Orgo Lab Midterm
Simple Distillation.
If you were trying to separate two liquids which boiled at 63 oC and 196 oC, which method would be preferred?
Fractional Distillation
If you were trying to separate two liquids which boiled at 75 oC and 96 oC, which method would be preferred?
Thermometer adapter, fractionating column and condenser.
In a fractional distillation, what 3 items are connected to the 3-way adapter?
Our yield of recrystallized product is reduced.
In a recrystallization experiment, if we use more than the minimum amount of hot solvent to dissolve our impure solid, what happens?
Perform a hot gravity filtration.
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?
Hot (boiling) solvent and the minimal amount of solvent.
In a recrystallization experiment, you should dissolve the impure solid in what?
The sum of the vapor pressures of the organic compound and water equals the prevailing atmospheric pressure.
In a steam distillation, the liquid boils when... ?
The sum of the individual vapor pressures from water and the volatile organic.
In a steam distillation, the vapor pressure above the boiling liquid is equal to what?
This creates a better seal and prevents solids from washing underneath the paper into the filter flask
In a vacuum filtration, why is it best to pre-wet the filter paper inside the Buchner funnel before applying a vacuum?
Filter vac.
What sits between a Buchner funnel and the filter flask in a vacuum filtration?
Fractional Distillation
What would be the best method to separate cyclohexane (b.p. 81 oC) from acetone (b.p. 56 oC)?
Simple Distillation
What would be the best method to separate cyclohexanol (b.p. 162 oC) from cyclohexane (b.p. 81 oC)?
A vacuum would be created inside the separatrory funnel causing the liquid to flow very slowly or not at all.
What would happen if you left the top stopper on the separatory funnel while attempting to drain a liquid layer through the stopcock?
Emulsion.
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?
Steam Distillation
When an immiscible organic compound is co-distilled with water, the process is called what?
Middle of the filter paper.
When pouring a liquid-solid mixture onto the filter paper during a vacuum filtration, you should pour the solids where?
The solid has some impurities.
When recording a melting point, a broad melting point range implies what?
The solid is relatively pure.
When recording a melting point, a sharp melting point range implies what?
No more than 2 degrees per minute.
When recording an accurate melting point, how quickly should you should increase the temperature?
Cold.
When rinsing an organic solid that was just collected during a vacuum filtration, what type of solvent should we use?
Flows downward into a receiving flask.
When the stopcock on an upright separatory funnel is opened, the liquid inside does what?
On a ring support.
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?
Simple Distillation
If you were trying to prepare drinkable water (free of salt) from salt water found in the ocean, which method would be preferred?
Simple Distillation.
If you were trying to prepare drinkable water (free of salt) from salt water found in the ocean, which method would be preferred?
Add activated charcoal to the hot solution, mix well, and hot filter
While performing a recrystallization, you realize that a soluble, colored impurity is difficult to remove. What can you do to remove it?
Because organic tend to be non-polar while water is polar.
Why are many organic solvents not miscible with water?
To separate a solid from a liquid.
Why do we perform a gravity filtration?
This allows for rapid removal of the heating source in case it is needed.
Why is it good practice to elevate the heating mantle on a lab jack rather than placing it directly on the bench top?
To add more water to the distilling flask.
Why would an addition funnel be utilized in a steam distillation?
C
You are asked to recrystallize 100 grams of an unknown, impure solid. You are not given the name of the organic solid but you are given the following solubility information: 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 Which solvent should you choose?
5-15
A typical fractional distillation proceeds with how many theoretical plates?
2-3
A typical simple distillation proceeds with how many theoretical plates?
Fractional Distillation
A mixture of 5 different liquids with 5 different boiling points between 100 and 165 oC would be best separated using what method?
Steam volatile and immiscible with water.
A steam distillation is designed to isolate what type of organic compounds?
Point the bottom tip of the separatory funnel into the hood and open the stopcock to release gasses.
After inverting a separatory funnel, what should you do before shaking the flask to mix the layers?
By extraction using an organic solvent followed by evaporation of the solvent and by physically separating the water and oil layers by pouring off the top organic layer or draining the bottom water layer.
After the distillate containing organic oil and water is collected in a steam distillation, how can the organic oil be isolated, free of water?
the m.p. will be lower and its range will increase
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?
The organic compound moved from the aqueous layer to the organic layer.
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?
One vaporization-condensation cycle.
By definition, a theoretical plate is equal to what?
Mostly butyl acetate.
Congratulations! You just completed a fractional distillation of a mixture containing ethyl acetate and butyl acetate. In doing so, you plotted the volume of distillate collected versus temperature and your plot is shown below. At about 125 oC, what was the composition of the distillate?
Mostly ethyl acetate
Congratulations! You just completed a fractional distillation of a mixture containing ethyl acetate and butyl acetate. In doing so, you plotted the volume of distillate collected versus temperature and your plot is shown below. From about 70 to 75 oC, what was the composition of the distillate?
Mostly ethyl acetate.
Congratulations! You just completed a fractional distillation of a mixture containing ethyl acetate and butyl acetate. In doing so, you plotted the volume of distillate collected versus temperature and your plot is shown below. From about 70 to 75 oC, what was the composition of the distillate?
1.33
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?
A glass funnel.
During a gravity filtration, the fluted filter paper is placed inside what?
To increase the yield of crystals and in attempt to grow more crystals.
During a recrystallization experiment, why do we place the flask containing the newly formed crystals on an ice bath?
Leave them on the filter paper with vacuum running, place them on a watch glass and let them air dry, and place them on a watch glass and then in a desiccator.
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?
Crystals may grow but they will be less pure than those grown with slow cooling and crystals may grow but they will be smaller than those grown with slow cooling.
During a recrystallization, you have dissolved your compound in hot solvent. What happens if you cool the solution quickly?
The droplets contain water plus an immiscible oil.
During a steam distillation, you notice that the droplets of distillate look cloudy, not clear. Why is this?
To dry the solids.
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?
A mixture containing significant quantities of both toluene and hexane.
During the fractional distillation of a 50:50 mixture of toluene and hexane, the following plot was prepared. At approximately 90 oC, what was the composition of the distillate?
55.56
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?
Insoluble in water.
For a steam distillation, the organic material to be isolated must be...
Hirsch funnel.
If you only have a very 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?
Methylene Chloride (denser).
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?
The flutes allow air to move inside and outside the receiving flask, equalizing pressures and allowing for a faster filtration.
Gravity filtration uses a piece of fluted filter paper. Why use "fluted" filter paper?
A variac that can carefully regulate applied voltage.
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?
The melting point will be lower and the melting point range will increase.
If a pure solid has a melting point of 130-131 degrees, what will be the effect of adding impurities?
Add a seed crystal and scratch the bottom of the flask.
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 and get this party started?
Steam distillation followed by extraction using a volatile organic solvent and finally evaporation of the solvent and steam distillation followed by physical separation of the aqueous layer and oil layers.
If you had plant material that contained 10% of a volatile organic oil with a boiling point of 185 oC, how could you isolate it?
A Vigreux condenser and a standard condenser with steel wool stuffed inside.
In order to increase theoretical plates, which of the following is used as a fractionating column in a fractional distillation...
416.67
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?
86 mole %
Look carefully at the distillation diagram below for a mixture of benzene and toluene. Xtoluene is the mole fraction of toluene and Xbenzene is the mole fraction of benzene. If the starting solution is 50 mole % benzene, what mole % benzene will the distillate be after 2 theoretical plates?
6 mole %
Look carefully at the distillation diagram below for a mixture of benzene and toluene. Xtoluene is the mole fraction of toluene and Xbenzene is the mole fraction of benzene. If the starting solution is 50 mole % toluene, what mole % toluene will the distillate be after 3 theoretical plates?
A is for water outlet; B is for water inlet
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?
~6-7%
Look carefully below at the fractional distillation diagram for mixtures of toluene and cyclohexane. An initial mixture is chosen that is 50 mole % toluene and 50 mole % cyclohexane. This mixture is fractionally distilled and undergoes 2 complete vaporization-condensation cycles before the first distillate is collected. What mole % of toluene is in the first distillate collected?
Lower than the boiling point of either component, lower than the boiling point of the highest boiling component, and lower than the boiling point of the lowest boiling component.
Mixtures of immiscible liquids will always boil at a temperature that is...?
To dry the organic layer of residual water.
Once the organic layer is recovered in a receiving flask like an Erlenmeyer flask, why is anhydrous Na2SO4 added?
The distillate droplets are clear, no longer cloudy.
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...
Water and the volatile organic compound.
The composition of the distillate collected in the receiving flask during a steam distillation is...
On the bottom.
The more dense liquid inside a separatory funnel will be located where?
Lower than its boiling point and near 100 degrees.
The organic compound isolated in a steam distillation vaporizes at a temperature...?
A little bit lower than the boiling point of water.
The temperature of the condensing vapors in a steam distillation will always be...
Just below 100 degrees.
The temperature of the distilling vapor that is measured throughout a steam distillation is...
Leave a little bit of liquid in the distillation flask.
Upon nearing completion of either a simple or fractional distillation, it is important to do what?
Heating Mantles
What are these?
Add boiling stones to the distillation flask and add a stirbar to the distillation flask and stir the solution while heating.
What can be done to avoid uneven heating, hot spots and the potential "bumping" of liquids in the distillation flask?
It vaporizes at a temperature below its boiling point and it vaporizes near the boiling point of water.
What can be said about the vaporization of the organic compound to be isolated during a steam distillation?
Creates a vacuum.
What does a water aspirator do?
To separate two immiscible layers.
What is a separatory funnel used for?
To add additional water as needed.
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)?
2-Neck Flask
What is this?
3-Way Adapter
What is this?
Beaker
What is this?
Buchner Funnel
What is this?
Clamp
What is this?
Condenser
What is this?
Distillation Flask
What is this?
Erlenmeyer Flask
What is this?
Filter Flask
What is this?
Fractionating Column
What is this?
Glass Funnel
What is this?
Heating Mantle
What is this?
Laboratory Jack
What is this?
Pear Shaped Flask
What is this?
Powder Flask
What is this?
Receiving Flask
What is this?
Ring Stand
What is this?
Round Bottomed Flask
What is this?
Rubber Hose for Water
What is this?
Seperatory Funnel
What is this?
Stirring Hot Plate
What is this?
Thermometer
What is this?
Vacuum Adapter
What is this?
Best for the separation of liquids with relatively close boiling points, generally less than about 40 degrees. Slower to perform than Simple Distillations.
What is true of Fractional Distillations?
Best for the separation of liquids with widely different boiling points, generally greater than about 40 degrees.
What is true of Simple Distillations?
Thick hose.
What kind of hose is needed to connect the filter flask to a vacuum in a vacuum filtration?
A hot gravity filtration to remove the activated charcoal with absorbed impurities.
When attempting a recrystallization, you dissolve your impure solid in hot solvent and your lab partner says, "Dude, we should add activated charcoal to that hot solvent." Although he was not your first choice for lab partner and you've never been impressed by this fellow, 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?
Meniscus.
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?
The sum of the vapor pressures of the pure liquids at a given temperature.
When immiscible liquids are co-distilled, the vapor pressure above the liquid mixture is equal to...
They mix together and do not seperate.
When liquids are freely miscible, what is true?
Because we want the solvent to stay hot and because the compound we're recrystallizing could otherwise start to recrystallize prematurely.
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?
At or slightly below the opening on the 3-way adaptor.
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?
74%
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?
It is impossible to know at this stage and another recrystallization would need to be performed in order to answer this question.
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?
50
You are given a mixture that is 80 mole % toluene and 20 mole % cyclohexane. Using a fractional distillation set-up, the mixture undergoes 1 and only 1 vaporization-condensation cycle before distillate is collected. What is the new mole percent of cyclohexane in the distillate?
We don't know because there is no literature m.p. for comparison and the relatively sharp m.p. range indicates that it could be pure but we won't know for sure until we recrystallize it again.
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?
The large m.p. and depressed m.p. indicates it is not pure.
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?
There are free flowing particles.
You know you have added enough drying agent to an organic solvent when...
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?
Steam distillation
You wish to isolate 100 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?
Simple distillation
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?
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?
To determine whether or not the drug is pure, you will dissolve it in a solvent and perform a hot filtration to get rid of any impurities. Then, you will place it on a hot plate and slowly raise the temperature. You will repeat this at least two times in order to determine the melting point. If there is a small range in the melting points, then the drug is free of impurities.
You're working for a pharmaceutical company that is trying to prepare a new drug compound that destroys the Covid-19 virus. They've got a drug candidate that shows great promise. Everyone is excited. Your co-workers in an adjacent lab synthesize a large quantity of the new drug, but it's impure. It needs to be recrystallized and purified. You have no idea what the melting point of the new drug is because it's never been made or studied before. Briefly explain how you could use recrystallization and melting points to determine if and when you have a purified version of this exciting new drug.
First, you will dissolve the dark brown solid in water, using the minimum amount of water needed to dissolve the solid. Then, you will remove the black insoluble particles by performing a gravity filtration. Once the impurities are removed, you will slowly cool the solution to room temperature in an ice bath. Once the crystals are separated and dried, you will be able to identify the compound by determining the melting point by repeatedly melting the substance until the melting point is sharp with a small range. You can now identify the compound.
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 explain how you would purify and identify this compound.
2
Your boss gives you 25.63 grams of an impure, unknown compound and asks you to purify it by recrystallization and then return it to her. You do not know the identity of the compound and your only means to determine purity is by a melting point determination. What is the minimum number of recrsytallizations you will need to perform?
First, you would dissolve the compound in the minimum amount of solvent and bring it to a boil. You would pour a little bit of solvent in at a time until the solid is completely dissolved to get the maximum yield. The solid and solvent mixture is placed on a hot plate. Once the solvent is clear, the solid has has been completely dissolved. In order to get rid of any impurities that did not dissolve, you will perform a hot gravity filtration since organic compounds dissolve better in hot solvents. Then, you will slowly cool the solution so crystalization can occur. The crystals will be separated and dried. Then, you will be able to determine the melting point by slowly increasing the temperature by no more than 2 degrees per minute. This needs to be repeated in order to determine the melting point. The melting point will be relatively consistent between replications if the substance is free of impurities. Once the melting point is determined, the identity of the compound can be determined as well.
Your boss gives you 25.63 grams of an impure, unknown compound that melts from 104-118 oC. She tells you that it must be either benzoic acid or 2-nitrobenzoic acid, but she's not quite sure which one. She asks you to purify it and identify it using a melting point determination. Carefully but briefly explain how you would purify and identify this compound.
