowering and broadening the melting point range?

because the impurity disrupts the crystalline lattice of the sample so less energy is required to convert the solid phase into a liquid phase

IR presence of benzene rings

small stretches from 1500-2000

How fast should a melting-point capillary sample be heated?

temperature rise of 5-10°C per minute.

Define "freezing point."

the temperature at which a liquid changes to a solid.

Define "melting point."

the temperature at which the first crystal just starts to melt until the temperature at which the last crystal just disappears.

How do you pick a good solvent for recrystallization?

when solvent is hot, it should dissolve both the desired product and impurities when solvent is cold, the desired compound is insoluble (forms a precipitate) while the impurities are soluble.