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PChem > Stereochemsitry > Flashcards

Stereochemsitry Flashcards

1
Q

Isomerism

A

Relates to structural differences between different molecules

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2
Q

Conformation

A

Different arrangements of the atoms of a SINGLE MOLECULE
- Happen as a result of rotation around single bonds

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3
Q

Configuration and Conformation

A

Configuration: rearrangment of bond/atoms in a molecule
Conformation: change of postiton of atom (they rotate or something)
- Taka a cat for example, it can strike poses and look cute all day by changing it’s conformation but if you swape a limb or something then you changed it’s configuration

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4
Q

Isomers

A

Have the same molecular formula but different structures
- Constitutional Isomers
- Stereoisomers

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5
Q

Constitutional Isomers

A

Two or more compounds that have the some constitution but differ in bond connectivity (differ in order of attachment of atoms)

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6
Q

Steroisomer

A

Same constitution (formula) and connectivity but differ in spatial arrangement

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7
Q

Olefin Isomer Nomenclature

A

Designated as Z (zusammen-together) when
- The double bond geometry in which the groups at each end of the bond have higher priority on the same side

Designated as E (entgegen-apart) when
- The double bond geometry in which the groups at each end of the bond have higher priority on the opposite side

Use E/Z only if there 3 or more unique substituents, other wise use cis (same side)/trans (opposite sides)

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8
Q

Cahn-Ingold Prelog (CIP) system

A

Priorities are assigned based on atomic numbers using
- The atom directly attached to the sterocenter that has the higher atomic number is assigned the higher priority
- For two identical atoms, the atomic number of the atoms attached to each determine priority
– If one is attached to ANY atom that has a higher atomic number than ANY of the atoms attached to the other one then it has higher priority
– Move away from sterochemisty in the direction that will first give a difference in priorities

Carbonyl (C=O) is higher priority than phenyl
- Because it is next to two oxygen which got higher atomic numbers than the 3 carbons

Phenyl (Ph) is higher priority thatn C=C bond
- The Ph is 3 carbon while the double bond is only 2 carbons

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9
Q

CIP System

Additional rules

A

When moving to find an atom that gives rise to difference in priorities
- Double bonds count as two single bonds
- Triple bonds count as three single bonds

A phenyl group is counted as if the C is attached to C,C and C (this is because the C is attached to a double bond (sp2) site and a single bond on the other side)
- Sp2 (aromatic)>Sp2 (alkene)
- for substituted phenyl rings ortho>meta>para (first point of difference, ortho is closer, followed by meta, and para)

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10
Q

CIP System

Isotopes, lone pair

A

Higher isotopes (more e-) have priority over lower isotope (stick to atomic mass order)
- T>D>H or (3)H>(2)H>H; (13)C>(12)C

A lone pair is the only group with a priority lower than H (almost no atomic mass)
- Do not assign E/Z to double bonds in aromatic rings smaller than 8-membered

I>Br>Cl

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11
Q

Enantiomer

Chirality

A

Two steroisomers that result from the reflection of the compound on the left in a missor
- Not superimposed on one another
- Becomes possible when compound contain a center of asymmetry; chiral center
– Chiral center has a central atom that is attached to 4 different group (thus lacking symmetry)
- Have identical chemical and physical properties as long as they remain in achiral environement
- They will be indentical with respect to:
– Melting point, Boiling point
– UV/vis, IR, NMR, mass spectra
– Chromatographic retention time
– Rate of reaction with achiral reagent

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12
Q

Why is Enantimoers optically active?

A

They are distinguished from one another by their ability to rotate the plane of polarized light when it passed through a solution of an enantiomer
- Optical rotation is measured in a POLARIMETER and is expressed as [alpha] which is called specific rotation
- Two enantiomers will rotate polarized light with equal magnitude but with opposite signs (ie, if one has [alpha] =+14.5 the other one got an [alpha] = -14.5°)

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13
Q

What is the optic rotation of a mixture of Equal amount of two enantimoer?

A

The [alpha] =0°, because their rotation will cancel each other out
- Such a mixture is called Racemic Mixture and is indicated using (±)- or (dl)- in the name
-

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14
Q

Non-racemic mixture

A

A non equal mixture of two enantiomers will have a non zero value for [alpha]
- It will be less than that of a pure enantiomer

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15
Q

Chiral Centers

A

May occur at atoms other than C
- Any configurationally stable tetrahedral atom can be a chiral center if attached to four different groups
– An example is seen with sulfur in the form of sulfoxides (Esomeprazole, Prilosec; for acid-reflux disease)
— If S is bonded with two different groups, the O and lone pair make up the four groups
- Omeprazole is the racemate

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16
Q

Chiral Center: Amines

A

Amine are NOT chrial center because they are NOT configurationally stable (otherwise an amine that has three different R goups and a lone pair attached to N would be a chiral center)
- At room temp they undergo rapid inversion of configuration that equilibrates the isomer => NOT optically active (Nitrogen never is)

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17
Q

What is the importance of steroisomerism to drugs?

A
  • For some drugs one enantiomer contain most or all of the activity
  • Administration of a racemic mixture of such drug only provide patients with 50% of an effective dose
  • In other instance two enantiomers can have entirely different medicicnal activities

Detromethorphan:Levomethorphan (enantiomers)
Non-addicting, antitussive, no analgestic potency, sold as OTC: Addicting, poten analgesic, Schedule I drug

Thalidomide
- (S) - toxic
- (R) - active, calming

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18
Q

R or S for Enantiomer

A

1) Assign priorities to all groups
2) Redraw so the lowest priority point back into the paper
- It is (R) if the other three groups decrease in priority in a clockwise direction
- It is (S) if the other three groups decrease in priority in a counterclockwise direction

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19
Q

What happens when the lowest priority group is oriented between you and the chiral center?

A

If it is between you and the chiral center (not dashed)
- Make use of opposite assignment
– R if group decrease counterclock wise
– S if group decrease clock wise

20
Q

What happens if the lowest priority group lie IN the plane of the paper

A

In this case you have two choices
- Imagine that you are in the plane of the paper and apply either of the first two methods above
- Alternatively, assigning priortities => mentally switch the lowest priority group with whatever group lies behind the plane (dashed), => make opposite assignment that you would normally make

21
Q

Fisher Projection

A
  • Groups on the horizontal line come out of the plane (bolded lines)
  • Groups on the vertical line go behind the plane (dashed lines)
  • Where the line cross is a carbon

Used extensively in carbohydrate and amino acid chemistry

22
Q

How to apply R- and S- designation to Fisher Projection?

A

Assign Priorities
- If the lowest priority group lies on the vertical line (top or bottom; behind the plane; dashed lines); observe the decreasing order of the 3 other groups and make NORMAL (CW=R, CCW=S) assignment
- If the lowest priority group lies on the horizontal line (right or left; come out of the plane; bolded lines); observe the decresing order of the 3 other group and make opposite assignment

23
Q

What do Fisher projection make easy?

Skip

A

It makes it easy to make comparison between two identically substituted chiral centers
- If the projection is rotated in either direction by 90° then the two structure are enantiomers

Skip

24
Q

What is the older method for specifying the absolute configuration?

A

It relies on the comparison with a reference compound
- The reference chosen is glyceraldehyde as it has only one chrial center
– The enantiomer that has a (+) optical rotation is designated as D (last of OH on the right)
– The enatiomer that has a (-) optical rotation is designated as L (last of OH on the left)

25
Q

Carbohydrates; Fisher Projection

A

Carbon at the highest oxidation state is at the top
- The H and OH are located on the horizontal line
– L-enantiomer has the OH group to the LEFT
– D-enantiomer has the OH group to the right

26
Q

Amino Acids: Fisher

A

Use the same system but the H and NH2 groups are on the horizontal line

27
Q

Fisher Projetion: Multiple Chiral Centers

A

It is drawn so that the highest oxidation sate carbon is as close as possible to the top position
- Again the orientation of OH on the furthest below determines whether it is D- or L-

28
Q

Why is D- and L- system not wide spread?

A

Mainly used in carbohydrate and amino acids
- It becomes less useful as the structures deviate significantly from that of glyceraldehyde
– Only for D-glyceraldehyde do the rotation have to be (+) while L-glyceraldehyde be (-)
— For other compounds D-enantiomer may have [alpha] = positive or negative value

29
Q

Opposite Configuration in Fisher Projection

A

When two compounds have many chiral center; if each chiral center has opposite configuration as the corresponding one as in the other compound => Enantimoers
- Assign S; R not D and L

30
Q

Diastereomers

A

Stereoisomers that are not enantiomers are disastereomer (not mirror images).
- May have completely different chemical and physical properties
- In achiral environment, if they are optically active, their [alpha] may vary in both sign and magnitude

When comparing two identically substituted compounds that have multiple chiral center
- If even one retain the same configuration when the other changes then it is not enantiomer

This means E- and Z- as well as cis- and trans- isomer are calssified as being diastereomer

31
Q

Meso-compound

Diastereomer

A

A plance of symmetry exist in the molecule
- Not optically active because they are no asymmetric
- If meso isomers are possible then for n-chiral centers, there will be fewer than 2^n steroisomers possible

32
Q

Prochirality

A

A Molecule that possesses enantiotopic groups is called prochiral

Ethanol has no chiral centers because the centeral C is attached to two equivalent H
- As long as it stays in an achiral environment, then they will be equivalent
- If it’s placed into a chiral environmen, then they may be differentiated from one another (enzyme react with ethanol; they react with the H that is less sterically hindered)
– These type of groups are called enantiotopic (they may be distinguishable in chiral environment)
— They react in a different way or a different rate in a chiral environment

33
Q

Saturated Rings

A

Cyclic compounds that have two or more substituents on the ring(s) may also exist as stereosisomers
- If substituents lie on the same face (both dashed or both bolded), then it is cis-
- If substituent lie on opposite faces, the trans

Ring Junctions of saturated rings can be cis- or trans-
- Cis- if both on the same side
- Trans- if on opposite sides

34
Q

Conformation

Acyclic Compounds

A

Conformation: different 3-dimensional arrangement of the atoms of a molecule as a result of rotation (without bond cleavage)

35
Q

Conformation

Eclipse

A

When H eclipsed each other there are non-bonding interactions (torsion strain) that increase the E of th emolecule
- Each H-H eclipsing interaction add 1 kcal/mol to total E (unfavorable interactions) Me-H add 1.3; Me-Me add 4
– The E difference between staggered (preferred) and eclipsed ethane conformers is 2.9 kcal/mol
— Staggered (antiperiplanar-60° away from each other) is most preferred, followed by gauche (only Me-Me add energy 0.9 kcal/mol), then partially eclipsed, and finally fully eclipsed

36
Q

Cyclohexane Two Conformation

A

Boat Conformation:
- Four pairs of eclipsing H that raise the E of the conformation

Chair Conformation:
- No eclipsing H and so is lower in E

Molecules prefer to exist in their lowest E conformation

37
Q

Cyclohexane; Hydrogen Sets

A

Two distinct sets:
- Equatorial: Each carbon has one H that is in the average plane of the ring
- Axial: Each carbon has one H that is perpendicular to the ring

Every other carbon has an axial H on the top face of the ring; while the axial H of the other C are on the bottom face

38
Q

Chair Conformation Inversion

A

Form a different chair
- When this happen, what ever occupied an Axial position in one chair form => Equatorial upon inversion

39
Q

Substituent when occupies the axial position

Chair Conformation

A

When that happens; it experiences steric interactions with the Axial H that reside at the 3- and 5- position
- These 1,3-diaxial interactions raise the E of that particular conformation
– The effect is more severe as the size increase

An Example is
- Axial Methyl got the CH3 on Axial at 1-position => 1,3 Diaxial interactions
– Gonna want to change the conformation
— Equatorial Methyl got the CH3 on (E) => No interaction => more favorable

Diaxial conformations are much higher in E and will be inverted into diequatorial conformation

40
Q

Equatorial Vs. Axial

A

Subtituent that is (E) is lower in energy than the same substituent that is (A)
- Two substituent on a ring can be cis- or trans-

Conformation have one axial and one equatorial substituent even when inverted
- The bulky (larger) group will get to be in th e (E) postition => lower in E

41
Q

Cyclohexenes (6-membered) and Conformations

A

Cyclohexenes have one double bond
- The two sp2 hybridized carbons => they and the two carbons adjacent to them reside in the same plane
– Only the furthest two carbons from the double bond are not in the same plane

42
Q

Cyclopentanes (5-membered ring) and conformation

A

Got Two Major Conformation:
- Envelope: four carbons in one plane with the fifth carbon out of the plane (like the flap of an envelope)
- Half-Chair: lower in energy; like the envelope but twisted to minimize eclipsing

43
Q

Cyclobutanes (4-membered rings) and Conformation

A

Not flat but are puckered with a torsion angle of about 35°
- The puckering help minimize eclipsing

44
Q

Cyclopropanes (3-membered rings) and Conformation

A

They are planar ( flat; a lot of drugs are in this form because they are strong) and they are high in energy:
- The C-C-C bond angles are small (60° as opposed to the normal tetrahedral bond angle of 109.5°)
- All of the hydrogens eclipse one another on each face

45
Q

Steroids

A

All got three 6-membered rings and one 5-membered ring (envelop)
- Trans- low E
- Series of chairs; Flat

PChem (6 decks)

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