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MCAT > Chem II: 3-5 > Flashcards

Chem II: 3-5 Flashcards

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1
Q

isomers

A

same molecular formula

different chemical structure

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2
Q

structural isomers

A

aka constitutional isomers

same molecular formula –> molecular weights are the same

different physical and chemical properties

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3
Q
A
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4
Q

physical properties

A

characteristics of processes that do not change the composition of matter

ex: MP, BP, solubility, odor, color, density

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5
Q

chemical properties

A

reactivity of molecule –> result in changes in chemical composition

generally attributable to functional groups in the molecule

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6
Q

stereoisomers

A

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

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7
Q

conformational isomers

A

type of stereoisomer

differ in rotation around single bonds

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8
Q

configurational isomers

A

type of stereoisomer

can be interconverted only by breaking bonds

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9
Q

newman projection

A

molecular is visualized along extending through C-C bond

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10
Q

staggered conformation

A

newman projection

groups are 60 deg apart

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11
Q

eclipsed conformation

A

newman projection

groups are 120 deg apart and directly overlap with atoms on adjacent carbon

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12
Q

anti staggered

A

two largest groups are 180 deg apart and strain is minimized

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13
Q

gauche staggered

A

two largest groups are 60 deg apart

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14
Q

totally eclipsed conformation

A

two largest groups are directly in front of each other and strain is maximized

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15
Q

the higher the energy, the ____ time the molecule will spend in the energetically unfavorable state

A

less

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16
Q

are conformation interconversion barriers easy to overcome? room temp? high/low temp?

A

yes at room temp bc they are small

at very low temp, interconversions are dramatically slow –> may not rotate at all

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17
Q

ring strain

A

angle strain, torsional strain, nonbonded strain

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18
Q

angle strain

A

results when bond angles deviate from their ideal values by being stretched or compressed

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19
Q

torsional strain

A

results when cyclic molecules must assume conformations that have eclipsed or gauche interactions

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20
Q

nonbonded strain

A

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
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21
Q

which main conformations does cyclohexane mainly exist in?

A

chair, boat, and twist boat forms

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22
Q

most stable conformation of cyclohexane

A

chair conformation

minimizes all 3 types of strain

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23
Q

axial

A

chair conformation

hydrogen atoms that are perpendicular to the plane of the ring (sticking up or down)

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24
Q

equatorial

A

chair conformation

hydrogen atoms that are parallel (sticking out) of the ring

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25
Q

chair flip

A

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
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26
Q

how can chair flip be slowed

A

if a bulky group is attached to the ring

ex: tert-butyl groups

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27
Q

for substituted rings, the bulkiest group will favor the _____ position to reduce nonbonded strain

A

equatorial

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28
Q

in rings with more than one substituent, the preferred chair form is determined by the _____ group, which will prefer the ______ position

A

larger

equatorial

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29
Q

cis

A

when both groups are located on the same side of the ring

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30
Q

trans

A

when both groups are on opposite sides of the ring

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31
Q

configurational isomers

A

can only change from one form to another by breaking and reforming covalent bonds

two types: diastereomers and enantiomers

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32
Q

optical isomers

A

different spatial arrangement of groups affects the rotation of plane-polarized light

ex: enantiomers and diasteromer

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33
Q

chiral

A

mirror image cannot be superimposed on the og object

molecule lacks internal plane of symmetry

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34
Q

achiral

A

have mirror images that can be superimposed

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35
Q

chiral center

A

lack a plane of symmetry

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36
Q

whenever you see a carbon with 4 different substituents, think ______

A

chirality

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37
Q

enantiomers

A

configurational isomers

non-superimposable mirror images

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38
Q

diastereomers

A

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

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39
Q

enantiomers

physical and chemical properties

A

have identical physical and chemical properties except for:

optical activity and rxns in chiral environments

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40
Q

a compound is optically active if

A

it has the ability to rotate plane polarized light

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41
Q

polarizer

A

allows light waves oscillating only in a particular direction to pass through, producing plane polarized light

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42
Q

optical activity

A

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

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43
Q

dextrortatory (d-)

A

compound that rotates the plane of polarized light to the right

labeled (+)

must be determined experimentally

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44
Q

levorotatory (l-)

A

compound that rotates the plane of polarized light to the left

labeled (-)

must be determined experimentally

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45
Q

optical activity

amount of rotation depends on

A

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

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46
Q

specific rotation eq

A
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47
Q

racemic mixture

A

when both (+) and (-) enantiomers are present in equal concentrations

rotations cancel each other out, and no optical activity is observed

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48
Q

for any molecule with n chiral centers, there are ____ possible stereoisomers

A

2n

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49
Q

diastereomers

chemical and physical properties

A
  • 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
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50
Q

cis-trans isomers

A

aka geometric isomers

subtype of diastereomers

substituents differ in their position around an immovable bond (double bond, ring structure)

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51
Q

meso compounds

A

molecular with chiral centers that has an internal plane of symmetry

two halves that are mirror images

optically inactive

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52
Q

configuration of a stereoisomer

A

spatial arrangement of the atoms or groups in the molecule

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53
Q

conformation of a chiral molecule

A

describes the exact spatial arrangement of these atoms or groups, independent of other molecules

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54
Q

E/Z nomenclature is used for

A

compounds with polysubstituted double bonds

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55
Q

Cahn Ingold Prelog priority rules

A

the higher the atomic number, the higher the priority

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56
Q

determine E/Z designation

A

cahn priority rules

higher atomic number, higher priority

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57
Q

Z

A

if two highest priority substituents on each carbon are on the same side of the double bond

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58
Q

E

A

if two highest priority substituents on each carbon are on the opposite side of the double bond

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59
Q

R/S nomenclature is used for

A

chiral (sterogenic) centers

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60
Q

assigning R/S

steps

A
  1. assign priority
    1. higher atomic number takes priority
  2. arrange in space
    1. orient the molecule so the atom w lowest priority is at the back of the molecule
  3. draw a circle from highest -> lowest priority
    1. S - counterclockwise
    2. R - clockwise
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61
Q

fischer projection

A

horizontal lines - bonds that project out from the page (wedges)

vertical lines - bonds going into the plane of the page (dashes)

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62
Q

assign R/S for fischer projection

A
  1. determine priority
  2. obtain R/S
  3. true designation is the opposite of what you obtained
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63
Q

R or S for the carbon labeled 1?

A

Neither. There are only 3 unique bonds.

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64
Q

R or S for the carbon labeled 2?

A

Neither. There are only 3 unique bonds (the 2 terminal methyls are equivalent).

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65
Q

What happens to plane-polarized light as it passes through a polarimeter tube filled with an optically active substance?

A

The plane-polarized light will rotate.

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66
Q

What is the difference between enantiomers and diasteriomers?

A

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.

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67
Q

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

A

(B) Epimer

An epimer is a type of diastereomer where the chirality only differs at one chirality center.

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68
Q

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

A

(C) Anomer

An anomer is a type of epimer that forms depending on how a ringed structure forms.

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69
Q

What is the difference between Constitutional Isomers and Conformational Isomers?

A

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.

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70
Q

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.

A

(B) The neighboring substituents on the close and distant carbon of a Newman Projection.

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71
Q

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
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72
Q

a) 0 deg
b) 180
c) 60
d) 120

A

b) 180

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73
Q
A

b

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74
Q

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

A

b) R and S

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75
Q

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

A

c) shine plane polarized light through the same

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76
Q

a) R and S only
b) S only
c) R only
d) R, S, Z

A

d) R, S, Z

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77
Q

the smaller the principal quantum number, the _____ the shell is to the nucleus, and the _____ its energy

A

closer

lower

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78
Q

as the azimuthal quantum number increases, energy _____

A

increases

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79
Q

p orbital is composed of

A

two lobes and a node

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80
Q

node

A

an area where the probability of finding an electron is 0

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81
Q

d orbital is composed of

A

4 symmetrical lobes with 2 nodes

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82
Q

principal quantum number describes…

A

size

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83
Q

azimuthal quantum number describes…

A

shape

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84
Q

magnetic quantum number describes…

A

orientation

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85
Q

principal quantum number

orientation level

A

shell

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86
Q

azimuthal quantum number

orientation level

A

subshell

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87
Q

magnetic quantum number

orientation level

A

orbital

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88
Q

principal quantum number

possible values

A

1 to infinity

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89
Q

azimuthal quantum number

possible values

A

0 to n-1

90
Q

magnetic quantum number

possible values

A

-l to +l

91
Q

when two atomic orbitals combine, they form ____

A

molecular orbitals

92
Q

if signs of wave functions are the same, a ____ orbital is produced

A

bonding

93
Q

if signs of wave functions are different, a ____ orbital is produced

A

antibonding

94
Q

single bonds are made of ___ bonds

A

sigma

95
Q

double bond is made of

A

pi bond and existing sigma bond

96
Q

triple bond is made of

A

sigma bond and 2 pi bonds

97
Q

can pi bond exist without a sigma bond

A

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

98
Q

what differences would be observed in a molecule containing a double bond compared to the same molecule containing only single bonds?

A
  • bond length - shorter in double bond
  • bond energy - higher in double bond
  • molecular rigidity - higher in double bond
99
Q

rank the following orbitals in decreasing order of strength: sigma bond, pi bond, double bond, triple bond

A

triple bond > double bond > sigma bond > pi bond

100
Q

sp3 hybridized orbitals

A

1 s, 3 p

tetrahedral geometry

no unhybridized p orbitals to form pi bonds

ex: carbon

101
Q

sp2 hybridized orbitals

A

1 s, 2 p

trigonal planar geometry

third p orbital is unhybridized -> can participate in pi bond

ex: alkenes

102
Q

sp hybridized orbitals

A

1 s, 1 p

linear geometry - 180 deg

2 unhybridized p orbitals -> for pi bonds

103
Q

what are resonance structures?

A

resonance structures differ in their placement of electrons in hybridized p orbital and require bond conjugation to delocalize electrons in a molecule.

104
Q

how does the true electron density of a compound relate to its resonance structures?

A

true electron density is a weighted avg of the resonance structures of a given compound, favoring the most stable structures

105
Q

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

A

(C) London Dispersion Forces

Londen Dispersion Forces result from the polarization of electron clouds between non-polar molecules.

106
Q

If Carbon only has two 2p electrons, how does it form four bonds to make Methane (CH4)?

A

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.

107
Q

What equation is used to determine the steric number in terms of the number of σ Bonds in a compound?

A

SN = σ + LP

SN = Steric Number
σ = # of Sigma Bonds
LP = # of Lone Pairs
108
Q

What is the equation for dipole moment (μ)?

A

μ = q d

μ = Dipole Moment
q = Charge
d = Distance
109
Q

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

(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)
110
Q

Compare the polarity of CCl4 and CHCl3. Why is the polarity the same/different?

A

CCl4 is non-polar due to its symetrical structure.

CHCl3 is polar due to its non-symetrical structure.

111
Q

Hey, CCl3 is a non-polar molecule! I thought you said that polarity is what will stabilize the carbanion? Please explain how this works.

A

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.

112
Q

Cholesterol has an important biochemical role in our bodies. How many sp3 carbons does it contain?

(A) 15
(B) 23
(C) 25
(D) 48

A

(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

113
Q

True or false? Because both carbocations and carbanions have an unhybridized p-orbital, they will have the same orbital geometry.

A

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.

114
Q

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

(A) σ , π

A σ bond can form using hybridized orbitals, whereas a π bond must use a non-hybridized p-orbital.

115
Q

Enantiomers are an example of which of the following? What are Enantiomers?

(A) Constitutional Isomers
(B) Conformational Isomers
(C) Stereoisomers
(D) Geometric Isomers

A

(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.

116
Q

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

A

b) all stereoisomers have chiral centers

117
Q

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

A

c) square pyramidal

118
Q

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

a) achiral, because it is superimposable on its mirror image

119
Q

a) 6 sigma, 3 pi
b) 6 sigma, 6 pi
c) 3 sigma, 6 pi
d) 12 sigma, 3 pi

A

d) 12 sigma, 3 pi

120
Q

a) 8
b) 10
c) 20
d) 24

A

d) 24

121
Q

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

a) BF3

122
Q

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

A

b) 7

123
Q

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

A

c) the geometry is sometimes trigonal pyramidal

124
Q

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

a) the same molecule because most of the sigma bonds can rotate freely

125
Q

a) carbon 3
b) carbon 1
c) hydrogen
d) carbon 5

A

c) hydrogen

126
Q

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

A

c) 2 unpaired electrons, 4 bonds

127
Q

a) sp2
b) nitrogen is unhybridized
c) sp3
d) sp

A

a) sp2

128
Q

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

A

d) london dispersion forces

129
Q

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

A

c) steric hindrance can be both an intermolecular or an intramolecualr interaction

130
Q

extraction

A
  • combines two immiscible liquids, one of which easily dissolves the compounds of interest
  • like dissolves like –> solubility based
131
Q

immiscible

A

two solvents that form two layers and do not mix

important in extractions

132
Q

aqueous phase (layer)

A

water layer

dissolves compounds with hydrogen bonding or polarity

133
Q

organic phase (layer)

A

nonpolar layer

dissolves nonpolar compounds

134
Q

how is extraction carried out

A
  • 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
135
Q

how to increase solubility in extraction

A

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

136
Q

wash

A

reverse of extraction

small amount of solvent that dissolves impurities is run over the compound of interest

137
Q

rotovap

A

evaporates the solvent

138
Q

intermolecular forces that affect solubility

A
  • 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
139
Q

filtration

A

isolates a solid (residue) from a liquid (filtrate)

140
Q

gravity filtration is used when

A

product of interest is in filtrate

141
Q

gravity filtration

A

solvent’s own weight pulls it through filter

hot solvent used to maintain solubility

142
Q

residue

A

at end of filtration, the solid left

143
Q

filtrate

A

at end of filtration, liquid that passes through filter

144
Q

vacuum filtration

A

solvent is forced through filter by vacuum connected to flask

145
Q

vacuum filtration is used when

A

the product of interest is the solid

146
Q

recrystallization

A

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

147
Q

what solvent should you choose for recrystallization

A

product is only soluble at high temperatures

148
Q

what must be true about the two solvents used for an extraction to work?

A

immiscible

different polarity or acid-base properties that allow compound of interest to dissolve more easily in one than the other

149
Q

when doing an extraction, would it be better to do 3 extractions with 10 mL of solvent or one extraction with 30 mL?

A

3 washes because more of the compounds of interest would be extracted

150
Q

would acid dissolve better in aqueous acid or aqueous base? why?

A

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)

151
Q

distillation

A

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

152
Q

distillate

A

liquid with lowest BP that vaporizes and is collected first in distillation

end product

153
Q

simple distillation used when

why

A

BPs are under 150 deg C andart at least 25 deg apart

prevents the temp from becoming so high that the compound degrades

154
Q

vacuum distillation used when

why

A

BPs are over 150 deg C to prevent degradation of product

155
Q

fractional distillation used when

why

A

BPs are less than 25 deg C apart because it allows for more refined separation of liquids by BP

156
Q

condensate

A

in distillation the vapor that rises up and condenses in condenser

157
Q

distilling flask

A

contains combined liquid soln

158
Q

superheating

A

occurs when a liquid is heated to a temp above its BP without vaporization

159
Q

vacuum distillation

vacuum

A

vacuum lowers ambient pressure, decreasing the temp that the liquid must reach in order to have sufficient vapor pressure to boil

160
Q

fractional distillation

fractional column

A

column in which surface area is increased by the inclusion of inert objects like glass beads or steel wool

161
Q

how does fractional distillation work?

A
  • 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.
162
Q

distillation separates compounds based on what property?

A

BP to separate solutions of miscible liquids

163
Q

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?

A

fractional distillation

164
Q

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?

A

vacuum distillation

165
Q

chromatograph

A

uses two phases to separate compounds based on physical or chemical properties

166
Q

stationary phase

A

aka adsorbent

in chromatography

polar solid

167
Q

mobile phase

A

chromatography

runs/elutes through stationary phase

usually liquid or gas

168
Q

chromatography

compounds w higher affinity for stationary phase have ____ retardation factors, and take a ____ amount of time to pass through

A

smaller

longer

169
Q

chromatography

compounds with higher affinity for mobile phase elute through more _____

A

quickly

170
Q

thin layer and paper chromatography used to

A

identify a sample

171
Q

thin layer and paper chromatography

stationary phase

mobile phase

A

stationary phase: polar material (silica, alumina, paper)

mobile phase: non polar solvent –> climbs the card through capillary action

172
Q

thin layer and paper chromatography

steps

A
  • 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
173
Q

reverse phase chromatography

A

uses nonpolar card with polar solvent

polar molecules move up the plate quickly, while nonpolar molecules stick more tightly to stationary phase

174
Q

column chromatography uses what to separate compounds?

A

uses polarity, size, or affinity to separate compounds based on their physical or chemical properties

175
Q

column chromatography

stationary phase

mobile phase

A

stationary phase: column containing silica or alumina beads

mobile phase: nonpolar solvent, which travels through column by gravity

176
Q

ion exchange chromatography

method for separating sample

A

column is given a charge, which attracts molecules with the opposite charge

177
Q

size exclusion chromatography

method of separating sample

A

small pores are used

smaller molecules are trapped, while larger molecules pass through the column

178
Q

affinity chromatography

method of separating sample

A

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

179
Q

gas chromatography

A

separates vaporizable compounds according to how well they adhere to the adsorbent in the column

can be combined with mass spectrometry

180
Q

gas chromatography

stationary phase

mobile phase

A

stationary phase: coil of crushed metal or a polymer

mobile phase: nonreactive gas

181
Q

mass spectrometry

A

ionizes and fragments molecules and passes these fragments through a magnetic field to determine molecular weight or structure

182
Q

high performance liquid chromatography (HPLC)

A

similar to column chromatography but uses sophisticated computer mediated solvent and temperature gradients

formerly called high pressure liquid chromatography

183
Q

HPLC used if

A

used if sample size is small or if forces such as capillary action will affect results

184
Q

thin layer and paper chromatography

the more nonpolar the solvent sample is, the _______ the plate it will move

A

further up

185
Q

volatile

A

low melting point, sublimable solids or vaporizable liquids

186
Q

retardation factor eq

A

Rf = distance spot moved / distant solvent front moved

187
Q

what properties of molecules do thin layer, paper, and standard column chromatography take advantaged of to separate compounds?

A

charge and polarity

188
Q

in what way is gas chromatography distinct from all of the other techniques we have discussed?

A

gaseous elutent instead of liquid

189
Q

how to check if enants

A

if you can switch two groups and you get the same other molecule, they’re enants

190
Q

Why use an oil bath as compared to a water bath when heating up the distilling flask during a distillation?

A

An oil bath is preferable because it will not evaporate and will also allow us to maintain a steady temperature throughout the distillation.

191
Q

What is the purpose of the condenser in a distillation?

A

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.

192
Q

Why does a vacuum lower the boiling point of a substance?

A

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.

193
Q

How does a fractionating column help you more accurately separate compounds with very close boiling points?

A

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.

194
Q

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.

A

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.

195
Q

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

A

(C) Distillation

As long as the two solvents have different boiling points, distillation could be used to remove the more volatile solvent.

196
Q

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?

A

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.

197
Q

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.

A

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!

198
Q

What is the difference between paper chromatography and Thin Layer Chromatography (TLC)?

A

They are the same thing, except that in TLC, the stationary phase is always Silica Gel.

199
Q

Is Silica Gel polar or non-polar?

A

polar

200
Q

In Normal Phase Chromatography, what is the typical Stationary Phase that is used?

(A) Wool beads
(B) Organic solvent
(C) Silica gel
(D) Paper

A

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.

201
Q

Why is it that the smaller compounds elute last in Size-Exchange Chromatography?

A

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.

202
Q

Is the Stationary Phase negatively-charged or positively-charged in Cation-Exchanged Chromatography? What about in Anion-Exchange Chromatography?

A

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.

203
Q

In Affinity Chromatography, what would a typical Stationary Phase be composed of? Mobile Phase?

A

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.

204
Q

What type of compounds will typically fluoresce using a UV lamp?

A

Compounds that are aromatic.

205
Q

Why is it important to keep the sand/beads of a chromatography column wet at all times?

A

If it runs dry, it may cause mixing of bands, which would result in a lack of separation between your compounds of interest.

206
Q

What would happen if your bands ended up slanted while doing a Column Chromatography?

A

The bands would mix and the compounds would not separate appropriately.

207
Q

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

A

(B) Liquid, Gas

In Gas Chromatography, the Stationary Phase is a Liquid while the Mobile Phase is a Gas.

208
Q

Why do compounds with lower boiling points travel further in a Gas Chromatography column?

A

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.

209
Q

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

(A) I Only

Longer coil, LOWER temperature, and LOWER pressure will result in greater separation of bands when running Gas Chromatography.

210
Q

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?

A

Compound B will reach the detector first because it is smaller and thus will be pushed faster by the Mobile Phase than compound A.

211
Q

Why is Gas Chromatography often coupled with other procedures such as Mass Spectrometry?

A

Because Gas Chromatography alone is not necessarily accurate enough in identifying exact compounds.

212
Q

What procedure can be used to separate enantiomers from each other?

A

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.

213
Q

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

A

c) gas chromatography

214
Q

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

a) increase coil length

215
Q

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

A

b) using a solvent that is not polar enough

216
Q

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

A

b) aromatic compounds are often fluorescent

217
Q

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

a) Normal-phase chromatography, the least polar compound would elute first.

218
Q

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

A

b) affinity chromatography

219
Q

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

a) compounds that are more attracted to the mobile phase move faster

220
Q

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

A

b) extraction

MCAT (26 decks)

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