CHEM 343 Final Exam
1. What are the common units for the x and y axes in an IR spectrum?
1. The common unit for the x axis is wavenumbers (cm-1) and the y axis is transmittance (%) in an IR spectrum.
In today's experiment, butan-2-ol is dehydrated to yield 3 isomeric butene. Butan-2-ol (MW = 74.12 g/mol, D = 0.810 g/mL) If 100 microliters of butan-2-ol are used, how many mmols of butan-2-ol are used
1.09 mmols
Where are the IR peaks of cinnmaldehyde?
1675 cm for carbonyl 2800-2900 cm for CH stretches (doublet)
Reactions rates are affected by the type of solvent that was used. Which of the following solvents is not a polar protic solvent 1) CH3OH 2) CH3CN 3) CH3COOH 4) H2O
2) CH3CN
Which of the following statements are true? 1) at the beginning of a distillation (t=0 where t represents time), the vapor phase contains more of the high boiling component of the mixture 2) Temperature is constant while a compound is distilling as long as the composition of the mixture isn't changing 3) The process of condensation, vaporization causes the vapor to become enriched in the less volatile component 4) Temperature will change during a phase change as long as the mixture is being heated
2) Temperature is constant while a compound is distilling as long as the composition of the mixture isn't changing
Which if the following statements is (are) not true about a distillation set-up? 1) the water hoses must be securely attached to the condenser and faucet 2) The water will enter from the top of the condenser and exit from the bottom of the condenser so that it flows "downward" 3) The water serves to cool the vapors causing them to condense 4) The lowest boiling liquid will be collected first
2) The water will enter from the top of the condenser and exit from the bottom of the condenser so that it flows "downward" water will flow "upward"
Dibromo product has __ stereocenters with___ stereoisomers.
2,4
One of the possible products the hydration of norbornene is shown below (exo-norborneol). how many chiral centers are present?
3
What are the IR peaks for eugenol?
3500-3200 cm for OH 1270 for C-O-C
A student starts with 1.000 g of a mixture of compounds A and B. After extraction, the student isolates 0.200 g of compound A and 0.201 g of compound B. What is the percent composition of his mixture based on his results
50% A and 50% B
For Williamson ether synthesis 25 mL of 21% (by weight) sodium ethoxide solution is used. The density of the solution is 0.868 g/mL and the molar mass of sodium ethoxide is 68.05 g/mL. How many mmols of sodium ethoxide were used
67 mmol
Which of the following situations would contribute to a lower yield of product? a) sodium ethoxide solution reacts with water in the air of solvents instead of 1-bromobutane b) sodium ethoxide solution reacts with the ethanol in the solution instead of 1-bromobutane c) some of the product is left in the distilling pot, because students are not supposed to distill to dryness
A & C
Explain the following statement: A pure liquid has a constant boiling point, but a constant boiling point doesn't necessarily imply a pure liquid.
A constant boiling point does not imply a pure liquid because of the potential of azeotropes. If two miscible liquids form an azeotrope, then the azeotrope has a consistent boiling point, but is not a pure liquid. An example of this is the water/ethanol azeotrope.
What are the properties of a good recrystallization solvent?
A good solvent dissolves the solid of interest when the solvent is hot, but not when the solvent is cold or at room temperature
If you have a dissolved colored impurity (and some solid contaminants) when you dissolve your unknown in the appropriate solvent, what should be done?
Add activated charcoal to the solution of unknown in your solvent before you do the gravity filtration
How will the melting point change if the solid sample is impure?
Broader than expected
Place the following in order of increasing bond stretching frequency (lowest to highest) C-H, N-H, O-H, F-H
C-H < N-H < O-H < F-H
When you collect your crystals at the end of lab, why do you wash with cold solvent (as opposed to room temperature solvent)?
Cold solvent will be much less likely to re-dissolve the solid during the wash
Which of the following is a good practice for obtaining an accurate melting point of a solid? A) ramp rate 10°c and 20°c below the expected melting point B) ramp rate 2°c and 15°c below the expected melting point C) ramp rate 10°c and 30°c below the expected melting point D) ramp rate 2°c and 20°c below the expected melting point
D) ramp rate 2°c and 20°c below the expected melting point
True or False: In distillation water will enter from the top and exit from the bottom so it flows down
False
What is the best way to remove an insoluble solid, like sand, from an impure solid sample for recrystallization?
Find a solvent to dissolve the desired compound and then do a hot gravity filtration
What does it mean if a mixed solvent system such as ethanol/water is suggested for a recrystallization
Heat in a minimal amount of hot ethanol and add drops of water until cloud point is reached then allow to cool
The product is recystallized using a mixed solvent system (ethanol/water). What does this mean
Heat in a minimal amount of hot ethanol and add drops of water until the cloud point is reached then allow to cool
What does it mean if a mixed solvent system such as ethanol/water is suggested for recrystallization?
Heat the sample in a minimal amount of hot ethanol and add drops of hot or cold water until the cloud point is reached then allow the solution to cool
The absolute configuration of 3 isomeric products were determined as follows: Isomer A: 3S, 4R, 5S Isomer B: 3S, 4S, 5S Isomer C: 3R, 4S, 5R which of the following statements is correct
Isomer B and Isomer C are diastereomers
Why is a filter paper fluted for a gravity filtration?
It allows more surface area of the paper to come in to contact with the mixture being filtered.
When you set up for microscale boiling point determination, you are asked to rubber band a thermometer to the test tube containing the unknown liquid. Where should you position the bulb of the thermometer?
It should be aligned with the curved bottom of the test tube containing your unknown
What is the mobile and stationary phase when the TLC plate experiment was done?
Mobile= mixture of ethyl acetate and hexanes (nonpolar) Stationary= TLC plate w/ Si-O and Si-OH (polar)
In the experiment with the TLC plate what are the polar and nonpolar phases?
Mobile= nonpolar Stationary= polar
If 1,4-dimethoxybenzene and NaOH react what happens?
NO REACTION
What is the precipitate of the Williamson ether synthesis reaction, and how do you get rid of it?
NaBr white salt; redissolve it and then it dissociates into Na+ and Br- ions and comes out in the aqueous layer
What functional groups are present on eugenol?
OH and methoxy group
In today's experiment, butan-2-ol is dehydrated to yield 3 isomeric butene. If 60 mmol of butan-2-ol is used and the product mixture contains 10.0 mmol butene #1, 30.0 mmol butene #2, and 20.0 mmol butene #3 what is the mole fraction of each in the product mixture
#1 = 0.167, #2 = 0.500, #3 = 0.333
What is the purpose of using a reflux condenser?
- enables continuous condensing of the liquid for reboiling - prevents solvent liquid from escaping- traps solvent vapors - keeps reactant concentration constant
What is the Rf value for a spot that traveled 2.0 cm up a TLC plate if the solvent front traveled to 8.0 cm
0.25
A dehydration reaction was done starting with: 2.00 g butan-2-ol. GC integ. alkene 1 = 65.43, alkene 2 = 46.02, alkene 3 = 13.25. total mass of alkene products = 1.3g. molar mass of alkenes = 84.12 g/mol. how many grams of each alkene 1, 2, and 3 were produced?
0.68g, 0.48g, 0.14g
A 10% solution of Br2/DCM by volume is used in the experiment. If 0.400 mL of the bromine solution are used in the reaction, how many mmols of Br2 are reacted? MW Br2= 159.8 g/mol d = 3.12 g/mol (pure bromine)
0.781 mmol
Which of the following are not appropriate for doing an extraction: 1) acetone and water 2) diethyl ether and water 3) methanol and water
1) Acetone and water 3) methanol and water solvents must be immiscible
When conducting a safe distillation in the lab which of the following should always be done?
1) a boiling chip should be placed in the distillation flask 2) The glassware joints should be lightly greased
In the Williamson Ether synthesis what is the electrophile?
1-bromobutane
In the Williamson Ether synthesis what is in the organic layer?
1-bromobutane and 1-ethoxybutane
The first step in the work up of Williamson Ether Synthesis was to add both water and ether to the reaction mixture to transfer the mixture to a sep funnel and then drain off the aqueous layer. Which of the following species were in the organic layer
1-bromobutane and 1-ethoxybutane
An impure compound was dissolved in hot methanol and a gravity filtration was done to remove activated charcoal that was added to the solution. During the filtration, crystals began to form on the filter paper. What should have been done next?
Rinse the filter paper with hot methanol
identify the mobile and stationary phases of gas chromatography in dehydration of 2-butanol
S: carbowax M: helium
A student reported the following amounts of her compound recovered with each of 3 successive extractions using the same volume of solvent each time. 0.45 g, 0.23 g, 0.30 g. Based on your understanding of the partition coefficient, why can't this be correct
She should have obtained smaller amounts of compound with each successive extraction
What is the reaction mechanism of the Williamson ether synthesis?
Sn2
Why do organic chemists often rinse glassware with an organic solvent, like acetone or an alcohol, after washing with soap and water?
Sometimes the compounds that we work with in an organic lab are not highly soluble in water, so even after scrubbing glassware with soap and water, the glassware may contain a residue. Using an organic solvent (like acetone) will rinse out any remaining nonpolar/slightly polar residues.
Suppose an extraction with methylene chloride (d = 1.33 g/mL) also called dichloromethane is performed with the desired compound initially in brine (d = 1.04 g/mL). In a sep funnel, which layer will be the organic layer?
The bottom layer
A microscale boiling point determination has been correctly set up and is heating. At the start a small stream of air bubbles exits from the capillary tube. Then the rate of bubbling becomes faster and faster. As the rate of bubbling becomes faster, what is causing the bubbling?
The bubbles are actually the vapor of the unknown liquid being forced out of the tube.
When using a mixed solvent system such as water and methanol to do a recrystallization, which of the following is TRUE?
The first step in the recrystallization process is to dissolve the compound in a minimum of one of the hot solvents
At the end of the reaction, there is a caution noted in the procedure. Do not remove the reservoir or allow the open end to break the surface of the water. Why?
The product will escape into the atmosphere. Goodbye yield
A melting point for a known substance was high and broad. Why might that be?
The ramp rate of the Meltemp instrument was too fast (sample heated too quickly) and there was too much sample.
What is a sign that a solid sample may contain an impurity
The solid has a broad melting point range( >2°c span) and a value that is lower than the literature value.
In the separatory funnel during extraction of essential oils, what is in the top and bottom layer?
Top= organic, DCM layer Bottom= Aqueous, water layer
True or False: In a reaction of benzoic acid and sodium hydroxide the products will be more soluble in water than the organic solvent.
True
Which of the following will result in the formation of few or no crystals when trying to recrystallize a solid?
Using an excessive amount of solvent to dissolve the impure solid
If you do all of the reading, then you know about the partition of a compound and how that affects the completeness of each extraction. Without doing calculations, what is the big take home message that the calculations show you for each successive extraction
Using multiple smaller volume extractions extracts greater mass of the compound than does one larger volume extraction
In today's experiment, butan-2-ol is dehydrated to yield 3 isomeric butene. How will the mixture of butene products be analyzed to see how much of each one was formed
We will do a gas chromatography and use the integration of the peaks to calculate the mole fraction of each isomer
When using Siwoloboff's method, when is the thermometer read to determine the boiling point of the unknown liquid?
When the entire setup is cooled down and the liquid unknown begins to enter the capillary tube
How could IR be used to distinguish between a carboxylic acid and a ketone?
While there will be nearly identical signal position and intensity when comparing a carboxylic acid and ketone, there are also specific traits that can be used to distinguish the two. Carboxylic acid has a very broad peak for OH (3500-2000 cm-1) in addition to a double bond between carbon and oxygen at around 1700 cm-1. Ketones have only a double bond between carbon and oxygen at around 1700 cm-1.
The bromination reaction is believed to proceed via the formation of an intermediate. How is the intermediate best characterized
a bromonium ion
In fractional distillation, a large surface area for condensation is provided through ____________ __________
a fractionating column
What is a forerun?
a low boiling liquid that sometimes distills over long before the boiling point of the liquid you want has started to boil.
What is an azeotrope?
a mixture of 2 or more liquids with a constant boiling point
What is an azetrope?
a mixture of two or more liquids with a constant boiling point
What is a polar protic solvent?
a solvent with OH or NH bonds (ex. ethanol, water, methanol, ammonia, acetic acid)
Using the proper terminology in lab work is important. Which is a correct statement about today's experiment? a) The acidic compound, benzoic acid, is being extracted into the sodium bicarbonate solution b) The neutral compound, 1,4-dimethoxybenzene, is being extracted into the sodium bicarbonate solution c) The neutral compound, 1,4-dimethoxybenzene, is being washed into the sodium bicarbonate solution d) The acidic compound, benzoic acid, is being washed into the sodium bicarbonate solution
a) The acidic compound, benzoic acid, is being extracted into the sodium bicarbonate solution
Which physical properties help determine identity of unknown solid? Select all that apply a) melting point b) boiling point c) color d) dielectric constant
a) melting point c) color
An agent used for decolorization is?
activated charcoal
What functional groups are present on cinnamaldehyde?
aldehyde
When preparing a gravity chromatography column, which of the following practices should be followed
all of the above
Which of the following demonstrates the proper use of a seperatory funnel during washings and/or extractions?
all of these
Several separation techniquies are used in organic chemistry such as extraction, TLC, column chromatography, distillation and gas chromatography. TLC and column chromatography often use a liquid as the mobile phase. What is the mobile phase in gas chromatography
an inert gas such as helium
Based on well-documented evidence, what is the dibrominated product the result of
anti-addition
What is the reaction type of the bromination of trans-cinnamic acid?
anti-additon
In the extraction of benzoic acid, where does the benzoate ion go in the separatory funnel?
aqueous layer (bottom)
What reaction conditions favor an addition reaction? a) reduce amount of alkene reactants b) increase the amount of water c) increase the amount of sulfuric acid d) nothing can be done to favor the addition reaction
b) increase the amount of water
After extraction compounds have ________ and ________ melting points.
broader and lower
If 10 mL of prepared 21% (by weight) sodium ethoxide solution, d=0.868 g/mL, is used, how many mmols of sodium ethoxide are reacted? (MW of sodium ethoxide= 68.05 g/mol) a) 35.6 mmol b) 60.7 mmol c) 26.8 mmol d) 3.56 mmol
c) 26.8 mmol
What type of reactive intermediate is formed in the reaction of an alkene with H2O and H2SO4 to give an alcohol? a) carbanion b) radical c) carbocation d) alkoxide
c) carbocation
If a compound contains an isolated ethyl group, CH3CH2, which of the following splitting patterns would you expect to observe in the proton NMR spectrum of the compound? a) quartet, singlet b) singlet, triplet c) quartet, triplet d) triplet, doublet
c) quartet, triplet
During gravity filtration, a student forgets to heat the solution before running it through the filter. After capturing the filtrate, the student analyzes the sample via infrared (IR) spectroscopy and finds none of the desired product in the filtrate. What likely occurred to the student's product? A. The product degraded because of a prolonged filtration time. B. The product evaporated with collection of the filtrate. C. The product precipitated and is present in the residue. D. The product was dissolved in the solvent.
c) the product precipitated and is present in the residue
how is the dehydration of 2-butanol worked up?
calibration of collection tube using water
What type of reactive intermediate is formed in the reaction of an alkene with H2O and H2SO4 to give an alcohol
carbocation
After crystals are collected, they should be washed with?
cold solvent
Distillate is a liquid that...
condenses after distillation
What is the purpose of performing NMR spectroscopy on the product of the Williamson ether synthesis? a) verify structure b) assess purity c) identify impurities d) all of the above
d) all of the above
Why is steam distillation used? a) used to purify temperature sensitive materials b) volatile co-distilled below boiling point of water c) avoids high temperature decomposition d) all of the above
d) all of the above
If proton A is closer to an EN atom than proton B, proton A will be more _ than proton B and appear _.
deshielded, downfield
Why are deuterated solvents used in NMR spectroscopy?
deuterated solvents do not contain protons so they do not give an NMR signal that could interfere with your compound's NMR signals
In steam distillation lab, what solvent is used to extract cinnamaldehyde and eugenol?
dichloromethane (DCM)
During the workup of the reaction, diethyl ether is used for the extraction process. Which of the following statements is true about the extraction
diethyl ether will be the top layer because it is less dense than water
The melting point of diastereomers generally will be ______. The melting point of enantiomers will definitely be _______.
different; the same
How could a water-insoluble, organic base be separated from a water-insoluble, neutral organic compound?
dissolve both compounds in ether and extract it with an acid
If proton A is closer to an EN atom than proton B, proton A will be more _ than proton B and appear _.
downfield, at a higher ppm
why are deuterated solvents used in NMR spectroscopy? a) they dissolve organic compounds better b) they do not contain protons so they do not give an NMR signal that could interfere c) they are less toxic d) they are cheaper e) all of the above
e) all of the above
When a solution is saturated it is said to be at ___________
equilibrium
Which of the following combinations of solvents would be good for an extraction? a) ethanol and water b) ether and water c) benzoic acid and methanol
ether and water
Fractional distillation under atmospheric pressure would most likely be used to separate which of the following compounds?
ethyl acetate (boiling point of 77°c) and ethanol (boiling point of 80°c)
When separating a solid from a liquid via filtration, what is the name given to the liquid that is collected?
filtrate
How are insoluble impurities from solution during crystallization removed?
filtration
Ethyl actate (bp= 77 C) and ethanol (bp= 80 C) would be best separated using what distillation technique?
fractional distillation
If the bp of water is 100 C and bp of ethanol is 78 C, what distillation technique should be used??
fractional, only a 22 degree difference
What is used to differentiate between the reagents of dehydration of 2-butanol?
gas chromatography
In which waste container should dichloromethane be disposed?
halogenated waste container
The product precipitates from the reaction mixture and is collected by vacuum filtration. If the crystals are orange, you are instructed to wash them with 30% aqueous sodium thiosulfate. Where should the filtrate be disposed
halogenated waster
Often, saturated sodium bicarbonate solution is used to do a wash during the extraction process. What is a saturated sodium bicarbonate solution?
it is NaHCO3 dissolved in water to a point that no more of it can dissolve in water
In today's experiment, butan-2-ol is dehydrated to yield 3 isomeric butene. What is the role of sulfuric acid
it is a catalyst. It makes the hydroxyl group a better leaving group and is not used up in the course of the reaction
During the workup of the reaction, diethyl ether is washed with brine solution. What is the purpose of this wash?
it is to"predry" the diethyl ether. It helps remove traces of water in the ether.
Where should the bulb of the thermometer be placed to get an accurate reading in distillation?
it should be aligned slightly below the opening of the 3-way adapter leading to the condenser
Norbornene is converted into norborneol products. How will you determine which products you obtained
melting point
What physical property will help you to determine the identity of your unknown solid?
melting point
Solvents in crystallization should be _________
miscible
what is the electrophile in the bromination reaction
molecular bromine
How can you tell which protons are more downfield?
more downfield = more deshielded = stronger electron-withdrawing groups
When benzoic acid and NaOH react, are the products more or less soluble in water than the organic solvent?
more soluble in water than organic solvent
If a sample of the dibrominated product is placed in a polarimeter, what will the result be and why
no rotation, racemic mixture formed
In which waste container should the liquid solvent, acetone be disposed?
non-halogenated waste container
Which of the following statements are true 1) diastereomers can be differentiated by melting point 2) nornornene is an example of a solid with a high vapor pressure at room temperature (it sublimes) 3) sulfuric acid will act as both a catalyst and nucleophile in the hydration of norbornene
only 1 and 2 are true
which of the following statements are false concerning today's reaction of the hydration of norbornene 1) infrared spectroscopy could be used to distinguish the two possible products 2) an alkene is being transformed into an alcohol 3) water is the electrophile
only 1 and 3 are false
When you set up for microscale boiling point determination, you are asked to put a capillary tube into a test tube containing your unknown liquid. How should the capillary tube be inserted?
open end down in the liquid
Benzoic acid is: 1) Water soluble 2) Organic soluble
organic soluble
Think of your experiment from last week and choose which of the following is true about extracting an organic compound into a given solvent.
organic substances with acidic functional groups may dissolve in water if treated with base
In the extraction of benzoic acid, where does the 1,4-dimethoxybenzene go in the separatory funnel?
organic, ether layer (top)
You will use two different kinds (thin layer and column) of chromatography as a means of separation. Both of the chromatographic experiments that you will perform will separate the components of a mixture based on which physical property?
polarity/solubility
Which of the following is not a step in the process of distillation? A) Conversion of liquid into a vapor B) Precipitation C) Condensation of purified liquid D) Collection of condensed liquid
precipitation
What would happen if the reactant concentrations were doubled in a Williamson ether synthesis reaction?
product concentrations would also double
Why are silica beads used in fractional distillaion?
provides surface area to recondense droplets as well as purify them
If a compound contains an isolated ethyl group (CH3CH2) which of the following splitting patterns would you expect to observe in the proton NMR spectrum of the compound
quartet, triplet
What is in the aqueous layer of the Williamson ether synthesis?
residual sodium ethoxide sodium ions bromide ions
The first step in the work of the Williamson Ether Synthesis up was to add both water and ether to the reaction mixture, to transfer the mixture to a separatory funnel, and then to drain off the aqueous layer. Which of the following species were in the water layer
residual sodium ethoxide, sodium ions, and bromide ions
For crystallization, the solution should be?
saturated
Where is benzoic acid extracted into?
sodium bicarbonate solution
In Williamson Ether synthesis what is the nucleophile?
sodium ethoxide
Why is sulfuric acid, H2SO4 used instead of HBr or HCl in the dehydration reaction of butan-2-ol
sulfuric acid is non-nucleophilic and therefore it doesn't give any substitution products
Compounds isolated after an extraction may be impure or wet. What effect will this have on the melting points?
the melting points will be broader and lower than expected
In the hydration of norbornene reaction, the melting point is obtained only after sealing the capillary tube containing the product. Why does the tube need to be sealed
the product sublimes
Norborneol products will be affected if the alcohol was left under the vacuum for an excessive period of time to remove all traces of solvent
the yield would be lower, because some of the product would be removed.
Which of the following would be the best procedure for extracting acetaldehyde from an aqueous solution?
three successive extractions with 33.3 mL portions of ether
Like the experiment last week, today you will perform a liquid-liquid extraction. The organic solvent you will use is hexane ("hexanes" to be specific...). After the hexane layer is separated, you will treat it with anhydrous sodium sulfate. what is the purpose of this?
to "dry" the organic layer by absorbing any water that remains in the hexane
A reflux condenser was used during the reaction. What was the purpose of using a reflux condenser
to prevent the ethanol solvent from boiling off
What is the point of using a boiling chip or stir bar when heating a liquid?
to provide a surface on which gas bubbles can form in the liquid to avoid superheating
What was the purpose of adding brine to the ether layer as a final wash in the last step of the extraction workup
to remove any residual water in the ether layer, further drying the organic layer
The first step of today's experiment is to grind spinach leaves in acetone with a mortar and pestle. The purpose of this step is:
to solubilize the organic plant pigments in the spinach leaves
Like the experiment last week, today you will perform a liquid-liquid extraction. The organic solvent you will use is hexane ("hexanes" to be specific...). When you mix the hexane with water, you will expect to see:
two layers: the top will be hexane, as hexane is less dense than water
Which technique should you use to collect your purified crystals?
vacuum filtration
Which is more dense, diethyl ether or water?
water (bottom layer)
When benzoic acid reacts with NaOH is the product water or organic soluble?
water soluble
In today's experiment, butan-2-ol is dehydrated to yield 3 isomeric butene. How will the crude mixture of butene products be isolated
we will collect them as a gas mixture over water
what makes for good leaving groups?
weak bases
When doing a microscale boiling point determination, when should the temperature be read for the boiling point measurement?
when the last bubble escapes the mp capillary tube and liquid is sucked back into the tube
When should you record the first boiling point range during a distillation?
when the vapor condenses to form a drop of liquid on the bulb of the thermometer and the temperature stabilizes
