Chem II: 3-5 Flashcards
isomers
same molecular formula
different chemical structure
structural isomers
aka constitutional isomers
same molecular formula –> molecular weights are the same
different physical and chemical properties
physical properties
characteristics of processes that do not change the composition of matter
ex: MP, BP, solubility, odor, color, density
chemical properties
reactivity of molecule –> result in changes in chemical composition
generally attributable to functional groups in the molecule
stereoisomers
same molecular formula, same atomic connectivity (Same structural backbone)
differ in how the atoms are arranged in space (wedge and dash pattern)
two groups: conformational isomers, configurational isomers
conformational isomers
type of stereoisomer
differ in rotation around single bonds
configurational isomers
type of stereoisomer
can be interconverted only by breaking bonds
newman projection
molecular is visualized along extending through C-C bond
staggered conformation
newman projection
groups are 60 deg apart
eclipsed conformation
newman projection
groups are 120 deg apart and directly overlap with atoms on adjacent carbon
anti staggered
two largest groups are 180 deg apart and strain is minimized
gauche staggered
two largest groups are 60 deg apart
totally eclipsed conformation
two largest groups are directly in front of each other and strain is maximized
the higher the energy, the ____ time the molecule will spend in the energetically unfavorable state
less
are conformation interconversion barriers easy to overcome? room temp? high/low temp?
yes at room temp bc they are small
at very low temp, interconversions are dramatically slow –> may not rotate at all
ring strain
angle strain, torsional strain, nonbonded strain
angle strain
results when bond angles deviate from their ideal values by being stretched or compressed
torsional strain
results when cyclic molecules must assume conformations that have eclipsed or gauche interactions
nonbonded strain
aka van der waals repulsion
- results when nonadjacent atoms or groups compete for the same space
- dominant source of steric strain in the flagpole interactions of the cyclohexane boat conformation
- cyclic molecules will usually adopt nonplanar shapes to minimize this strain
which main conformations does cyclohexane mainly exist in?
chair, boat, and twist boat forms
most stable conformation of cyclohexane
chair conformation
minimizes all 3 types of strain
axial
chair conformation
hydrogen atoms that are perpendicular to the plane of the ring (sticking up or down)
equatorial
chair conformation
hydrogen atoms that are parallel (sticking out) of the ring
chair flip
one chair form is converted to the other
- all axial groups become equatorial , and all equatorial groups become axial
- all dashes remain dashes and all wedges remain wedges
- during this process, the cyclohexane briefly passes thru a 4th confirmation called half chair conformation
how can chair flip be slowed
if a bulky group is attached to the ring
ex: tert-butyl groups
for substituted rings, the bulkiest group will favor the _____ position to reduce nonbonded strain
equatorial
in rings with more than one substituent, the preferred chair form is determined by the _____ group, which will prefer the ______ position
larger
equatorial
cis
when both groups are located on the same side of the ring
trans
when both groups are on opposite sides of the ring
configurational isomers
can only change from one form to another by breaking and reforming covalent bonds
two types: diastereomers and enantiomers
optical isomers
different spatial arrangement of groups affects the rotation of plane-polarized light
ex: enantiomers and diasteromer
chiral
mirror image cannot be superimposed on the og object
molecule lacks internal plane of symmetry
achiral
have mirror images that can be superimposed
chiral center
lack a plane of symmetry
whenever you see a carbon with 4 different substituents, think ______
chirality
enantiomers
configurational isomers
non-superimposable mirror images
diastereomers
configurational isomers
non mirror images
any stereoisomer that is not an enantiomer
occur when a molecule has 2+ sterogenic centers and differs at some, but not all, of these centers
enantiomers
physical and chemical properties
have identical physical and chemical properties except for:
optical activity and rxns in chiral environments
a compound is optically active if
it has the ability to rotate plane polarized light
polarizer
allows light waves oscillating only in a particular direction to pass through, producing plane polarized light
optical activity
refers to the rotation of this plane polarized light by a chiral molecule
lack plane of symmetry
one enantiomer will rotate plane polarized light to the same magnitude but in the opposite direction of its mirror image
dextrortatory (d-)
compound that rotates the plane of polarized light to the right
labeled (+)
must be determined experimentally
levorotatory (l-)
compound that rotates the plane of polarized light to the left
labeled (-)
must be determined experimentally
optical activity
amount of rotation depends on
number of molecules that a light weight encounters
depends on: conc of optically active compounds and length of the tube through which the eligh passes
specific rotation eq
racemic mixture
when both (+) and (-) enantiomers are present in equal concentrations
rotations cancel each other out, and no optical activity is observed
for any molecule with n chiral centers, there are ____ possible stereoisomers
2n
diastereomers
chemical and physical properties
- different chemical properties
- might behave similarly in particular reactions because they have the same functional groups
- different physical properties
- rotate plane polarized light
- knowing the specific rotation of one diasteromer gives no indication of the rotation of another
cis-trans isomers
aka geometric isomers
subtype of diastereomers
substituents differ in their position around an immovable bond (double bond, ring structure)
meso compounds
molecular with chiral centers that has an internal plane of symmetry
two halves that are mirror images
optically inactive
configuration of a stereoisomer
spatial arrangement of the atoms or groups in the molecule
conformation of a chiral molecule
describes the exact spatial arrangement of these atoms or groups, independent of other molecules
E/Z nomenclature is used for
compounds with polysubstituted double bonds
Cahn Ingold Prelog priority rules
the higher the atomic number, the higher the priority
determine E/Z designation
cahn priority rules
higher atomic number, higher priority
Z
if two highest priority substituents on each carbon are on the same side of the double bond
E
if two highest priority substituents on each carbon are on the opposite side of the double bond
R/S nomenclature is used for
chiral (sterogenic) centers
assigning R/S
steps
- assign priority
- higher atomic number takes priority
- arrange in space
- orient the molecule so the atom w lowest priority is at the back of the molecule
- draw a circle from highest -> lowest priority
- S - counterclockwise
- R - clockwise
fischer projection
horizontal lines - bonds that project out from the page (wedges)
vertical lines - bonds going into the plane of the page (dashes)
assign R/S for fischer projection
- determine priority
- obtain R/S
- true designation is the opposite of what you obtained
R or S for the carbon labeled 1?
Neither. There are only 3 unique bonds.
R or S for the carbon labeled 2?
Neither. There are only 3 unique bonds (the 2 terminal methyls are equivalent).
What happens to plane-polarized light as it passes through a polarimeter tube filled with an optically active substance?
The plane-polarized light will rotate.
What is the difference between enantiomers and diasteriomers?
Enantiomers are stereoisomers that are nonsuperimposable mirror images and have opposite configurations at ALL chirality centers.
Diasteriomers are stereoisomers that are nonsuperimposable NON-mirror images and have opposite configurations at SOME chirality centers.
CRB Which of the following is a type of diastereomer where the chirality only differs at one chirality center?
(A) Enantiomer
(B) Epimer
(C) Anomer
(D) Meso Compound
(B) Epimer
An epimer is a type of diastereomer where the chirality only differs at one chirality center.
CRB Which of the following is a type of epimer that is formed depending on how a ringed structure forms?
(A) Diastereomer
(B) Enantiomer
(C) Anomer
(D) Meso Compound
(C) Anomer
An anomer is a type of epimer that forms depending on how a ringed structure forms.
What is the difference between Constitutional Isomers and Conformational Isomers?
Constitutional Isomers have the same make up but a different arrangement of bonds.
Conformational Isomers have the same make up, the same arrangement of bonds, but their bonds are rotated differently from one another.
Dihedral/Torsional Angle is the angle between which substituents?
(A) The opposing substituents on the close and distant carbon of a Newman Projection.
(B) The neighboring substituents on the close and distant carbon of a Newman Projection.
(B) The neighboring substituents on the close and distant carbon of a Newman Projection.
Cyclohexane is the most commonly tested cycloalkane on the MCAT. Draw its “chair”, “boat” and “twist-boat” conformations, and predict which conformations have the highest and lowest energy.
a) 0 deg
b) 180
c) 60
d) 120
b) 180
b
which of the following can be used to differentiate between conformational isomers?
a) staggered and eclipsed
b) R and S
c) enantiomer and diastereomer
d) cis and trans
b) R and S
A student is unsure whether a compound they are working with is chiral. What can be done to experimentally determine this?
a) size exclusion chromotography
b) high performance liquid chromotography
c) shine plane polarized light through the same
d) shine unpolarized light through the sample
c) shine plane polarized light through the same
a) R and S only
b) S only
c) R only
d) R, S, Z
d) R, S, Z
the smaller the principal quantum number, the _____ the shell is to the nucleus, and the _____ its energy
closer
lower
as the azimuthal quantum number increases, energy _____
increases
p orbital is composed of
two lobes and a node
node
an area where the probability of finding an electron is 0
d orbital is composed of
4 symmetrical lobes with 2 nodes
principal quantum number describes…
size
azimuthal quantum number describes…
shape
magnetic quantum number describes…
orientation
principal quantum number
orientation level
shell
azimuthal quantum number
orientation level
subshell
magnetic quantum number
orientation level
orbital
principal quantum number
possible values
1 to infinity
azimuthal quantum number
possible values
0 to n-1
magnetic quantum number
possible values
-l to +l
when two atomic orbitals combine, they form ____
molecular orbitals
if signs of wave functions are the same, a ____ orbital is produced
bonding
if signs of wave functions are different, a ____ orbital is produced
antibonding
single bonds are made of ___ bonds
sigma
double bond is made of
pi bond and existing sigma bond
triple bond is made of
sigma bond and 2 pi bonds
can pi bond exist without a sigma bond
pi bonds cannot exist independently of sigma bond
only after the formation of a sigma bond will the p orbitals of adjacent carbons be parallel and in position to form the pi bond
what differences would be observed in a molecule containing a double bond compared to the same molecule containing only single bonds?
- bond length - shorter in double bond
- bond energy - higher in double bond
- molecular rigidity - higher in double bond
rank the following orbitals in decreasing order of strength: sigma bond, pi bond, double bond, triple bond
triple bond > double bond > sigma bond > pi bond
sp3 hybridized orbitals
1 s, 3 p
tetrahedral geometry
no unhybridized p orbitals to form pi bonds
ex: carbon
sp2 hybridized orbitals
1 s, 2 p
trigonal planar geometry
third p orbital is unhybridized -> can participate in pi bond
ex: alkenes
sp hybridized orbitals
1 s, 1 p
linear geometry - 180 deg
2 unhybridized p orbitals -> for pi bonds
what are resonance structures?
resonance structures differ in their placement of electrons in hybridized p orbital and require bond conjugation to delocalize electrons in a molecule.
how does the true electron density of a compound relate to its resonance structures?
true electron density is a weighted avg of the resonance structures of a given compound, favoring the most stable structures
Which intermolecular force is a result of polarization of electron clouds between non-polar molecules?
(A) Dipole-dipole Interaction
(B) Hydrogen Bonding
(C) London Dispersion Forces
(D) Covalent Bonding
(C) London Dispersion Forces
Londen Dispersion Forces result from the polarization of electron clouds between non-polar molecules.
If Carbon only has two 2p electrons, how does it form four bonds to make Methane (CH4)?
One of the 2s orbital electrons jumps up to one of the 2p orbitals. The three 2p orbitals then hybridize with the single 2s orbital, forming four sp3 orbitals with a lone electron in each.
What equation is used to determine the steric number in terms of the number of σ Bonds in a compound?
SN = σ + LP
SN = Steric Number σ = # of Sigma Bonds LP = # of Lone Pairs
What is the equation for dipole moment (μ)?
μ = q d
μ = Dipole Moment q = Charge d = Distance
Cl- and H+ have partial charges of -4.76⋅10^-34 C and 4.76⋅10^-34 C, respectively. they are separated by a distance of 2.21⋅10^28 m. What is the dipole moment for HCl based on these numbers?
(A) 1.05⋅10^-5
(B) 4.62⋅10^-6
(C) 3.98⋅10^-7
(D) 9.45⋅10^-8
(A) 1.05⋅10^-5
μ = q d μ = (4.76⋅10^-34 C)(2.21⋅10^28 m) μ = approx. 10⋅10^-6 (actual: 10.52⋅10^-6)
Compare the polarity of CCl4 and CHCl3. Why is the polarity the same/different?
CCl4 is non-polar due to its symetrical structure.
CHCl3 is polar due to its non-symetrical structure.
Hey, CCl3 is a non-polar molecule! I thought you said that polarity is what will stabilize the carbanion? Please explain how this works.
For this stabilizing inductive effect to occur, there must be Polar Bonds, not necessarily a polar molecule! All that matters is that the carbanion has less electron density, not where the electron density moves to or if it is evenly distributed amongst the other atoms in the molecule.
Cholesterol has an important biochemical role in our bodies. How many sp3 carbons does it contain?
(A) 15
(B) 23
(C) 25
(D) 48
(C) 25
Cholesterol contains 25 sp3 hybridized carbons.
Do not forget the two carbons from wedges and one from dashes! Only the two carbons in the double bond are not sp3 hybridized here
True or false? Because both carbocations and carbanions have an unhybridized p-orbital, they will have the same orbital geometry.
False. Carbocations have a vacant p-orbital, so they can adopt a trigonal planar orbital geometry. Carbanions have a lone pair of electrons there that will repel the other three orbitals, preventing a trigonal planar orbital geometry.
Fill in the blanks: A ____ bond can form using hybridized orbitals, whereas a ____ bond must use a non-hybridized p-orbital.
(A) σ , π
(B) π , σ
(C) σ , θ
(D) θ , π
(A) σ , π
A σ bond can form using hybridized orbitals, whereas a π bond must use a non-hybridized p-orbital.
Enantiomers are an example of which of the following? What are Enantiomers?
(A) Constitutional Isomers
(B) Conformational Isomers
(C) Stereoisomers
(D) Geometric Isomers
(C). Stereoisomers
Enantiomers are an example of Stereoisomers, which are defined as non-superimposible mirror images.
Conformational isomers can be interconverted by only rotating bonds.
Which of the following statements is not correct?
a) chiral carbons must be sp3 hybridized
b) all stereoisomers have chiral centers
c) all molecules with only a single chiral center are enantiomers
d) all enantiomers are stereoisomers
b) all stereoisomers have chiral centers
Tetrahedral centers are not the only geometry that can form chiral centers. What other geometry can be a chiral center?
a) t shaped
b) bent
c) square pyramidal
d) square planar
c) square pyramidal
a) achiral, because it is superimposable on its mirror image
b) chiral, because it is not superimposable on its mirror image
c) achiral, because it has chiral centers
d) chiral, because it has two chiral centers
a) achiral, because it is superimposable on its mirror image
a) 6 sigma, 3 pi
b) 6 sigma, 6 pi
c) 3 sigma, 6 pi
d) 12 sigma, 3 pi
d) 12 sigma, 3 pi
a) 8
b) 10
c) 20
d) 24
d) 24
Which of the following molecules’ central atom(s) has bonding electrons only in sp2 hybridized orbitals?
a) BF3
b) NH3
c) O3
d) CH2H4
a) BF3
Xenon can form several stable compounds when reacted with fluorine. What is the steric number for xenon in XeF6?
a) 4
b) 7
c) 6
d) 8
b) 7
Because steric number is related to hybridization, it can also give insight into the geometry of an atom. What does a steric number of 4 reveal about the molecular geometry surrounding that atom?
a) the bond angles are all 109.5 deg
b) the geometry is tetrahedral
c) the geometry is sometimes trigonal pyramidal
d) the geometry cannot be linear
c) the geometry is sometimes trigonal pyramidal
a) the same molecule because most of the sigma bonds can rotate freely
b) isomers because most of the sigma bonds can rotate freely
c) isomers because none of the sigma bonds can rotate freely
d) the same molecule because all the sigma bonds can rotate freely
a) the same molecule because most of the sigma bonds can rotate freely
a) carbon 3
b) carbon 1
c) hydrogen
d) carbon 5
c) hydrogen
Electrons are vital to how chemical reactions and bonding proceeds. How many unpaired electrons - based on the electron configuration - does atomic carbon have, and how many bonds can atomic carbon form?
a) 4 unpaired electrons, 4 bonds
b) 2 unpaired electrons, 2 bonds
c) 2 unpaired electrons, 4 bonds
d) 4 unpaired electrons, 2 bonds
c) 2 unpaired electrons, 4 bonds
a) sp2
b) nitrogen is unhybridized
c) sp3
d) sp
a) sp2
which interaction is primarily responsible for the tertiary structure on the inside of a protein?
a) induced dipole dipole
b) dipole dipole
c) hydrogen bonding
d) london dispersion forces
d) london dispersion forces
steric hindrance is an example of what type of interaction?
a) intramolecular
b) steric hindrance is neither an intermolecular nor an intramolecular interaction
c) steric hindrance can be both an intermolecular or an intramolecualr interaction
d) intermolecular
c) steric hindrance can be both an intermolecular or an intramolecualr interaction
extraction
- combines two immiscible liquids, one of which easily dissolves the compounds of interest
- like dissolves like –> solubility based
immiscible
two solvents that form two layers and do not mix
important in extractions
aqueous phase (layer)
water layer
dissolves compounds with hydrogen bonding or polarity
organic phase (layer)
nonpolar layer
dissolves nonpolar compounds
how is extraction carried out
- carried out in a separatory funnel
- one phase is collected, and the solvent is then evaporated
- gravitation forces cause the more dense layer to sink to bottom of funnel where it can be removed
how to increase solubility in extraction
acid base properties
when acid dissociates, anion formed will be more soluble in aqueous layer than original protonated acid because it is charge
adding a base will help extract an acid into aqueous phase
wash
reverse of extraction
small amount of solvent that dissolves impurities is run over the compound of interest
rotovap
evaporates the solvent
intermolecular forces that affect solubility
- hydrogen bonding: compounds will move most easily into aqueous layer
- ex: alcohols, acids
- dipole-dipole interactions: less likely to move into aqueous later
- van der waals forces: least likely to move into aqueous later
filtration
isolates a solid (residue) from a liquid (filtrate)
gravity filtration is used when
product of interest is in filtrate
gravity filtration
solvent’s own weight pulls it through filter
hot solvent used to maintain solubility
residue
at end of filtration, the solid left
filtrate
at end of filtration, liquid that passes through filter
vacuum filtration
solvent is forced through filter by vacuum connected to flask
vacuum filtration is used when
the product of interest is the solid
recrystallization
further purifies crystals
produce is dissolved in a minimum amount of hot solvent
when soln cools, only desired product will recrystallize out of solution, excluding impurities
what solvent should you choose for recrystallization
product is only soluble at high temperatures
what must be true about the two solvents used for an extraction to work?
immiscible
different polarity or acid-base properties that allow compound of interest to dissolve more easily in one than the other
when doing an extraction, would it be better to do 3 extractions with 10 mL of solvent or one extraction with 30 mL?
3 washes because more of the compounds of interest would be extracted
would acid dissolve better in aqueous acid or aqueous base? why?
acid dissolves better in aqueous base because it will dissorciate to form conjugate base, and being more highly charged, will become more soluble
like dissolves like only applies to polarity not acid base (opposite)
distillation
separates liquids according to differences in their BPs
liquid with lowest BP vaporizes first, and vapors will rise up the distillation column to condense in condenser
uses evaporation and condensation
distillate
liquid with lowest BP that vaporizes and is collected first in distillation
end product
simple distillation used when
why
BPs are under 150 deg C andart at least 25 deg apart
prevents the temp from becoming so high that the compound degrades
vacuum distillation used when
why
BPs are over 150 deg C to prevent degradation of product
fractional distillation used when
why
BPs are less than 25 deg C apart because it allows for more refined separation of liquids by BP
condensate
in distillation the vapor that rises up and condenses in condenser
distilling flask
contains combined liquid soln
superheating
occurs when a liquid is heated to a temp above its BP without vaporization
vacuum distillation
vacuum
vacuum lowers ambient pressure, decreasing the temp that the liquid must reach in order to have sufficient vapor pressure to boil
fractional distillation
fractional column
column in which surface area is increased by the inclusion of inert objects like glass beads or steel wool
how does fractional distillation work?
- As the vapor rises up the column, it condenses on these surfaces and refluxes back down until rising heat causes it to evaporate again, only to condense again higher in the column.
- Each time the condensate evaporates, the vapor consists of a higher proportion of the compound with the lower BP.
- By the time the top of the column is reached, only the desired product drips down to the receiving flask.
distillation separates compounds based on what property?
BP to separate solutions of miscible liquids
If we are given a solution of ether, with a boiling point of 308 K, and methylene chloride, with a boiling point of 313 K, which type of distillation should be used to separate them?
fractional distillation
If we are given a solution of bromobenzene, with a boiling point of 156°C, and camphor, with a boiling point of 204°C, which type of distillation should be used to separate them?
vacuum distillation
chromatograph
uses two phases to separate compounds based on physical or chemical properties
stationary phase
aka adsorbent
in chromatography
polar solid
mobile phase
chromatography
runs/elutes through stationary phase
usually liquid or gas
chromatography
compounds w higher affinity for stationary phase have ____ retardation factors, and take a ____ amount of time to pass through
smaller
longer
chromatography
compounds with higher affinity for mobile phase elute through more _____
quickly
thin layer and paper chromatography used to
identify a sample
thin layer and paper chromatography
stationary phase
mobile phase
stationary phase: polar material (silica, alumina, paper)
mobile phase: non polar solvent –> climbs the card through capillary action
thin layer and paper chromatography
steps
- card is spotted and developed
- solvent creeps up plate by capillary action, carrying the various compounds in sample with it at varying rates
- Rf values calculated and compared to reference values
reverse phase chromatography
uses nonpolar card with polar solvent
polar molecules move up the plate quickly, while nonpolar molecules stick more tightly to stationary phase
column chromatography uses what to separate compounds?
uses polarity, size, or affinity to separate compounds based on their physical or chemical properties
column chromatography
stationary phase
mobile phase
stationary phase: column containing silica or alumina beads
mobile phase: nonpolar solvent, which travels through column by gravity
ion exchange chromatography
method for separating sample
column is given a charge, which attracts molecules with the opposite charge
size exclusion chromatography
method of separating sample
small pores are used
smaller molecules are trapped, while larger molecules pass through the column
affinity chromatography
method of separating sample
specific receptors or antibodies can trap the target into the column
column is made to have high affinity for a compounds by coating the beads with a receptor or antibody to the compound
the target must then be washed out using other solutions
gas chromatography
separates vaporizable compounds according to how well they adhere to the adsorbent in the column
can be combined with mass spectrometry
gas chromatography
stationary phase
mobile phase
stationary phase: coil of crushed metal or a polymer
mobile phase: nonreactive gas
mass spectrometry
ionizes and fragments molecules and passes these fragments through a magnetic field to determine molecular weight or structure
high performance liquid chromatography (HPLC)
similar to column chromatography but uses sophisticated computer mediated solvent and temperature gradients
formerly called high pressure liquid chromatography
HPLC used if
used if sample size is small or if forces such as capillary action will affect results
thin layer and paper chromatography
the more nonpolar the solvent sample is, the _______ the plate it will move
further up
volatile
low melting point, sublimable solids or vaporizable liquids
retardation factor eq
Rf = distance spot moved / distant solvent front moved
what properties of molecules do thin layer, paper, and standard column chromatography take advantaged of to separate compounds?
charge and polarity
in what way is gas chromatography distinct from all of the other techniques we have discussed?
gaseous elutent instead of liquid
how to check if enants
if you can switch two groups and you get the same other molecule, they’re enants
Why use an oil bath as compared to a water bath when heating up the distilling flask during a distillation?
An oil bath is preferable because it will not evaporate and will also allow us to maintain a steady temperature throughout the distillation.
What is the purpose of the condenser in a distillation?
The condenser will cause the compound with the lowest boiling point (lower than the temperature the mixture has been heated to) to condense and collect as liquid in the receiving flask.
Why does a vacuum lower the boiling point of a substance?
A vacuum decreases the atmospheric pressure which pushes gas particles down into the liquid phase. With zero atmospheric pressure, these gas particles are more easily able to escape the liquid phase.
How does a fractionating column help you more accurately separate compounds with very close boiling points?
The fractionating column contains steel wool, beads, or another substance to which the gases can condense on. This way, the gases will have to condense and evaporate multiple times further away from the heat source in order to escape the distilling flask and make it to the condenser. This way, only the compound with the lower boiling point will be able to escape.
How could superheating adversely affect distillation?
(A) It decreases the vapor pressure of the mixture being distilled.
(B) It could allow multiple liquids to reach boiling point, leading to an impure distillate.
(C) It can prevent a distillate from forming altogether, or lead to breaking glassware.
(D) It decreases the total yield of the distillate.
B) It could allow multiple liquids to reach boiling point, leading to an impure distillate.
Superheating often occurs when gas bubbles cannot overcome atmospheric pressure and surface tension. That necessary increase in temperature then could cause multiple liquids to boil at the same time.
After not paying attention when doing the pre-lab exercises, two miscible solvents were mixed! Which of the following could be a viable way to separate them?
(A) Extraction
(B) Affinity Chromatography
(C) Distillation
(D) Recrystallization
(C) Distillation
As long as the two solvents have different boiling points, distillation could be used to remove the more volatile solvent.
If you were trying to minimize the amount of extractions needed to isolate one compound in a solvent, would you want that solvent to be on the top or bottom? Why?
To minimize the amount of extractions and pouring back into the separatory funnel, the compound to isolate should stay in the top layer. That way, you can just add the second solvent, shake, pour off that lower solvent, and repeat.
You add ether and water to the Separatory Funnel. You then add a mixture of alcohol, carboxylic acid, and an alkane to the Separatory funnel. Describe how you might isolate the carboxylic acid from the other two compounds.
Add a base to deprotonate the carboxylic acid. This way, your carboxylic acid is far more polar than the other two substances. Once you’ve opened the stopcock and your carboxylic acid is in the receiving flask, evaporate the water and there you have it!
What is the difference between paper chromatography and Thin Layer Chromatography (TLC)?
They are the same thing, except that in TLC, the stationary phase is always Silica Gel.
Is Silica Gel polar or non-polar?
polar
In Normal Phase Chromatography, what is the typical Stationary Phase that is used?
(A) Wool beads
(B) Organic solvent
(C) Silica gel
(D) Paper
They typical Stationary Phase in Normal Phase Chromatography is Silica Gel, which is polar.
Normal Phase Chromatography features a polar stationary phase and nonpolar mobile phase.
Why is it that the smaller compounds elute last in Size-Exchange Chromatography?
The Stationary Phase used in Size-Exchange Chromatography consists of beads with small holes that run through them. The smaller compounds will have to make their way through these small holes while the big compounds will go around the beads, quickly making their way through the column.
Is the Stationary Phase negatively-charged or positively-charged in Cation-Exchanged Chromatography? What about in Anion-Exchange Chromatography?
In CATION-Exchange Chromatography, the Stationary Phase is negatively charged; Thus, CATIONS will be retained in the column.
In ANION-Exchange Chromatography, the Stationary Phase is positively charged. Thus, ANIONS will be retained in the column.
In Affinity Chromatography, what would a typical Stationary Phase be composed of? Mobile Phase?
In Affinity Chromatography, a typical Stationary Phase would be composed of an enzyme. The Mobile Phase would something the substrate is even more attracted to in order to wash it out in the end.
What type of compounds will typically fluoresce using a UV lamp?
Compounds that are aromatic.
Why is it important to keep the sand/beads of a chromatography column wet at all times?
If it runs dry, it may cause mixing of bands, which would result in a lack of separation between your compounds of interest.
What would happen if your bands ended up slanted while doing a Column Chromatography?
The bands would mix and the compounds would not separate appropriately.
In Gas Chromatography, the Stationary Phase is a __________ while the Mobile Phase is a _____________.
(A) Gas, Liquid
(B) Liquid, Gas
(C) Gas, Solid
(D) Solid, Gas
(B) Liquid, Gas
In Gas Chromatography, the Stationary Phase is a Liquid while the Mobile Phase is a Gas.
Why do compounds with lower boiling points travel further in a Gas Chromatography column?
Compounds with low boiling points will vaporize more readily and thus get dragged along by the gas Mobile Phase while compounds with higher boiling points will not vaporize as easily and will thus remain dissolved in the liquid Stationary Phase.
Which will result in greater separation between bands when running Gas Chromatography?
I. Longer Coil
II. Higher Temperature
III. Higher Pressure
(A) I Only
(B) I and III Only
(C) II and III Only
(D) I, II, and III
(A) I Only
Longer coil, LOWER temperature, and LOWER pressure will result in greater separation of bands when running Gas Chromatography.
Compound A (MW = 342.8) and compound B (MW = 45.82) have the same boiling points. Which will reach the detector first in Gas Chromatography? Why?
Compound B will reach the detector first because it is smaller and thus will be pushed faster by the Mobile Phase than compound A.
Why is Gas Chromatography often coupled with other procedures such as Mass Spectrometry?
Because Gas Chromatography alone is not necessarily accurate enough in identifying exact compounds.
What procedure can be used to separate enantiomers from each other?
You do Affinity Chromatography (Column or Gas) in which the Stationary Phase contains a chiral enzyme that only binds to one of the two enantiomers.
gasoline is a volatile mixture of alkanes of varying length. which technique would be most useful for separating the components of gasoline?
a) affinity chromatography
b) normal phase HPLC
c) gas chromatography
d) reversed phase column chromatography
c) gas chromatography
a student is running gas chromatography on 2 compounds that are very similar and finds that the peaks overlap significantly in the chromatogram. what can the student do to increase the resolution between these peaks?
a) increase coil length
b) inc temp
c) use a more polar gas for the mobile phase
d) inc pressure
a) increase coil length
in normal phase column chromatography, the goal is to separate compounds of interest. which of the following mistakes would not cause the possible mixing of compounds in column chromatography?
a) allowing the silica bead to run dry
b) using a solvent that is not polar enough
c) using a solvent that is too polar
d) separating with slanted bands
b) using a solvent that is not polar enough
A student runs TLC on a sample. When put under a UV lamp, some components of the sample can be visualized because they immediately emit light. What allows many aromatic compounds to be visualized in this way?
a) aromatic compounds often react w UV light to form visible light and a new compound
b) aromatic compounds are often fluorescent
c) aromatic compounds are often phosphorescent
d) aromatic compounds often reflect UV light
b) aromatic compounds are often fluorescent
A chromatography column is packed with silica gel. What type of chromatography would this be and what would elute first from this column?
a) Normal-phase chromatography, the least polar compound would elute first.
b) Reversed-phase chromatography, the least polar compound would elute first.
c) Reversed-phase chromatography, the most polar compound would elute first.
d) Normal-phase chromatography, the least polar compound would elute first.
a) Normal-phase chromatography, the least polar compound would elute first.
Before insulin could be synthesized, it had to be purified from the pancreas of animals. What method would be best for separating insulin from a pancreas?
a) TLC
b) affinity chromatography
c) gas chromatography
d) HPLC
b) affinity chromatography
A student is using paper chromatography to compare two compounds. What determines the retention factor in paper chromatography?
a) compounds that are more attracted to the mobile phase move faster
b) compounds that are more attracted to the stationary phase move faster
c) polar compounds move farther
d) non polar compounds move farther
a) compounds that are more attracted to the mobile phase move faster
A student is trying to separate two substances with different polarities for use in further reactions. Which technique will be most useful for the student?
a) gas chromatography
b) extraction
c) filteration
d) NMR
b) extraction
