ORGO Final Lab Exam
A typical small-scale fractional distillation performed in an organic chemistry lab proceeds with how many theoretical plates?
about 5-15
If during an attempted recrystallization, you are cooling the solvent but nothing is happening. There is no sign of crystals even after 10-15 minutes. How can you induce recrystallization? Choose all that apply.
add a seed crystal scratch the bottom of the flask
What can be done to avoid uneven heating, hot spots and the potential "bumping" of liquids inside the distillation flask? Choose all that apply.
add a stirbar to the distillation flask and stir the solution while heating add boiling stones to the distillation flask
While performing a recrystallization, you realize that a soluble, colored impurity is difficult to remove. What could you do to remove it? Choose all that apply.
add activated charcoal to the hot solution, mix well, and hot filter
When you "spot" a TLC plate, what are you doing?
applying a spot of the sample to be analyzed to the bottom of the TLC plate using a capillary tube
A typical simple distillation proceeds with how many theoretical plates?
about 2-3
Your boss gave you 5.0 grams of an unknown sample that contains either benzoic acid or 2-nitrobenzoic acid. Your job is to purify and identify the compound. Your unknown is a dark brown solid including numerous black particles that are insoluble in all solvents including water. You are told that you should recrystallize using water, but otherwise you're on your own. Carefully but briefly (30 words or less) explain the steps you would take to purify and identify this compound. For every word of explanation beyond 30 words, I will automatically deduct 1 point. All words count including "a", "the", "and", etc. Think carefully and choose your words wisely.
(1) dissolve in the minimum quantity boiling water (2) hot gravity filter (3) recrystallize (4) record m.p.; compare to literature values (5) repeat steps 3-4 until literature value achieved
If during development of a TLC plate, the mobile phase moved 20 cm up the plate and your compound of interest moved 14 cm, what is your compound's Rf value?
0.7
If a molecule contains 6 protons that are chemically equivalent to one another, then those 6 protons will give rise to how many 1H NMR signals?1
1
Your boss gives you 25.63 grams of an unknown compound with unknown purity. It could be completely pure already or it could have significant impurities. You just don't know. Your boss asks you to purify it, if necessary, and return it to her. Your only means to purify is by recrystallization and your only means to determine purity is by melting point determination. What is the minimum number of recrystallizations that you will need to perform?
1
What would happen if you left the top stopper on the separatory funnel while you attempted to drain a liquid layer through the stopcock?
A vacuum would be created inside the separatory funnel causing the liquid to flow very slowly or not at all
Densities are expressed in units of grams per milliliter (g/mL) or grams per cubic centimeters (g/cm3). What the density of methylene chloride, CH2Cl2?
1.33
Linalool is a naturally occurring alcohol with a pleasant odor that is found in a variety of plants including lavender. You have decided to start your own Aromatherapy business and your main product will be 100% pure Linalool. You are going to isolate linalool from lavender buds using a steam distillation, and the expected yield of volatile organics is 4% by weight. The oil you will collect is not pure linalool. It will be 60% linalool, the remaining 40% due to other volatile organics. You'll have to isolate the linalool from these other organics later in order to achieve 100% pure linalool for your business. If you wish to ultimately isolate 10 grams of pure linalool, how many grams of lavender buds do you need to steam distill?
416.7
Eugenol has been appreciated by humans for thousands of years. It is a potent antioxidant with anti-microbial activity. It has a pleasant odor and is found naturally in a variety of plants including cloves, nutmeg, cinnamon, basil and bay leaf. You are going to isolate eugenol from clove buds using a steam distillation, and the expected yield of volatile organics is 20% by weight. The oil you will collect is not pure eugenol. It will be 90% eugenol, the remaining 10% due to other volatile organics. You'll have to isolate the eugenol from these other organics later in order to achieve 100% pure eugenol. If you wish to ultimately collect 10 grams of pure eugenol, how many grams of clove bud do you need to steam distill?
55.6
What is the chemical shift region expected for aromatic protons in a 1H NMR spectrum?
6.5-8.5
You are asked to recrystallize 36.50 grams of an impure solid that melts from 142.5 - 147 oC. You do so. After drying, you record a mass of 27.01 grams of beautiful beige, fluffy crystals. What is your percent yield?
74
One of the most important reasons to use TLC in an organic chemistry laboratory is to monitor the progress of a reaction, like the one depicted below for which detailed concentration-time profiles are known for each A, B, C and D: You just took took a small sample of the reaction mixture (called an "aliquot") after 5 hours of reaction, and used this to spot a TLC plate. The TLC plate you ran has 5 lanes including one lane each for pure A, B, C and D, and one lane for the reaction aliquot containing the crude reaction mixture after 10 hours of reaction. Assuming the concentration-time profile shown above is accurate, the lane containing the reaction aliquot should include spots corresponding to which components? Choose all that apply.
A B C D
During a gravity filtration, the fluted filter paper is placed inside what?
A glass funnel
Look carefully at the partial assembly of glassware for a simple distillation below. What is the role for the two rubber hoses labelled A and B, respectively?
A is for water outlet; B is for water inlet
When performing a simple or fractional distillation, it is important to record the temperature of the vapor reaching the condenser at all times. In order to obtain an accurate temperature of the vapor, where should the thermometer bulb be located?
At or slightly below the opening on the 3-way adaptor
For a melting point determination, the solids should be located where in the capillary tube?
At the bottom, near the sealed end
Why are many organic solvents not miscible with water?
Because organic tend to be non-polar while water is polar
You are asked to recrystallize 100 grams of an unknown, impure organic solid using one of five solvents (A, B, C, D or E). You must choose one of these 5 solvents. To help you choose, you are given the following solubility information for your compound in each of the 5 solvents: Solvent A: 11 g/L cold; 20 g/L hot Solvent B: 50 g/L cold; 104 g/L hot Solvent C: 23 g/L cold; 223 g/L hot Solvent D: 52 g/L cold; 235 g/L hot Solvent E: 0.5 g/L cold; 5.2 g/L hot Clearly identify which solvent you choose for this recrystallization and briefly (30 words or less) explain your rationale. For every word of explanation beyond 30 words, I will automatically deduct 1 point. All words count including "a", "the", etc. Think carefully and choose your words wisely.
C
During a recrystallization, you have dissolved your compound in hot solvent. What will happen if you cool the solution quickly? Choose all that apply.
Crystals may grow but they will be smaller than those grown with slow cooling Crystals may grow but they will be less pure than those grown with slow cooling
Assuming that compounds A, B, C and D all have the same solubility in the eluant, which one is most polar?
D ( the one that travels the shortest distance)
Each molecule below gives rise to 1 and only 1 1H NMR signal. Think carefully about the chemical shifts associated with each signal. Choose the one that would appear furthest downfield. A. (CH3)4C B. CH3OH C. CH4 D. CH3CH3 E. H2C=CH2
E
When 2 liquids inside a separatory funnel separate cleanly, we see a well defined interface between the layers. When they don't separate cleanly, we see what between the two layers?
Emulsion
What sits between a Buchner funnel and the filter flask in a vacuum filtration?
Filter vac
When the stopcock on an upright separatory funnel is opened, what does the liquid inside the separatory funnel do?
Flows downward into a receiving flask
If you were trying to separate two liquids which boiled at 75 oC and 96 oC, which method would be preferred?
Fractional Distillation
What would be the best method to separate cyclohexane (b.p. 81 oC) from acetone (b.p. 56 oC)?
Fractional Distillation
Which method would be best to separate a mixture of 1-pentanol and 1-hexanol?
Fractional Distillation
Which method would be best to separate a mixture of methanol, 1-butanol and 1-hexanol?
Fractional Distillation
When liquids are freely miscible, what is true?
They mix together and do not separate
Choose all correct statements below:
Fractional Distillations are best for the separation of liquids with relatively close boiling points, generally less than about 40 degrees -Simple Distillations are best for the separation of liquids with widely different boiling points, generally greater than about 40-50 degrees - Fractional Distillations have a greater number of Theoretical Plates as compared to Simple Distillations -Fractional Distillations are slower to perform than Simple Distillations
If you only have a small quantity of solids to collect via a vacuum filtration, then you may wish to use what type of funnel instead of a Buchner funnel?
Hirsch funnel
You are asked to recrystallize 36.50 grams of an impure solid that melts from 142.5 - 147 oC. You do so. After drying, you record a new m.p. of 147-147.5 oC. Is the recrystallized solid pure? Choose all that apply.
It is impossible to know at this stage Another recrystallization would need to be performed in order to answer this question
What is true about the ring of condensate that moves up the fractionating column in a fractional distillation? Choose all that apply.
It is more highly enriched in the lower boiling component of the mixture, relative to the contents of the distillation flask It is lower in temperature than the liquids in the distillation flask
Upon nearing completion of either a simple or fractional distillation, it is important to do what?
Leave a little bit of liquid in the distillation flask
Which of the following procedures are associated with a safe fractional distillation? Choose all that apply.
Leave the apparatus open to the air to avoid a pressure build-up Elevate the distillation flask using a block of wood or a cork ring or Never distill to dryness
When draining 2 liquids from a separatory funnel, the interface between the layers moves down into the narrowest part of the funnel and can now be called what?
Meniscus
Given the densities of water and methylene chloride, which one will be in the bottom layer when they are both added to a separatory funnel?
Methylene Chloride
When pouring a liquid-solid mixture onto the filter paper during a vacuum filtration, you should pour the solids where?
Middle of the filter paper
1H NMR spectra provide what type of information? Choose all that apply
Multiplicities of signals Integral values Chemical Shift values
When recording an accurate melting point, how quickly should you should increase the temperature?
No more than 2 degrees per minute
Where do we place the separatory funnel after shaking the layers together multiple times (with proper venting) as we wait for the layers to separate?
On a ring support
After inverting a separatory funnel, what should you do before shaking the flask to mix the layers? There could be more than 1 correct answer. Choose all that are correct.
Open the stopcock to release gas pressure Point the bottom tip of the separatory funnel toward the back of the hood
If we use more than the minimum amount of hot solvent to dissolve an impure solid in a recrystallization experiment, what happens? Choose all that apply.
Our yield of recrystallized product is reduced
The components of a mixture can be separated on a TLC plate and then identified by comparing what value to known compounds that were run up the same plate, albeit in different lanes?
R(f) value
If you were trying to prepare drinkable water (free of salt) from salt water found in the ocean, which separation and purification method would be preferred?
Simple Distillation
If you were trying to separate two liquids which boiled at 63 oC and 196 oC, which method would be preferred?
Simple Distillation
What would be the best method to separate cyclohexanol (b.p. 162 oC) from cyclohexane (b.p. 81 oC)?
Simple Distillation
Which method would be best to separate a mixture of methanol and 1-hexanol?
Simple Distillation
Why is it good practice to elevate the heating mantle on a lab jack rather than placing it directly on the bench top?
This allows for rapid removal of the heating source in case it is needed
Gravity filtration uses a piece of fluted filter paper. Why use "fluted" filter paper?
The flutes allow air to move inside and outside the receiving flask, equalizing pressures and allowing for a faster filtration
You have successfully recrystallized your compound which is known in the literature to have a melting point of 134 oC. You determine the m.p. for your recrystallized compound to be 124-128 oC. What can you say about the purity of your recrystallized compound? Choose all that apply.
The large m.p. range indicates it is not pure The depressed m.p. indicates it is not pure
If a pure solid has a melting point of 130-131 degrees, what will be the effect of adding impurities?
The melting point will be lower and the melting point range will increase
Assume a polar organic compound is dissolved in water and the water is placed inside a separatory funnel that already contains methylene chloride. With proper technique including shaking, the organic compound is "extracted". What happened?
The organic compound moved from the aqueous layer to the organic layer
When recording a melting point, a broad melting point range implies what?
The solid has some impurities
When recording a melting point, a sharp melting point range implies what?
The solid is relatively pure
You know you have added enough drying agent to an organic solvent when..
There are free flowing particles
In a vacuum filtration, why is it best to pre-wet the filter paper inside the Buchner funnel before applying a vacuum?
This creates a better seal and prevents solids from washing underneath the paper into the filter flask
Once the organic layer is recovered in a receiving flask like an Erlenmeyer flask, why is anhydrous Na2SO4 (sodium sulfate) added?
To dry the organic layer of residual water
During a vacuum filtration, once the solids have been collected on top of the filter paper, why do we leave them there for a few extra minutes with the vacuum on?
To dry the solids
Why do we perform a gravity filtration?
To separate a solid from a liquid
What is a separatory funnel used for?
To separate two immiscible layers
You have successfully recrystallized a previously unknown compound. There is no literature m.p. because this is the first time anyone has ever isolated the compound. You record a melting point of 183-183.5 oC. Is it pure?
We don't know because there is no literature m.p. for comparison The relatively sharp m.p. range indicates that it could be pure but we won't know for sure until we recrystallize it again
Heating mantles are generally preferred to open flames like a Bunsen burner for the purpose of heating the liquid mixture inside a distillation flask. Prior to use, the heating mantle should be plugged into what?
a variac that can carefully regulate applied voltage
Which of the following is typically used as a fractionating column in a fractional distillation? Choose all that apply.
a Vigreux condenser a standard condenser with steel wool stuffed inside
What is used to spot your known or unknown sample onto the TLC plate?
a capillary tube
The temperature of the condensing vapors in a steam distillation will always be...
a little bit lower than the boiling point of water
Choosing the correct mobile phase for a TLC experiment can be tricky. You just spotted a known polar compound onto a silica gel TLC plate and observed that it hardly moved during development with methylene chloride as mobile phase (Rf = 0.1). You'd like to find conditions whereby this polar compound would move further up the plate such that the Rf value was something closer to 0.5. What type of mobile phase should try next?
a more polar mobile phase
If you are going to spot multiple compounds or unknowns onto a single TLC plate, you will want to place them in separate lanes, all the same distance from the bottom of the plate. To help do this, you will draw a line across the plate as a guide for where each spot should be placed. What should you draw this line with?
a pencil
In order to construct a TLC plate, a thin layer of silica gel is placed atop what?
a solid support or substrate like glass or plastic
When performing a hot gravity filtration, it is best to keep the Erlenmeyer flask (containing your impure compound dissolved in the chosen solvent) on a hot plate. Why? Choose all that apply.
because we want the solvent to stay hot because the compound we will soon recrystallize could otherwise start to recrystallize prematurely
When placing a spotted TLC plate inside the TLC chamber containing a pool of solvent at the bottom, the solvent level in the chamber should be where, relative to the spots on the TLC plate?
below the spots
After the distillate containing organic oil and water is collected in a steam distillation, how can the organic oil be isolated, free of water? Choose all that apply.
by extraction using an organic solvent followed by evaporation of the solvent by physically separating the water and oil layers by pouring off the top organic layer or draining the bottom water layer
What drives the mobile phase up the TLC plate?
capillary action
When rinsing an organic solid that was just collected during a vacuum filtration, what type of solvent should we use?
cold
An Rf value can be defined as what?
distance moved by the spot divided by distance moved by the solvent
A mixture of 5 different liquids with 5 different boiling points between 100 and 165 oC would be best separated using what method?
fractional Distillation
You wish to isolate 100 grams of an organic compound from water. The compound has a boiling point of 120 oC and is miscible with water. What is your best option to separate the organic compound?
fractional distillation
In a recrystallization experiment, you should dissolve the impure solid in what?
hot (boiling) solvent the minimal amount of solvent
For a steam distillation, the organic material to be isolated must be... Choose all that apply.
insoluble in water
When distilling a mixture of liquids with different boiling points, what is always true about the temperature of the vapor reaching the condenser?
it is equal to the boiling point of the distillate collected at that moment
What can be said about the vaporization of the organic compound to be isolated during a steam distillation? Choose all that apply.
it vaporizes at a temperature below its boiling point it vaporizes near the boiling point of water
The speed with which a particular compound moves up the TLC plate during development depends upon what?
its affinity for both the mobile and stationary phases
The temperature of the distilling vapor that is measured throughout a steam distillation is...
just below 100 degrees
During a recrystallization experiment, you have grown beautiful crystals and collected them by vacuum filtration. But they are still wet. What is a good way to dry them? Choose all that apply.
leave them on the filter paper with the vacuum running place them on a watch glass and let them air dry place them in a desiccator
Mixtures of immiscible liquids will always boil at a temperature that is...? Choose all that apply.
lower than the boiling point of the highest boiling component lower than the boiling point of either component lower than the boiling point of the lowest boiling component
the more dense liquid inside a separatory funnel will be located where?
on the bottom
By definition, a theoretical plate is equal to what?
one vaporization-condensation cycle
In a recrystallization experiment, you have dissolved most of your solids but there are a few specs of brown particles that just won't dissolve, even after the addition of more solvent. What should you do next? Choose all that apply.
perform a hot gravity filtration
When attempting a recrystallization, you dissolve your impure solid in hot solvent and notice that the solution is brown. Your lab partner says, "Dude, we should add activated charcoal to that hot solvent." Although he was not your first choice for lab partner, you realize that this is actually excellent advice. Activated charcoal is frequently used to remove colored impurities. So you add a small scoop of activated charcoal, a black solid, to your hot solvent. The activated charcoal is doing its job but it doesn't dissolve, and now your solution has jet black particles floating all around. What should you do next?
perform a hot gravity filtration to remove the activated charcoal with absorbed impurities
You've just spotted your TLC plate and now you're preparing to develop it. However, you're not sure if you've spotted enough material because its colorless and you cannot see it. What should you do?
place the plate under a UV light to see if the size of the spot
When you develop a TLC plate, what are you doing?
placing the TLC plate into a chamber containing the solvent (eluant) so that the solvent climbs up the plate until nearly reaching the top
When using silica gel as the stationary phase and hexane as the mobile phase, one would expect which type of compound to travel more slowly up the plate, resulting in a smaller Rf value?
polar
TLC is what kind of method for the identification of organic compounds?
rapid, qualitative
Some compounds separated on a TLC plate are colorless and cannot be seen by the naked eye. In these cases, what two methods are utilized to see the separated compounds? Choose any 2 below.
shine UV light on the plate after solvent has evaporated use a visualization reagent like iodine to darken the spots where the compound(s) lie on the TLC plate
A thin coating of this stationary phase is most commonly utilized for TLC.
silica gel
The standard stationary phase used in TLC is what?
silica gel
You wish to isolate 100 grams of an organic compound from water. The compound has a boiling point of 180 oC and is miscible with water. What is your best option to separate the organic compound?
simple distillation
During development, the wet edge of the silica gel is observed to slowly ascend the TLC plate. What is this boundary between the dry and wet silica called?
solvent front
Common methods to see or visualize a developed TLC plate include what? Choose all that apply.
stain the plate using an iodine chamber place the plate under a UV light
When an immiscible organic compound is co-distilled with water, the process is called what?
steam distillation
You wish to isolate 10 grams of an organic compound from water. The compound has a boiling point of 150 oC and is immiscible with water. What is your best option to separate the organic compound?
steam distillation
You wish to isolate 100 grams of an organic compound from water. The compound is heat sensitive and will decompose at about 170 oC. It is also immiscible with water. What is your best option to separate the organic compound?
steam distillation
If you had plant material that contained 10% of a valuable, volatile organic oil with a boiling point of 185 oC, how could you isolate it? Choose all that apply
steam distillation followed by extraction using a volatile organic solvent and finally evaporation of the solvent steam distillation followed by physical separation of the aqueous layer and oil layers
One can determine whether or not a steam distillation has successfully removed all volatile organics from the distillation flask by visually observing the distillate droplets. The steam distillation is finished when...
the distillate droplets are clear, no longer cloudy
During a steam distillation, you notice that the droplets of distillate look cloudy, not clear. Why is this?
the droplets contain water plus an immiscible oil
An organic solid shows a melting point range of 123-126 oC. You now intentionally add an impurity and mix it well into the solid. What will happen to the melting point after the addition of impurity? Choose all that apply.
the m.p. range will increase the m.p. will be lower
In a steam distillation, the vapor pressure above the boiling liquid is equal to what?
the sum of the individual vapor pressures from water and the volatile organic
In a steam distillation, the liquid in the boiling flask boils when...
the sum of the vapor pressures of the organic compound and water equals the prevailing atmospheric pressure
When immiscible liquids are co-distilled, the vapor pressure above the liquid mixture is equal to..
the sum of the vapor pressures of the pure liquids at a given temperature
When performing a distillation, it is important to know that the temperature observed on the thermometer is not always an accurate reflection of the actual temperature of the vapor. This is true even if your thermometer is properly calibrated. In order to trust that the temperature observed on the thermometer accurately reflects the temperature of the vapor that is reaching the condenser, what must be true? Choose all that apply
the vapor-condensation ring must be covering 100% of the thermometer bulb the vapor must be condensing on the thermometer bulb with liquid droplets dripping back down to the distillation flask at a steady rate
In a fractional distillation, what 3 items are connected to the 3-way adapter?
thermometer adapter, fractionating column and condenser
What kind of hose is needed to connect the filter flask to a vacuum in a vacuum filtration?
thick hose
A steam distillation is designed to isolate what type of organic compounds? Choose all that apply.
those that are volatile just below 100 C those immiscible with water
What is the role of the addition funnel when a steam distillation is run using a 2-neck or a 3-neck distillation flask (with one of the necks connected to the addition funnel)?
to add additional water as needed
Why is a piece of wicking filter paper added to the TLC chamber?
to create an atmosphere inside the chamber that is saturated with solvent vapors
Why is a cover placed atop the TLC chamber? Choose all that apply.
to help maintain an atmosphere inside the chamber that is saturated with solvent vapor to prevent evaporative losses of solvent
Why do we use steel wool or glass beads (or some other) packing inside the fractionating column? Choose all that apply.
to increase the number of theoretical plates to increase the number of vaporization-condensation cycles
During a recrystallization experiment, why do we place the flask containing the newly formed crystals on an ice bath? Choose all that apply.
to increase the yield of crystals in an attempt to grow more crystals
Using the "n+1" rule, multiplets can be predicted based upon the number of chemically unique protons that are adjacent (within 3 bond lengths) to the proton(s) in question. What is the expected 1H NMR multiplicity for the methyl group in the compound below: CH3CH2CH2OH
triplet (3 line pattern)
When moving a TLC plate either into or out of the TLC chamber, what should be used?
tweezers or forceps
The distillate collected in the receiving flask during a steam distillation contains... Choose all that apply.
volatile organic compound water
