l 7 structural-isomerism_pch217_2013_2014
TRANSCRIPT
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Organic Chemistry
Course Number: PCH 1120-217
Lecture # 7
Sunday September 22, 2013
Structural Isomerism-Stereoisomerism with One and Two Chiral Carbons
Prof. Oludotun A. Phillips
Room # 2-81, 2nd Floor Pharmacy Building
Email: [email protected]
Tel: 24986070
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Learning ObjectivesAt the end of the class students should be able to:
describe structural and stereo-isomerism.
identify chiral carbon / stereocenter and achiral carbon.
draw structures of stereoisomers.
identify enantiomers and diastereomers.
distinguish between diastereomers and meso compounds.
describe optical activity in stereoisomers.
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Relationship Among Isomers
Geometric isomersRigidity in a molecule leads to isomerism
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Structural (Constitutional) Isomers
Constitutional isomers: compounds with the same molecular formula but different connectivity (order of attachment) of their atoms.
There are two constitutional isomers with molecular formula C4H10.
CH3CH2CH2CH3 CH3CHCH3
CH3
Butane(bp -0.5°C)
2-Methylpropane(bp -11.6°C)
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Structural (Constitutional) Isomers The potential for constitutional isomerism is
enormous:
4,111,846,763
4,347
75
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Constitutional Isomers
MolecularFormula
CH4
C5H12
C10H22
C15H32
C30H62
36,797,588C25H52
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Assignment
Submit to me during the next class:-
Questions:
1. How many constitutional isomers would you expect for each of the molecular formulas: C5H12, C6H14, C2H6O and C4H10O?
2. Draw all the possible constitutional isomers for the molecular formulas above.
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Stereoisomerism Stereoisomers: compounds with the same
molecular formula and same connectivity (order of attachment) of their atoms, but different 3D
(3-dimensional) orientations of their atoms in space.
Stereoisomerism relates to the three dimensional structure of the molecule in space.
Stereochemistry is the study of the orientation of molecules in space, relating how the atoms are arranged in space relative to each other.
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Stereoisomerism Three aspects of Stereochemistry:
1. Chirality of molecules: with right- or left-handed arrangement of the atoms around a carbon atom.
Chiral (from the Greek: cheir; hand) molecules are compds that are non-superimposable on their mirror images are stereoisomers; e.g Enantiomers
2. Geometric isomers: relates to how rigidity in the molecule can lead to isomerism.
3. Conformation of molecules: relates to how the change in the shapes of molecules can lead to isomerism.
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Stereoisomers with One Chiral Carbon Atom
A carbon with four different bonded groups is referred to as a Chiral Carbon (a Stereocenter, Chiral center; Stereocenter, Stereogenic center):
I C Cl
F
Br
H3C C CH2CH3
CH2CH2CH3
CH2CH2CH2CH3
Chiral Carbons(Stereocenter)
all four groups are different
H3CH2C C CH3
F
H
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Stereoisomers with One Chiral Carbon Atom
On the other hand, a carbon with all the four or two of the groups the same is referred to as an Achiral carbon:
H C H
H
H
achiral Carbons
all the four groups are the same
H3C C CH3
CH3
CH3
H3CH2C C CH2CH3
CH2CH3
CH2CH3
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Stereoisomers with One Chiral Carbon Atom
A Chiral carbon can exist in either of two 3D structural arrangements, because of the tetrahedral geometry.
The Two 3D representations are non-superimposable, mirror-image configurations referred to as Enantiomers.
Configuration refers to the orientation of the groups around the chiral carbon (stereocenter).
For example the two structural representation of Lactic acid, shown in next slide are in 3D, tetrahedral geometry of the chiral carbon:…
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Stereoisomers with One Chiral Carbon Atom
The two structural forms
are referred to as a pair
of Enantiomers.
Structure (i) is different from Structure (ii), they are both non-superimposable mirror images.
Each structural form exists independently: e.g. isomer (i): L(+)-lactic acid - is produced in muscles of the
body during exercise and responsible for the soreness.
while isomer (ii): D(+)-lactic acid is found in sour milk.
CO2H
C
HH3C
OH
CO2H
C
HCH3
HO
Lactic Acid: mirror images, both structures are different
Structure (i) Structure (ii)
(S) (R)
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Stereoisomers with One Chiral Carbon Atom Enantiomers of
2-butanol shown below:
To confirm that the mirror image of 2-butanol is not
superimposable on the original; rotate the mirror image by 180o
OH
CH3C CH2CH3
H
HO
CCH3
HCH3CH2
Original molecule Mirror image
Original molecule
OH
CH3C CH2CH3
H
OH
CCH3
HCH3CH2
OH
CH3C H
CH2CH3
Mirror image The mirror imagerotated by 180°
180°
rotate themirror imageby 180° about
the C-OH bond
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Stereoisomers with One Chiral Carbon Atom now try to fit one molecule (original on mirror image rotated
180o) on top of the other so that all groups and bonds match or align exactly:
The original and mirror images are non-superimposable…. they are different molecules with identical physical properties They are enantiomers (non-superimposable mirror images)
OH
CH3C CH2CH3
H
OH
CH3C H
CH2CH3
The original molecule
The mirror imageturned by 180°
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Stereoisomers with One Chiral Carbon Atom
Enantiomers:
are non-superimposable on their mirror images
are chiral molecules, having a center of chirality (the carbon with all the four different groups attached is the center of Chirality) – i.e. show handedness
contain stereogenic center, stereocentre, chiral center or asymmetric carbon atom.
do not have a plane of symmetry
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Stereoisomers with One Chiral Carbon Atom
The Physical properties of Enantiomers, such as melting point, boiling point, and refractive index are identical.
Enantiomers differ in only one physical property, the direction in which they rotate Plane-Polarized light.
Hence, Enantiomers are Optically active molecules.
Racemic mixture or Racemate: a mixture of equal (1:1) parts of enantiomers.
Racemic mixtures are Optically Inactive.
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Plane of Polarization of Plane Polarized Light Ordinary light waves - oscillate in all E-vector planes
perpendicular to its direction of propagation.
Plane-Polarized Light waves – oscillate in a single E-vector plane to its direction of propagation.
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Schematic diagram of a polarimeterMeasurement of Optical Activity – Polarimeter:
Polarimeter: instrument for measuring the rotation of plane-polarized light by a chiral compound.
Optically active: indicates that a compound rotates plane-polarized light.
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Rotation of Plane Polarized LightOptical Activity:
Dextrorotatory: clockwise rotation of plane-polarized light
Levorotatory: counterclockwise rotation of plane-polarized light
Specific rotation (): the observed rotation of an optically active substance at a concentration of 1g/100mL in a sample tube 10cm long; for a pure liquid, concentration is in g/mL (density):
Specific rotation ()= ________observed rotation “degrees”__________
length of sample tube “dm” X conc of sample “g/cm3”
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Stereoisomers with One Chiral Carbon Atom
Enantiomerism: is the presence of a carbon with four different groups bonded to it.
A compound having one chiral carbon with four different attached groups will exist in two non-superimposable mirror-image forms.
However, If a compound and its mirror image are superimposable, they are identical and there is no possibility of enantiomerism.
Such molecules are referred to as Achiral (without chirality):
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Isomers without Chiral Carbon Atom
A carbon having the two or more identical groups around it is referred to as Achiral carbon:
E.g. Propanoic acid:
Here two groups on the center carbon are similar, therefore both compounds are identical.
CO2H
C
HH
CH3
Propanoic acid: Both structures are identical
Structure (i) Structure (ii)
CO2H
C
HH
H3C
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Stereoisomers with Two Chiral Carbon Atoms
1. Molecules with Two Dissimilar Chiral Carbon Atoms – will give Enantiomers and Diastereomers:
e.g. 2,3,4-Trihydroxybutanal have two stereocenters.
Maximum possible number of stereoisomers = 2n; (n= number of chiral carbons)
22 = 4 stereoisomers are possible
HOCH2-CH-CH-CHOHOH
O* *
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Stereoisomers with Two Chiral Carbon Atoms1. Molecules with Two Dissimilar Chiral Carbon Atoms:
Enatiomers and Diastereomers:
Four Stereoisomers of 2,3,4-Trihydroxybutanal are possible:
Structures AC, AD, BC and BD are Diastereomers.
C
C
H OH
CHO
OH
CH2OH
H
C
C
HHO
CHO
HO
CH2OH
H H
CH2OH
HO
C
C
H OH
CHO
C
C
HHO
CHO
H
CH2OH
OH
A pair of enantiomers(Erythrose)
A pair of enantiomers(Threose)
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Stereoisomers with Two Dissimilar Chiral Carbon Atoms
Diastereomers are stereoisomers having more than one chiral carbon atom, but they are not mirror images.
Diastereomers differ in physical properties such as:
melting points, boiling points, densities refractive indices, and if they are chiral, specific rotations can differ.
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Stereoisomers with Two Chiral Carbon Atoms
2. Molecules with Two Similar Chiral Carbon Atoms - gives Enatiomers, Diastereomers and Meso Compounds:
Meso compound: an achiral compound having two or more stereocenters that is superimposable on its mirror image.
e.g. as may be found in
tartaric acid
Have two stereocenters; hence the expected number of stereoisomers 2n = 22 = 4; but only three stereoisomers truly exist
CC
OH
COOH
OH
H
HOOC
H
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Stereoisomers with Two Similar Chiral Carbon Atoms
Meso compounds (E and F) are achiral compounds therefore they are optically inactive molecules (they are the same molecule).
Compounds (G and H) are Enantiomers. While compounds (E and G), and (E and H) are Diastereomers.
E. F. G. H.
C
C
H OH
COOH
OH
COOH
H
C
C
HHO
COOH
HO
COOH
H
C
C
H OH
COOH
H
COOH
HO
C
C
HHO
COOH
H
COOH
OH
A pair of enantiomersA meso compound(plane of symmetry)
Has a Plane of Symmetry!!
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Rotation of Plane Polarized Light
Optical Activity: Like melting point and boiling point, Specific
rotation is a physical property of a compound e.g.
DD
H3CC
OHH
COOH
CH3
C
HOH
COOH
[]21 = -2.6°= +2.6°21
[]
(R)-(-)-Lactatic acid(S)-(+)-Lactic acid
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Questions - Stereoisomerism
Submit to me during the next class:-
How many stereoisomers are possible for the carbohydrate molecule 2-Deoxyribose shown below?
Draw all the possible stereoisomers of 2-Deoxyribose shown above, and identify the enantiomers and diastereomers.
CH2
OH
HC
OH
HC
OH
CH2 C H
O
2-Deoxyribose [A carbohydrate that is a structural component of the genetic material Deoxyribonucleic acid (DNA)]