OChem Lab Midterm (George)
Terms in this set (180)
Pure substances typically have a melting range of ____________.
Less than 2 degrees Celsius
The upper limit of the melting temperature range occurs when ___________.
the last remaining crystal melts
The lower limit of the melting temperature range occurs when ___________.
the first crystal starts to melt
For a pure substance, the melting temperature range is _________ than that of an impure substance.
For an impure substance, the melting point is _______ than that of a pure substance.
What is a eutectic composition?
the composition of a mixture of two components that generates the lowest possible melting point
What is the melting point for acetylsalicylic acid (Aspirin)?
135 degrees Celsius
How does the size of a sample affect the melting point measurement?
the sample should not be large, because a large sample would produce a higher and broader mp range
How does the rate of heating affect the melting point measurement?
a slow rate of heating produces a more accurate measurement
How does the degree of chemical purity of a substance affect the melting point measurement?
impurities make the melting point range lower and broader
How wide is the typical melting point range for a pure compound?
0.5 - 1.5 degrees Celsius
Which types of molecular interactions determine a molecule's melting range?
-Van der Waals
A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. What would provide the most conclusive evidence that the two samples are the same compound?
perform a mixture melting pont
List the seven procedural steps, from start to finish, that are required to perform a TLC separation.
1. make a pencil mark on the TLC plate 10mm from the bottom
2. use a capillary tube to deliver a 2mm diameter spot onto the pencil mark
3. add just enough developing solvent to cover the bottom 3mm of the TLC chamber
4. stand TLC plate upright in the chamber, and replace the lid
5. remove the TLC plate when solvent is near the top of the plate and mark the solvent front with a pencil before the TLC plate dries
6. mark the outline of the spots on the plate with a pencil. Unless the spots are colored, some visualization method will be needed.
7. measure the distance that the spot and the solvent have moved
In TLC, if you have two compounds, what can be said about the less polar substance?
it travels the longer distance
A TLC plate was run in 1:1 ethyl acetate:hexane, but when developed, the plate only showed baseline spots. What change could be made to get the spot(s) to move further up the plate?
spot a new plate and run in 2:1 ethyl acetate/hexane
In an eluent solution of 70% hexanes and 30% ethyl acetate, compound A moves farther on the TLC plate than compound B. Compound A is _________ polar than compound B.
How do you calculate the Rf for a compound on the TLC plate?
distance to spot from starting point / distance to solvent line from starting point
What is important about handling sample plates for the IR spectrometer?
-plates are fragile and will crack or chip if mishandled
-never handle them with bare hands
What factors govern the position of an IR absorption peak?
-type of vibration being observed
-masses of the atoms involved in the bond
-strength of the bond
In a typical IR spectrum, what name describes the units shown on the x-axis?
In a typical IR spectrum, what name describes the units shown on the y-axis?
Where in an IR spectrum would you find a strong C=O peak?
A region of the IR spectrum can be used to differentiate between similar compounds that contain the same functional groups. What is this region called?
In Fourier Transform Infrared (FTIR) spectroscopy, which type of vibration requires the most energy?
In infrared spectroscopy, what must chemically occur within a compound to have a signal appear in the IR spectrum?
What physical properties prevent a sample mixture from being analyzed by gas chromatography (GC)?
-one or more of the components is thermally unstable
-one or more of the components is not volatile
How is it possible to separate two liquids with the exact same boiling point by gas chromatography (GC) when it cannot be done by fractional distillation?
The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility.
The carrier gas in a GC separation is usually an inert gas such as nitrogen, helium, or argon, but can also be hydrogen. What are the pros and cons of using hydrogen as a carrier gas in a GC separation?
-cheaper than helium
-can be generated at the GC instead of from gas tanks
-greater difference in mass between carrier gas and sample than nitrogen
A young researcher set out to develop a method for quantifying the polycyclic aromatic hydrocarbon (PAH) content in a contaminated soil sample by gas chromatography. During the initial stages of method development, the researcher was unable to separate anthracene and naphthalene with baseline resolution. What are the possible modifications the researcher could make to improve the resolution?
-Increase the column length
-Choose a different stationary phase
-Use a column with a smaller inner diameter
Which procedures(s) can be used to identify a compound in GC?
-Measuring and analyzing retention times
-Analyzing with other spectroscopic methods
The stationary phase in GC is...
The mobile phase in GC is....
One type of detector used in gas chromatography is a thermal conductivity detector (TCD). What two elements commonly make up the wire (filament) used in TCDs?
A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. Which of the following would provide the most conclusive evidence that the two samples are the same compound?
Perform a mixture melting point
In TLC, if compound A has a smaller Rf value than compound B, we can conclude that compound A is _______ polar than B
In TLC, less polar compounds travel ______ on a TLC plate than more polar compounds.
Why is it necessary use a pencil when writing on a TLC plate?
The ink from the pen will dissolve in the developing solvent.
How does the solvent move in TLC?
by capillary action
What happens to the spot in a TLC if the concentration of the component is too high?
it will become a smear or begin to tail
A TLC plate was run in 1:1 toluene and ethyl acetate, but when developed the plate only showed baseline spots. What change could be made to get the spot(s) to move further up the plate?
Spot a new plate and run in 1:4 toluene/ethyl acetate
Assume your sample spots are 1 cm above the bottom of the TLC plate. What happens if you put the plate in a developing chamber that has 1.2 cm of developing solvent?
The compounds to be analyzed will end up in the developing solvent.
A mixture of two compounds on a TLC plate will move a short distance (Rf = 0.1) and appear as a single spot only if a very polar solvent is used. Which two ways should the system be adjusted for better results (Rf = 0.4 and separation)?
-using a less polar eluent
-using a less active adsorbent (stationary phase)
Rank the following solvents from most polar to least polar.
water > acetonitrile > ethyl acetate > dichloromethane > hexanes
After removing a TLC plate from the developing chamber, what is the first thing to do with the TLC plate?
Draw the solvent front line.
A student analyzes their TLC plate after a reaction and finds that the Rf value is 0.15. The student would like to adjust their Rf value to be 0.40. What could the student do to the TLC system conditions to adjust the Rf value?
Make the eluent more polar
A student uses a TLC plate laced with fluorescent dye to spot their compounds. In order to visualize these compounds under a UV light, the compound must
-absorb UV light
-quench the fluorescence of the adsorbent
In an eluent solution of 70% hexanes and 30% ethyl acetate, compound A moves farther on the TLC plate than compound B. Compound A is _________ polar than compound B.
What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?
The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.
T/F: Distillation is a physical separation, not a chemical reaction.
T/F: Over time, the amount of the most volatile component in the distillation flask increases.
T/F: Simple distillation is effective at separating mixtures of liquids with very similar volatilities.
T/F: Over time, the amount of the most volatile component in the distillate increases.
What word describes a mixture that cannot be purified by distillation? e.g. water and ethanol
When the vapor pressure equals the ______ , a liquid will boil.
Why is a flask never distilled to dryness?
-could crack or break
-could become charred
-could explode because of residue
After a distillation a student runs an IR of their compound. The student thinks it is acetone. Which stretches in the spectrum might lead the student to make this conclusion?
C=O stretch at 1710 cm-1
C-H stretch at 2990-2850 cm-1
In what ways can simple distillation be useful in a laboratory setting?
-To remove dissolved, non-volatile components from a liquid.
-As the last step in the purification of a liquid.
-To remove solids from a liquid.
-When two components of a system have a boiling point difference of 60 °C or more.
-To purify more than 60%.
What is the minimum boiling point range gap between two solvents that simple distillation can successfully separate?
Which one uses fewer theoretical plates? Simple or fractional distillation
During a fractional distillation of acetone and toluene, a student notices that their data shows a drop in temperature from 56 °C to 51 °C. What does the student conclude?
There is no more acetone vapor and the toluene has not started to distill.
A fractionating column is more efficient than simple distillation because it provides ________ where several liquid-vapor cycles can occur.
more surface area
During a solvent extraction, a student lost track of which layer was the aqueous layer and which was the organic layer. What simple, non-destructive test could be used to determine which layer is which?
Add a little distilled water to see which layer the water adds to.
A layer will form between the aqueous and organic phases if the extraction is shaken too vigorously. What is this the term used to describe this layer?
Which would be the more effective extraction route for a mixture of compounds X and Y: a single extraction using 75 mL of ether or combining the extracts from three separate extractions using 25 mL of ether?
3 × 25 mL
What is extraction?
The process of transferring a solute from one solvent into another solvent.
separation of substances
physically blocking certain objects or substances while letting others through
pouring off a liquid but keeping the solid
When a solid turns into a gas without turning into a liquid
If you have an organic layer that contains an organic acid, an organic base and a neutral organic compound, what is in the organic layer after complete treatment with NaOH(aq)?
-neutral organic compound
If there is a reaction mixture in an organic solvent that contains an organic acid, an organic base, neutral organic compounds, and base containing impurities, what is in the aqueous layer after complete treatment with NaOH(aq)?
If you have an organic layer that contains an organic acid, an organic base and a neutral organic compound, what is in the organic layer after complete treatment with NaOH(aq), followed by complete treatment with HCl(aq)?
neutral organic compounds
At what point during the process of extraction is a "drying agent" used?
before the solvent is evaporated to yield the final product
An organic compound was extracted into dichloromethane (DCM) and then the aqueous layer is shaken with saturated sodium chloride solution. What is the purpose of the sodium chloride?
to decrease the solubility of the organic product in water.
Arrange the following solvents from least dense to most dense.
hexanes < diethyl ether < water < dichloromethane < chloroform
If aniline, benzoic acid, and benzil are dissolved in a solution of diethyl ether, how can benzoic acid be extracted into the aqueous layer?
Benzoic acid can be extracted into the aqueous layer using 3M HCl.
If aniline, benzoic acid, and benzil are dissolved in a solution of diethyl ether, how can aniline be extracted into the aqueous layer?
Aniline can be extracted into the aqueous layer using 3M NaOH.
A solution contains aniline, benzoic acid, and benzil, dissolved in a solution of diethyl ether. After an extraction with 3M NaOH benzoic acid is now a salt. Which compounds are left in the organic layer after the extraction?
A solution contains aniline, benzoic acid, and benzil, dissolved in a solution of diethyl ether. After an extraction with 3M HCl aniline is now a salt. Which compounds are left in solution after the extraction?
benzil, benzoic acid
A reaction was performed, and the dichloromethane solvent was dried by adding magnesium sulfate drying agent. When the reaction flask was shaken, it was observed that the magnesium sulfate swirled around freely at the bottom of the flask. What does this observation indicate?
The correct amount of drying agent has been used.
What will facilitate a normal Diels-Alder reaction?
An electron withdrawing group on the dienophile.
Consider the Diels-Alder reaction. If we use anthracene and maleic anhydride, what is the molar mass of the product?
Which distillation technique would provide the best method to separate anthracene from cyclohexanol.
Antracene: (MP: -95) (BP: 56)
Cyclohexanol: (MP: 25) (BP: 161)
How can the injection time be indicated on a GC?
-use a pen to mark on the recorder base line immediately after the injection
-if the detector is a thermal conductivity detector, inject several microliters of air with the sample
-if the GC has analysis software it will automatically be shown on the print out
During lab, a closed system should not be heated because...
the setup could explode
In what situation should a water bath be used?
keeping a flask or test tube containing a chemical substance at a specific, constant temperature
While heating a vessel at approximately 65 °C, which contains a reaction using a hexanes and water solution, a student notices the volume in the vessel has significantly decreased. What should the student add to the reaction to continue the reaction properly?
What is the purpose of DMAP in an esterification reaction?
Rank the carboxylic acid derivatives according to their reactivity, from the most reactive to the least reactive.
Why must the cyclopentadiene be freshly distilled and kept cold prior to a Diels-Alder reaction?
cyclopentadiene dimerizes at room temperature
What type of electromagnetic radiation is associated with the peaks in IR spectroscopy?
What is measured in IR spectroscopy?
the vibrational frequency of a bond
What type of information is given by IR spectroscopy?
the functional groups in a molecule
Stronger bonds absorb at ________ wavenumber.
In IR spectroscopy, what must chemically occur within a compound to have a signal appear in the IR spectrum?
the distance between two crests of an electromagnetic wave
the rate at which electromagnetic waves oscillate
What are the correct units of wavenumber in IR spectroscopy?
What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?
The IR spectrum of a compound shows several weak bands from 1750-1950 cm-1. What is the origin of these bands?
aromatic overtones - combination bands
One technique for taking the IR spectrum of a solid same is by making a mull . A mull is made by mixing Nujol, a heavy paraffin oil, and the solid sample. The mull is then placed between two salt plates and the IR spectrum measured. What is true about a mull?
When the solid sample is mixed with the Nujol, a mull is formed and has a thick consistency like that of toothpaste. IR signals from Nujol can be subtracted out by measuring a blank of pure Nujol.
The vibration of which bond gives an IR absorption that distinguishes between the following compounds?
Rank the following molecules in terms of their carbon-halogen stretching frequency in the infrared spectrum.
In mass spectrometry, the _______ is assigned a relative intensity of 100.
In mass spectrometry, the _______ is the most intense peak in the mass spectrum.
In mass spectrometry, the _______ represents the original molecule that has only lost an electron.
molecular ion peak
In mass spectrometry, _______ and ________ could be/represent a smaller charged fragment of the original molecule.
In mass spectrometry, _____, _____, _____, and _____ represent a positively charged species.
Benzene is studied via electron ionization mass spectrometry. What is the most likely m/z value for the molecular ion peak of benzene.
IR: Only infrared radiation with energy _________ that of the bond with an induced vibration will be absorbed.
IR: Plank's constant (h) =
6.626 x 10^-34 [(m^2)(kg)]/s
IR: As wavelength gets smaller energy gets ________.
IR: What are the two kinds of fundamental vibrations?
IR: ___________ vibrations alter the distance between two atoms in a rhythmic manner.
IR: __________ vibrations alter the positions of the atoms relative to the bond axis.
Linear molecules have _________ possible fundamental vibrations
Nonlinear molecules have _________ possible fundamental vibrations.
IR: Conjugation _______ the frequency in IR spectroscopy.
sp3 hybridized has ______ s character
sp2 hybridized has ______ s character
sp hybridized has ______ s character
IR: The C-H bond of an sp3 carbon will be slightly __________ than the C-H of an sp2 or an sp carbon.
IR: Peak size vertically has to do with the _________ of radiation absorbed.
IR: Peaks representing bonds of large dipole moments will be called _______ peaks and will absorb at _________ % of radiation.
IR: Peaks representing bonds of small dipole moments will be called _______ peaks and will absorb at _________ % of radiation.
IR: Peak size horizontally has to do with the consistency of _____________.
static bond length
IR: If a bond has a fairly static bond length like a C-H bond, the peak shape will be _______.
IR: If a bond has a fairly inconsistent bond length like a O-H bond, the peak shape will be _______.
IR Frequency: O-H (alcohol)
3300 cm-1, always broad
IR Frequency: N-H (amine, amide)
3300 cm-1, may be broad, sharp , or broad with spikes
IR: Describe O-H bonds vs. N-H bonds
-both at 3300 cm-1
-OH is broader for entirety of the peak
-NH is broad only at the base of the peak and will have 1 or 2 sharp peaks based on being a primary (2 spikes) or secondary (1 spike) amine
IR Frequency: C-O double bond (ketone)
IR Frequency: C-O double bond (aldehyde)
2700 cm-1 and 2800 cm-1
IR Frequency: C-O double bond (ester)
IR Frequency: C-O double bond (carboxylic acid)
2500 cm-1 to 3500 cm-1 (broad and monstrous)
IR Frequency: C-O double bond (amide)
IR Frequency: C-H (alkane)
just below 3000 cm-1
IR Frequency: C-H (alkene)
just above 3000 cm-1
IR Frequency: C-H (alkyne)
3300 cm-1, always sharp, usually strong
IR Frequency: C-C triple bond (alkyne)
just below 2200 cm-1
IR Frequency: C-N triple bond (nitrile)
just above 2200 cm-1
IR Frequency: C-C double bond (alkene)
IR Frequency: C-N double bond (imine)
MS: Only ________ fragments can be observed in a mass spectrometer.
MS: _______________ ionizes a reagent gas (commonly methane, mw = 16) that interacts with and ionizes the vaporized (gaseous) inserted sample.
MS: ______________ ionizes a liquid sample (dissolved in solvent) via spraying the liquid sample into the instrument using a needle-like device with a voltage attached to it.
MS: _____________ uses a laser focused on a solid matrix that can absorb the laser's energy and transfer it to a sample (ionizes it) placed in the matrix.
matrix assisted laser desorption/ionization (MALDI)
MS: Describe a magnetic sector mass analyzer and how it identifies ions present.
-uses an electromagnet to direct ions of different m/z ratios around a corner one m/z at a time
-the # of ions that it the detector at the end of the corner are counted
-each m/z range is scanned by adjusting the magnetic field strength
-neutral molecules are not affected by the magnet and never "make the corner"
MS: Describe a quadrupole mass analyzer and how it identifies ions present.
-sends the stream of ions into the center of 4 parallel rods arranged in a square orientation
-rods have opposing DC voltages and superimposed high-frequency AC voltages placed on the rods
-voltages cause ions to oscillate in space
-every individual m/z can be sifted through by altering the voltage and frequency of the DC and AC placed in the rods
-detector at the end that counts the number of molecules that hit it
MS: The y-axis is ____________.
MS: The x-axis is ____________.
MS: The taller the peak, the ____ the relative abundance of that specific m/z relative to the others.
MS: Radicals are not charged (only electron deficient) therefore they __________.
will never be seen
MS: Radical cations/anions are charged, therefore they __________.
can be seen
MS: What is the mw of a methyl (CH3)?
MS: What is the mw of a methylene (CH2)?
MS: What is the mw of an ethyl (CH3CH2)?
MS: What is the rule of 13 for hydrocarbons?
-divide the m/z of the parent peak by 13
-the integer is the number of carbons in the sample molecule
-add the remainder to the integer to get the number of hydrogens in the sample molecule
MS: What is the rule of 13 for molecules with heteroatoms (oxygen)?
-divide mw by 13
-integer = carbons
-remainder + integer = hydrogens
-subtract 1 C and 4 H
-add 1 O
MS: What is the rule of 13 for molecules with heteroatoms (nitrogen)?
-divide mw by 13
-integer = carbons
-remainder + integer = hydrogens
-subtract 1 C and 2 H
-add 1 N
MS: In a MS, what peak pattern indicates a Bromine?
m+ and m+2 are equal in height
MS: In a MS, what peak pattern indicates a Chlorine?
m+2 and m+ have a 3:1 ratio, respectively
MS: _________ positions (carbon attached to vinylic carbon) are stable with a charge due to resonance.
MS: _______ position is just an extremely stable allylic position.
MS: Fragmentation of benzylic position resulting in a benzylic carbocation often undergoes a rearrangement (methylene insertion) to a _______ carbocation.
MS: _______ may not have a parent peak.
MS: ____________ to the carbonyl position is a favorable occurrence due to the stability of the resulting cation fragment.
MS: ____________ rearrangement occurs when the bond cleavage is between the alpha, beta positions due to gamma deprotonation.
MS: In nitrogen-containing species, the parent peak will have an ______ mw. Any fragments still containing nitrogen will be _______. Fragments not containing nitrogen will be _________.
odd, even, odd
How do you calculate the area beneath the peak in a GC?
peak area = (height) * (width at 1/2 height)
How do you calculate the total vapor pressure of a system?
Ptotal = (Xa x Pa) + (Xb x Pb)
How do you calculate the fractions of a mixture in the vapor phase if the fractions in the liquid phase are known?
Xvapor = (Pb * Xliquid) / Ptotal
How do you calculate the number of theoretical plates used in distillation?
Nt = L / HETP
How do you calculate the retention factor (Rf) for a compound on a TLC plate?
Rf = distance traveled by spot / distance traveled by solvent front
How do you calculate the total % recovery of an extraction?
% recovery = (total mass recovered / mass of sample)]* 100
How do you calculate the amount of an organic compound remaining in the aqueous layer after an extraction?
x = (volume of original solvent x initial mass of compound) / [(partition coefficient x volume of extracted solvent) + (volume of original solvent)]
How do you calculate theoretical yield?
theoretical yield = moles of reactant * mw of product
How do you calculate % yield?
% yield = (actual yield / theoretical yield) * 100
How do you calculate wavelength?
wavelength = 1 / wavenumber
How do you find the molecular formula of CnHx using mass spectrometry?
12n + 1(2n+2) = mw
How do you calculate relative abundance?
relative abundance = (relative abundance of m+1) / (relative abundance of m)
How do you calculate relative abundance using isotopes?
relative abundance = (natural abundance of less common isotope / natural abundance of common isotope) * (number of carbon atoms)
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