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Orgo 1 > Ch 5 - Stereoisomerism > Flashcards

Ch 5 - Stereoisomerism Flashcards

1
Q

3 phases of drug testing

A
  • phase I(20-100 people)
    - phase II(a few hundred people)
    - phase III(a few thousand people)
    - all after animal testing
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2
Q

stereoisomers

A

compounds that differ from each other only in the three dimensional spatial arrangement of their atoms but NOT in the connectivity of their atoms

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3
Q

isomer

A

compounds that are constructed from the same atoms(same molecular formulas) but still differ from each other

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4
Q

cis stereoisomer exhibits groups on

A

the same side of a double bond

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5
Q

the trans stereoisomer exhibits groups on

A

opposite sides of the double bond

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6
Q

pie bonds are formed from the overlap of

A

two p orbitals

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7
Q

when two identical groups are connected to the same position there

A

cannot be cis-trans isomerism(either way its flipped it’s the exact same)

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8
Q

superimposable

A

an object and its mirror image are the exact same

- pair of sunglasses

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9
Q

nonsuperimposable

A

an object and its mirror image are NOT the exact same

	- a pair of sunglasses missing one lens
	- a left and right hand are mirror images of each other BUT they will not fit in the same glove
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10
Q

chiral objects

A

objects which are mirror images but are not superimposable

- right and left hands

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11
Q

achiral objects

A

mirror images which are superimposable

- pair of sunglasses

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12
Q

the most common source of molecular chirality is the presence of a carbon atom bearing four different groups

A
  • they are different compounds even though all the parts are the same
    - stereoisomers because the only difference is the spatial arrangement
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13
Q

chirality center

A

a tetrahedral carbon bearing four different groups

- other common names: chiral center, stereocenter, ctereogenic center, and asymmetric center

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14
Q

sp2 hybrids can not be chirality centers

A

only 3 groups

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15
Q

enantiomer

A

when a compound is chiral it will have one nonsuperimposable mirror image call the enantiomer

	- the compound and its mirror image are a pair of enantiomers
	- each compound is said to be the enantiomer of the other
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16
Q

in most cases it is easiest to draw an enantiomer by placing the mirror

A

behind the molecule

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17
Q

3 ways to draw an enantiomer

A
  • mirror behind the molecule
    - mirror next to the molecule
    - mirror below the molecule
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18
Q

mirror behind the molecule

A
  • the skeleton of the molecule is drawn the exact same except all dashed become wedges and all wedges become dashes
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19
Q

mirror next to the molecule

A

draw the mirror image, all dashes remain dashes and all wedges remain wedges

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20
Q

mirror under the molecule

A
  • draw the mirror image, all dashes stay dashes and all wedges stay wedges
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21
Q

there are 3 ways to mirror a molecule but they still only produce

A

one enantiomer(produce the same one just at different angle of view)

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22
Q

bicyclic compounds will need to be drawn with the mirror next to the molecule or below it

A

no wedges and dashes for a mirror to be placed behind

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23
Q

Cahn-Ingold-Prelog System

A

system of nomenclature for identifying each enantiomer individually

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24
Q

5 steps for nomenclature for identifying each enantiomer individually

A
  • identify the four atoms directly attached to the chirality center
    - assign a priority to each atom based on its atomic number. The highest atomic number receives priority 1 and the lowest atomic number(often a Hydrogen atom) receives priority 4
    - if two atoms have the same atomic number, more away from the chirality center looking for the first point of difference. When constructing lists to compare remember that a double bond is treated as two separate single bonds
    - rotate the molecule so that the fourth priority is on a dash(going behind the plane of the page)
    - determine whether the sequence is 1-2-3 follows a clockwise(R) order or a counterclockwise(S)
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25
Q

enantiomers exhibit identical physical properties

A

same melting and boiling points etc

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26
Q

enantiomers do exhibit different behavior when exposed to

A

plane-polarized light

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27
Q

light is an electric and

A

magnetic perpendicular set of planar waves

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28
Q

polarization

A

the orientation of the electric field(shown in red) for a light wave

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29
Q

plane-polarized light

A

light passing through a polarizing filter allows only photons of a particular polarization to pass through the filter

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30
Q

optically active

A

certain organic compounds rotate the plane of plane polarized light

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31
Q

optically inactive

A

organic compounds which cannot rotate the plane polarized light

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32
Q

polarimeter

A

device which measures the rotation of plane polarized light caused by optically active compounds

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33
Q

optical activity is a direct consequence of chirality

A

chiral compounds are optically active while achiral compounds are not

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34
Q

Enantiomers(nonsuperimposable mirror images) will

A

rotate the plane polarized light in equal amounts but in opposite directions

35
Q

observed rotation(symbolized by alpha)

A

when a solution of a chiral compound is placed in a polarimeter the observed rotation will be dependent on the number of molecules that the light encounters as it travels through the solution

36
Q

if the concentration of the solution is doubles then

A

the observed rotation will double

37
Q

if the pathlength is doubled then

A

the observed rotation is doubled

38
Q

specific rotation

A

the observed rotation under standard conditions

	- concentration = 1g/mL
	- pathlength = 1dm
39
Q

Specific rotation =

A

[alpha(symbolized observed rotation] = (alpha)/(c)(l)

	- [alpha] = specific rotation
	- alpha = the observed rotation
	- c = concentration(g/mL)
	- l = pathlength(1dm = 10cm)
	- allows the calculation of nonstandard conditions
40
Q

the specific rotation of a compound is a physical constant much like

A

melting and boiling points

41
Q

specific rotation is temperature and wavelength but these factors can not be incorporated into the equation

A
  • these two factors are reported as follow:
    - [alpha]^T wavelength(upsidedown Y)
    - T = Celsius
    - Y is the wavelength of the light used
42
Q

specific rotations for enantiomers are equal in magnitude but

A

opposite in direction

43
Q

dextrorotatory(d)

A

a compound exhibiting a positive rotation(+)

44
Q

levorotatory(l)

A

a compound exhibiting a negative rotation(-)

45
Q

a chirality center can be R/S indefinitely but the optical activity can

A

change from +/- as other factors change

46
Q

optically pure(enantiomerically pure)

A

a solution containing a single enantiomer

47
Q

racemic mixture

A

solution containing equal amounts of both enantiomers

	- will be optically inactive
	- the net rotation will be zero(cancels out)
		- the individual compounds are active the net effect is not
48
Q

a solution containing both enantiomers in unequal amounts will be optically active

A 
enantiomeric excess(ee) – the difference between the % of enantiomer in solution(70%-30% = 40% excess
			-  %ee = ((absolute value alpha)/(alpha of pure enantiomer))(100%)
49
Q

enantiomeric excess(ee)

A

the difference between the % of enantiomer in solution(70%-30% = 40% excess
- %ee = ((absolute value alpha)/(alpha of pure enantiomer))(100%)

50
Q

enantiomers are stereoisomers that are

A

mirror images of one another

51
Q

diastereomers are stereoisomers that are not

A

mirror images of one another

52
Q

cis-trans are diastereomers

A

they are not mirror images of each other

53
Q

enantiomers have the same physical properties while diastereomers have

A

different physical properties

54
Q

maximum number of stereoisomers =

A

2^n

55
Q

any compound with a single chirality center must be

A

chiral

56
Q

with two chirality centers

A
  • trans isomer is chiral

- cis isomer is not chiral

57
Q

two types of symmetry

A
  • rotational

- reflectional

58
Q

trans isomer exhibit rotational symmetry

A

think an axis of a spinning top which can spin

59
Q

axis of symmetry

A

the imaginary stick which a molecule can be rotated around for rotational symmetry

60
Q

reflectional symmetry splits a molecule in half with each half being

A

the same as the other half

61
Q

plane of symmetry

A

split a molecule in half and see if reflected which creates reflectional symmetry

62
Q

chirality is only dependent on the presence or absence of

A

reflectional symmetry

63
Q

any compound that possesses a plane of symmetry in any conformation will be achiral

A

the absence of a plane of symmetry DOES NOT necessarily mean the compound is chiral

64
Q

plane of symmetry is only one type of reflectional symmetry

A

the presence of any kind of reflectional symmetry renders the compound achiral

65
Q

inversion

A

reflection about a point(rather than a plane)

	- still reflection symmetry
		- renders compound achiral
66
Q

the presence or absence of rotational symmetry is

A

irrelevant to chirality

67
Q

a compound that has a plane of symmetry will be

A

achiral

68
Q

a compound that lacks a plane of symmetry will most likely be chiral

A

there are rare exceptions, which can mostly be ignored

69
Q

meso compounds

A

a compound that exhibits reflectional symmetry will be achiral even though it has chirality centers

70
Q

meso compounds is a family of stereoisomers containing fewer than 2^n stereoisomers

A

if a structure has 2 chirality centers we expect 2^2 = 4 however if one is meso there would be a total of 3(a pair of enantiomers + the 3rd with reflectional symmetry which makes 3 instead of 4)

71
Q

fischer projections

A

for compounds with multiple chirality centers these drawings are quick

	- the horizontal lines are considered to be coming out of the page
	- the vertical lines are considered to be going behind the page
72
Q

fischer projections are primarily used for

A

analyzing sugars

73
Q

fischer projections are helpful in comparing the relationship between stereoisomers

A
  • if a componds 2 images have mirrored groups then its an enantiomer
    - all chirality centers have opposite configurations
74
Q

if a compound has 2 groups trans in one and 2 groups cis in the other then

A

it’s a diastereomer

75
Q

butane can adopt two staggered conformations with gauche interactions

A
  • nonsuperimposable and therefore enantiomeric
    - however butane is not chiral compound and optically inactive
    - the two conformations are constantly interconverting via single bond rotation
76
Q

a chirality center cannot invert configuration via single bond rotations

A

(R)-2-Butanol cannot be converted into (S)-2-Butanol via a conformational change

77
Q

if a plane of symmetry is apparent then the compound is

A

not optically active

78
Q

enantiomers have the same

A

physical properties

79
Q

resolution

A

the separation of enantiomers

80
Q

Resolution

Crystallization

A
  • Pasteur allowed tartrate salts to crystalize in 1847 and noticed the enantiomers created a mix of two shaped crystals
    - he sorted them by hand with a pair of tweezers
    - then dissolved each pile in water and placed the solutions in a polarimeter
    - discovered the specific rotations were equal in amount but opposite in sign
    - concluded the molecules must be nonsuperimposable mirror images of each other
    - most racemic mixtures are not easily resolved via crystallization
81
Q

Resolution

Chiral Resolving Agents

A

when a racemic mixture is treated with a single enantiomer of another compound, the resulting reaction produces a pair of distereomers(as opposed to enantiomers)

  • diastereomers have different physical properties and can then be separated by conventional means(like crystallization)
82
Q

Resolution

Chiral Resolving Agents

resolving agent

A

often acids which can split an enantiomer into its diastereomers

83
Q

Resolution

Chiral Column Chromatography

A
  • column chromatography – compounds which are enantiomers are separated from each other based on a different way they interact with the medium(the absorbent) the which they are passed
    - a traditional column they travel at the same rate because their properties are the same
    - if a chiral absorbent is used the enantiomers interact with the absorbent differently, causing them to travel through the column at different rates
    - thus separating the enantiomer

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