Organic Chemistry 234 Lab Quiz
How are the relative percentages in a mixture calculated based on their individual GC peak areas?
%A = (area of peak A/total area of the peaks) x 100%
What would be the maximum amount that could be recovered if you cooled the solution to 0°C?
(100mL/6.80g)x(3.40g) = 50mL (.02)(50mL/100mL) = .01g lost 3.40g - . 01g = 3.39g 3.39g maximum amount of solid recovered at 0C
The solubility of benzoic acid is 6.80g per 100 ml of water at its b.p. of 100°C. Its solubility is 0.34g per 100 ml of water at 25°C, and only 0.02g per 100 ml of water at 0 °C. Recall that solubility refers to the amount of solid that goes into the solution (dissolved), and any un-dissolved solid stays out of the solution. If you are to recrystallize 3.40g of benzoic acid, what would be the maximum amount of solid benzoic acid you could recover at a room temperature of 25°C?
(100mL/6.80g)x(3.40g) = 50mL (.34g)(50mL/100mL) = .17g lost 3.40g - .17g = 3.23g 3.23g maximum amount of solid recovered at 25C
Compare the Rf values compiled in Run #1 (previous page), rank order the polarity of the four standards. Give a brief rationale on how the polarity of each standard is related to their corresponding Rf value.
**In run one it is hard to determine the polarity because two of the standard values are equal** Most polar: Caffeine Middle: Acetaminophen Least polar: Aspirin and Ibuprofen (tied in run #1) [If we look at run two, we can determine that aspirin is slightly more polar than ibuprofen, so the order would be from most à least polar Ca > Ac > Asp > Ib] The more polar a molecule is, the less distance it traveled because it is more able to bond to the stationary phase. The less polar (or lack thereof) allows the molecule to travel a further distance because it doesn't get stuck in the stationary phase.
How much solid should you pack into a m.p. tube?
A few mg (height of 1-2 mm).
What are acceptable heating devices for heating a round bottom flask containing organic solvent?
A heating mantle is the best option, but you could also use a sand bath with a hot plate and stirrer.
Why is hot solvent instead of warm solvent used to recrystallize a solid?
A hot solvent is used because the solid dissolves more in a recrystallizing solvent when it's hot, so we can use less of the solvent. We don't want to lose too much of the solvent because we want to be able to bring as much back as possible!
How does mp value and range tell us about the purity of a solid?
A melting point is used to assess the purify of a known compound or identify an unknown crystal in solid. When a solid is impure, it melts at a lower temperature and melts over a wider temperature range.
What defines the difference between non-halogenated and halogenated organic wastes?
A non-halogenated compound doesn't have a halogen attached to it (Iodine, Chlorine, Bromine, Fluorine) A halogenated compound does have a halogen attached to it
What is an extraction?
A separation of compounds by filtration
How does a liquid-liquid extraction work?
A separatory funnel is used to perform vacuum filtration to separate immiscible liquids
How is it affected by the solvent used to develop the plate?
A solvent that does not cause any compounds to move from the original spot is not polar enough (thus small Rf values), whereas a solvent that causes all the spotted material to move with the solvent front is too polar (thus too large of Rf values). Trial and error to find appropriate solvent.
Explain why acetaminophen has a higher Rf value in one run than the other run, basing your arguments on intermolecular force competitions between acetaminophen and the plate coating vs. the solvent and the plate coating.
Acetaminophen has a slightly higher Rf value in run one than two because the more polar solvent used in run one can dissolve acetaminophen more easily and carry it in the mobile phase. Acetaminophen still travels a very small distance because it is capable of h-bonding with the silicone coating on the TLC plate, which wants to try and keep it in the stationary phase. The solvent and the plate coating are both polar, but it is more so in run, so there is greater competition to pull the molecule up in the mobile phase.
Why does recrystallization lead to a purer solid?
An impure solid is dissolved in a hot solvent, and as this solution is cooled, the pure product crystallizes out and the impurities stay dissolved.
What functional groups are considered hydrophobic?
Aromatic rings and Ether
How does the composition of organic structure (e.g. increase in carbon length, number of OH's, NH's) affect its polarity?
As the chemical structure has more carbon chain surface area, the solubility decreases. (more carbons and less OH groups = insoluble). More carbons = less polar. When there are four carbons per OH, the molecules will be soluble in H2O.
What's the purpose of boiling stone or a stirring bar during a distillation?
Boiling stones or a stirring rod are used during distillation to promote more even heating of the liquid.
Why do we add boiling stones to the distillation flask before heating it?
Boiling stones should not be added to a solution that is already hot, because it can cause the solution to boil over.
The mobile phase of column chromatography is eluent solvent. The choice of the solvent polarity is extremely important to the success of a column chromatography run. Why are solvents of different polarity chosen as the eluting solvents for different compounds such as carotene and xanthophyll?
By changing the polarity, you change how fast or slow different materials travel. In this case, we should expect carotene to travel further in a hexane solvent, since it is more easily separated in a non-polar solvent. On the other hand, xanthophyll is very polar and it will be better separated in the other two solvent solutions of 10:90 acetone:hexane and 30:70 acetone:hexane.
There are two phases involved in CC and TLC. What are the mobile and stationary phases involved in each of these two techniques?
CC--- Mobile phase = eluent solvent / stationary phase = Silica or Alumina TLC--- Mobile phase = solvent (moves up plate) / stationary phase = Silica or Alumina (solid coatings)
What does a liquid-liquid extraction involve (e.g. number of solvents and choices)?
Can be between two liquids (two solvents) or between liquid and solid (one solvent)
What functional groups are considered hydrophilic?
Carboxylic acids and Hydroxyl
What determines which layer is the bottom or top layer in a separatory funnel?
Density determines which layer is on top or bottom (more dense = bottom layer)
Estimate the relative polarity of these solvents, ethanol, ethyl acetate, and hexane.
Ethanol: 25 ethanol is more polar than ethyl acetate even if ethyl acetate has two O's because the OH's on ethanol are more polar and able to hydrogen bond Ethyl Acetate: 4.4 Hexane: .1
While the plate is in the solvent (it is being developed), the analyte molecules are attracted to the TLC coating (white solid) but the solvent molecules are moving up by capillary action trying to "push" the analyte molecules upwards. The "push" power of the solvent is related to its polarity. We will use a mixture of hexanes and ethyl acetate as our developing solvent. (a) What functional groups is in ethyl acetate?
Ethyl acetate contains an ester functional group.
What is a GC used for?
Gas chromatography is used for four main reasons: 1) to test the purity of a substance and separate the components of a mixture; 2) to determine the relative amount of the components in a mixture; 3) to identify a compound; 4) as a preparative method to isolate pure compounds from a small amount of a mixture.
Why should a hot plate not be used to heat organic solvents?
Hot plates are able to reach very high temperatures which are hot enough to ignite low boiling solvents.
Compounds with different bp or polarity can be separated on a GC. What is the principle of separation? (i.e. how do these compounds become separated when travelling through the GC column)
How volatile a component is will affect its speed in a GC column. If a component has a lower boiling point, it will travel through the column faster because it is released first. On the other hand, if a component has a high boiling point, it will travel slower through the column because it will release later.
On the same TLC plate shown above where would you expect the spot of xanthophyll to be at? Explain your reasoning based on the chemical structures and IMF.
I would expect xanthophyll to be in that location, because it is less soluble in the solvent, and it has the ability to form H-bonds with the silica gel, which traps it more in the stationary phase, resulting in less distance traveled.
When the developing solvent is too polar, what would happen to the Rf value of the compound being analyzed on a TLC plate?
If a solvent is too powerful, it causes all the spotted material to move with the solvent front and thus is too polar ( too large of Rf values).
To effectively separate two liquids by a simple distillation, how different should their bp be?
If the boiling point difference is greater than 60-70°C, the separation may be accomplished to a reasonable extent using a simple distillation.
During the extraction process, the spinach leaves were ground and its pigments extracted using ~10 ml of acetone solvent (ref. to Lab Manual procedure). The solvent was then evaporated and hexane added before the dark green mixture was used in column separation. What would have happened if the dark green solution of pigments in 10 ml acetone were used directly in the column separation? Explain why the results would have been the way you said.
If the dark green solution of pigments with acetone was used directly in column separation, we would see a lack of separation between the pigments. This is because acetone would act as a polar solvent, and it is very good at dissolving the polar pigment. Therefore, we start with hexane. Since it is non-polar, we can add solvents after it that increase in polarity slightly to pull out different pigments gradually.
What if the developing solvent is too non-polar?
If the solvent is too non-polar, no compounds will move with the solvent (too small of Rf values).
Why should a hot saturated solution be cooled slowly to obtain crystals instead of quickly being immersed in ice bath immediately (assuming the flask won't break with the sudden temperature change)?
If the temperature is increased too rapidly, an inaccurate melting point will be determined. The melting temperature would appear higher. To avoid this problem, the sample should be heated slowly.
How should you respond if you spill acid or base on your skin?
Immediately wash with water.
What should you do when chemicals get on your skin?
Immediately wash your skin for 15 mins, use safety shower or faucet depending on size
If the vacuum is applied to the distillation system, does the liquid distill at lower or higher temperature?
In vacuum distillation, the liquid can boil at a lower temperature.
One of the steps involved in running a TLC analysis is to spot samples on the plate. A dull pencil is used to gently mark the locations of the spots. (a) Why should an ink pen not be used?
Ink pen shouldn't be used because it will dissolve in the developing solvent.
Why must glass waste (e.g. capillary tubes, broken beakers) not be disposed into a regular trash container?
It can rip through bag and hurt the person handling it.
Why is the Rf value useful?
It is a physical characteristic of the compound, can determine how polar a compound is
Why should the spots be kept as small as possible?
It is difficult to separate mixtures on a TLC plate when the spots are too big.
How fast should the heating be when measuring mp?
It should be heated slowly
Why should it be inverted and vented often after two solvents mixed in a separatory funnel are shaken?
It should be inverted and vented often after two solvents are mixed in a separatory funnel and the funnel is shaken so that it released the pressure. This is necessary because if this isn't done, the pressure build-up might push the stopper out of the compound which would result in losing the compound.
When doing a recrystallization, it is undesirable to add the solid to the solvent. Why?
It's not a good idea to add the solid to the solvent because if you use more than the minimum almost of solvent, the crystals won't be able to form if too much solvent is used. It's hard to know what the minimum amount of solvent is without adding the solvent to the solid gradually because then we wouldn't be able to tell when the solid is dissolved when adding the solid to the solvent.
Why is it undesirable to use too much solvent?
It's undesirable to use more than the minimum amount of solvent because then we would lose too much of our solid resulting in our solution being too dilute for crystals to form.
What's the definition of a solid's m.p.?
MP is the temp at which a solid changed into a liquid.
Why should Bunsen burner be avoided in an organic lab?
Many organic compounds are flammable so heating them with a Bunsen burner could ignite the compound and cause a fire or explosion.
What is difference between melting and sublimation?
Melting = solid to liquid Sublimation = directly from solid to gas without going through liquid phase
What is the difference between bond polarity vs. molecular polarity?
Molecular polarity is the polarity of the whole covalent molecule Bond polarity is the polarity of a covalent bond
The more polar the compound analyzed on TLC or CC, the faster or the slower it moves? Why?
More polar compound on TLC = slower movement since TLC plate is polar More polar compound on CC = slower movement because they get absorbed into the stationary phase
Rank order the polarity of these solvents.
Most polar: 30:70 acetone:hexane > 10:90 acetone:hexane > hexane Least Polar
What is Rf value in TLC analysis?
Ratio of the distance each compound has traveled on the plate relative to the distance the solvent has traveled
What should you do when chemicals get in your eye?
Remove your contacts/wash your eyes out at eyewash station for 15 mins
What is its Rf value (two sig. figures)? Explain that Rf value for any given analyte can be as large as 1.0 but as small as zero.
Rf = distance molecule moved/distance solvent moved à 3.1cm/3.5cm = 0.89 The Rf value for any given analyte can be as large as one, because the analyte may dissolve extremely well in the mobile phase, and it will be pulled by the solvent front, and may be the same length as the solvent front. On the other hand, the Rf value may be as small as zero because the analyte may not dissolve in the solvent and have IMF forces (H-bonding) that allow it to bond to the stationary phase, which will keep it front traveling any distance or travel at all.
What is a separatory funnel used for during an extraction?
Separatory funnel is used to separate immiscible liquids.
Are silica gel and alumina polar or non-polar?
Silica gel and alumina are polar
How is it affected by the polarity of the spotted compounds on a TLC plate coated with silica gel?
Silica gel is a polar substance. The more polar the compound spotted, the less it traveled up the polar TLC plate, the smaller the Rf value. The more nonpolar the compound spotted, the farther it traveled up the polar TLC plate, the larger the Rf value.
What two chemicals are most typically used as the solid coatings (the stationary phase)?
Silica or alumina are most typically used as the solid coatings.
Benzoic acid does not dissolve in cold water but dissolves in dilute aqueous NaOH. Why?
Since "like dissolves like", benzoic acid doesn't dissolve in water because water is polar and benzoic acid is non polar. Additionally, benzoic acid is a weak acid, therefore it can only partially dissolve in water. It dissolves in dilute aqueous NaOH because NaOH is a strong base, it will attack the weak acid to form a salt.
How is Rf value defined?
The Rf value is the ratio of the distance moved by the solute to the distance moved by the solvent. To calculate the Rf of the unknown, take the solute distance traveled (2.7cm) divided by the distance of the solvent front (3.7cm). This calculation results in an Rf value of 0.73
What is boiling point of a liquid?
The boiling point is the temperature at which its vapor pressure is equal to the pressure of the gas above it.
After the samples are spotted, the plate will be "developed" using a developing solvent. The amount of solvent is important. (a) What is the problem with too much or too little solvent?
The depth of the solvent needs to be below the spotting line on the plate, otherwise the material will be dissolved before it is analyzed. There needs to be enough solvent present to create a saturated atmosphere within the container.
What quantitative physical property is a good measurement for solvent polarity?
The dielectric constant (higher constant = stronger polarity)
Hexanes are non-polar while ethyl acetate is moderately polar. What property of the solvent is a good measurement of its polarity? Provide numerical values to substantiate your answer.
The dielectric constant is a good measure of polarity. Ethyl acetate has a dielectric constant of 6 while hexanes have a dielectric constant of roughly 2.
The more polar the eluting solvent (used in CC) or developing solvent (used in TLC), the more or less the compound being analyzed (spotted) move down the column/up the TLC plate?
The eluting strength of a solvent is primarily related to how strongly it adsorbs onto the adsorbent and because typical adsorbents are highly polar; thus, eluting strength increases with solvent polarity
A piece of filter paper is also inserted into the developing chamber to keep the chamber saturated with the solvent vapor. What does it achieve?
The filter paper helps to speed up the development of the plate as well as improve the quality. In addition to that, it also helps to stabilize the plate.
What is used as the stationary phase in GC?
The liquid stationary phase is a high boiling organic compound absorbed on the solid inert packing in the column.
What is the main purpose of running a column chromatography?
The main purpose of column chromatography is to purify chemicals by taking advantage of the different polarities of compounds to separate the mixture.
What is the mp range?
The melting point range is the span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid
What does mp range magnitude tell us?
The melting point range magnitude tells us if our sample is 100% pure.
What can cause the mp of a solid to be lower than the literature reported value?
The melting points of compounds may be lower than the reported values because it may contain small amounts of the impurities or solvents. Impurities in a solid cause a melting point depression because the impurity disrupts the crystal lattice energies. It's not 100% pure yet.
What is the mobile phase and its purpose?
The mobile phase consists of the molecules that want to stay in solution. The mobile phase moves, which allows for separation, which we can then calculate an Rf value off of.
The chemical analyzed on a GC partitions thousands of times between the mobile and the stationary phases. What is typically used as the mobile phase?
The mobile phase is usually an inert gas or an unreactive gas such as helium, argon, nitrogen, or hydrogen.
How is retention time affected by: mobile phase flow rate, oven temperature, the compound's b.p., and the GC column length?
The mobile phase, also referred to as a carrier gas, carries the sample toward the detector. So, if the gas flows faster, the sample will reach the detector faster. The higher the temperature, the shorter the retention time. If it is too hot the compound will not spend enough time interacting with the column. If it is too cool the compound will spend too much time in the stationary phase and retention times will be long. If the compound has a lower boiling point, it will make its way through the column before compounds with higher boiling points, which results in a lower retention time. If the column is longer the retention time will increase and if the column is shorter the retention time will be shorter.
What is the most common error associated with a melting point determination?
The most common error is increasing the temperature too rapidly, which would result in an inaccurate melting point determination of a higher temperature than it actually is.
Similar to TLC, column chromatography is based on analytes being partitioned between a stationary phase (not moving) and a mobile phase (moving). What is most commonly used as stationary phase in a column? Is this material polar or non-polar?
The most used solids as the stationary phase are silica gel and alumina. Both of these compounds are polar.
What's the vapor pressure of a liquid at the temperature of its boiling point?
The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to one atmosphere (760 torr).
What do the areas of GC peaks tell us?
The peaks tell us the relative amount of that compound present in the solution that was injected.
What determines the solubility of an organic compound in water?
The polarity of a substance (more polar = more solubility)
Why is retention time useful?
The retention time of a compound found in the lab can be compared to a list of standard retention times to identify the unknown compound. Retention time can also be used to estimate the time of a run.
What is the retention time of a compound?
The retention time of a compound is defined as the interval between the injection of a sample and the detection of a substance in that sample.
Choosing a correct solvent is critical to the success of a recrystallization. When you choose a solvent to recrystallize a solid, what property should the solvent have in regard to the solid's solubility at different temperatures?
The solvent should dissolve when hot but not as much when cold.
The two TLC plates shown on the previous page were developed in different solvents. State which solvent used is more polar and why. How were the Rf values affected by the polarity of solvents used?
The solvent used in run one 90%:10% ethyl acetate/hexane is more polar than 60%:40% ethyl acetate/hexane, because it contains more of the polar solvent (ethyl acetate) than run two. The Rf values increased when the solvent was more polar.
As is obvious from the two tables on the previous page, different analgesics have different Rf values. After analgesics were spotted on the TLC plate, they interact with the stationary phase via intermolecular force (IMF). A stronger IMF in this case would mean a compound is "stuck" more strongly to the TLC coating. A weaker IMF means a compound would move more easily. In the space below, state what makes up the stationary phase of TLC plates, and explain how IMF causes caffeine and aspirin to have different Rf values (hint: review Prelab related readings/video, and IMF).
The stationary phase of the TLC plates is made up of silica gel SiO2, and is highly polar. In both tests, caffeine has the smallest Rf value because it is also a polar molecule, and it has many interactions with the silica gel, which causes it not to move very far. Aspirin on the other hand, has a higher Rf value and can travel further because the lack of polarity in the Aspirin doesn't allow it to get stuck to the stationary phase, therefore it is pulled by the mobile phase.
To accurately measure the bp of the distillate, where should the thermometer be placed in the distillation set-up (in relation to the three-way adapter)?
The thermometer should be placed so that the bulb (the blue end) is just below the arm of the adaptor.
In this experiment, you will use different solvents to run column chromatography separation of different pigments extracted from spinach leaves. These solvents are a mixture of acetone and hexanes. (a) What is the composition of these solvents?
The three solvents that we are using in this experiment are hexane, 10:90 acetone:hexane, and 30:70 acetone hexane. Hexane is an alkane with no functional groups and it is non-polar. 10:90 acetone:hexane has both acetone and hexane present. As stated previously, hexane is non-polar, but acetone is polar with ketone functional group. 30:70 acetone:hexane also contains acetone and hexane, but acetone is in higher concentration than the previous solution.
Why should the distillation flask not be filled up more than 2/3 with the liquid being distilled?
There needs to be sufficient clearance above the surface of the liquid so that when it starts to boil the liquid does not immediately go into the condenser, because that would impact the quality of the sample.
Organic compounds are commonly hazardous. What make them inherent hazardous in the lab?
They can be flammable/toxic.
During the column separation in the lab, your goal was to separate all pigments so that each colored pigment eluted at different time. You accomplished this by starting with pure hexane, then 10:90 acetone:hexane solvent mixture, followed by switching to 30:70 acetone:hexane mixture. Explain what this approach did, and why this approach works.
This approach takes advantage of polarity. Hexanes were first put into the column and allowed to flow out before the crude was added. After that, we added the crude. The crude got stuck near the top of the column, under the sand layer. Just before all the pigment went below the top of the sand layer, 10:90 acetone:hexane was added. This solvent is slightly more polar than hexane, and it begins to pull molecules through the column. The first pigment to go through was the b-carotene because it is non-polar. After all of that pigment was collected, we switched to 30:70 acetone:hexane because the xanthophyll is a polar molecule, and is more easily dissolved and pulled down the column in this solvent.
What is the purpose of a recrystallization?
To purify solids.
How many layers are usually involved?
Two layers are involved
What is the appropriate way to put out a fire caused by an organic solvent? Why is water not a good choice?
Use the fire extinguisher "ABC", water is not a good choice because some flammable materials react violently with water and could make it worse.
What is the proper thing to do shall your clothing catch on fire in the lab?
Use the safety shower
Conclusion: (use 3 sentences or less, state the most significant results/findings for this experiment)
Using TLC, we were able to identify the active ingredient of our unknown analgesic X as aspirin.
Why is vacuum distillation useful?
Vacuum distillation is used for compounds with high boiling points (+200°C). It prevents distilled compounds from decomposing by lowering the gas pressure above a liquid
Why is water not a good choice?
Water is not a good choice because some flammable materials react violently with water and could make it worse
What value can you look up on the internet to give you a good idea how polar they are?
We can look up the Rf value. (more polar = smaller Rf value)
How did you ensure in the lab that minimum amount of solvent was actually used?
We ensured that the minimum amount of solvent was used by adding little by little, one pipette of solvent at a time.
Which unknown did you have and what was its identity based on the TLC results? Briefly state your reasoning.
We had unknown X, and we determined that its identity is Aspirin. This was determined by looking at the TLC results and seeing which dot lined up with the unknown, so since the aspirin dot and the unknown dot line up, they are likely the same.
What is the difference between a solid melting and a solid dissolving in a solvent?
When a solid melts, heat is added. Solvent is added to a solid sample to turn into a dissolved sample.
If two unknown solids have identical m.p., can you conclude that they are the same compound? Why or why not?
You can't conclude that they're the same compound without grinding the solids together and recording the melting point. If melting point doesn't change, they are the same. If melting point reduces, the compounds are different.
b-Carotene moved up the plate fastest, almost at the solvent front. How can you explain this observation based on Intermolecular Force (IMF).
b-carotene moved the farthest because it is soluble in the hexane part of the solvent, and it does not form any hydrogen bonds with the silica gel, so it is not bound to the stationary phase.
How can you find out if they are indeed the same or different by using mp technique only?
the unknown can be mixed with one of the known compounds and a MP taken of the mixture. If the MP range is lowered and widened, it means that the two are different compounds. If the MP stays the same it means that the two compounds are likely identical.
