05 - stereochemistry - wade 7th
DESCRIPTION
Organic Chemistry, 7th Edition L. G. Wade, JrTRANSCRIPT
![Page 1: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/1.jpg)
Chapter 5
©2010,Prentice Hall
Organic Chemistry, 7th EditionL. G. Wade, Jr.
Stereochemistry
![Page 2: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/2.jpg)
Chapter 5 2
Chirality
• “Handedness”: Right glove doesn’t fit the left hand.
• Mirror-image object is different from the original object.
![Page 3: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/3.jpg)
Chapter 5 3
Achiral
• Objects that can be superposed are achiral.
![Page 4: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/4.jpg)
Chapter 5 4
Stereoisomers Enantiomers: Nonsuperimposable mirror
images, different molecules with different properties.
![Page 5: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/5.jpg)
Chapter 5 5
Chiral Carbons
• Carbons with four different groups attached are chiral.
• It’s mirror image will be a different compound (enantiomer).
![Page 6: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/6.jpg)
Chapter 5 6
Achiral Compounds
Take this mirror image and try to superimpose it on the one to the left matching all the atoms.
Everything will match.
When the images can be superposed the When the images can be superposed the compound is compound is achiralachiral..
![Page 7: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/7.jpg)
Chapter 5 7
Planes of Symmetry
• A molecule that has a plane of symmetry is achiral.
![Page 8: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/8.jpg)
Chapter 5 8
Cis and Trans Cyclic Compounds
• Cis-1,2-dichlorocyclohexane is achiral because the molecule has an internal plane of symmetry. Both structures above can be superimposed.
• Trans-1,2-dichlorocyclohexane does not have a plane of symmetry so the images are nonsuperimposable and the molecule will have two enantiomers.
![Page 9: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/9.jpg)
Chapter 5 9
(R) and (S) Nomenclature
• Different molecules (enantiomers) must have different names.
• Usually only one enantiomer will be biologically active.• Configuration around the chiral carbon is specified
with (R) and (S).
![Page 10: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/10.jpg)
Chapter 5 10
Cahn–Ingold–Prelog Rules
• Assign a priority number to each group attached to the chiral carbon.
• Priority is assigned according to atomic number. The highest atomic number assigned is the highest priority #1.
• In case of ties, look at the next atoms along the chain.
• Double and triple bonds are treated like bonds to duplicate atoms.
![Page 11: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/11.jpg)
Chapter 5 11
Assign (R) or (S)
• Working in 3-D, rotate the molecule so that the lowest priority group is in back.
• Draw an arrow from highest to lowest priority group.• Clockwise = (R), Counterclockwise = (S)
![Page 12: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/12.jpg)
Chapter 5 12
Assign Priorities
Atomic number: F > N > C > H
Once priorities have been assigned, the lowest priority group (#4) should be moved to the back if necessary.
![Page 13: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/13.jpg)
Chapter 5 13
Assign Priorities
Draw an arrow from Group 1 to Group 2 to Group 3 and back to Group 1. Ignore Group 4. Clockwise = (R) and Counterclockwise = (S)
CounterclockwiseCounterclockwise
((SS))
![Page 14: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/14.jpg)
Chapter 5 14
Example
C
OH
CH3CH2CH2H
CH2CH3
1
23
4
C
CH2CH3
CH3CH2CH2OH
H
1
2
3
4
rotate
When rotating to put the lowest priority group in the back, keep one group in place and rotate the other three.
ClockwiseClockwise ( (RR))
![Page 15: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/15.jpg)
Treatment of Multiple Bonds
Chapter 5 15
![Page 16: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/16.jpg)
Chapter 5 16
Example (Continued)
CH3
CH3CH2CH=CH
CH2CH2CH2CH3
H11
22
33
44
CounterclockwiseCounterclockwise ( (SS))
![Page 17: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/17.jpg)
Chapter 5 17
Draw the enantiomers of 1,3-dibromobutane and label them as (R) and (S). (Making a model is particularly helpful for this type of problem.)
The third carbon atom in 1,3-dibromobutane is asymmetric. The bromine atom receives first priority, the (–CH2CH2Br) group second priority, the methyl group third, and the hydrogen fourth. The following mirror images are drawn with the hydrogen atom back, ready to assign (R) or (S) as shown.
Solved Problem 1
Solution
![Page 18: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/18.jpg)
Chapter 5 18
Properties of Enantiomers• Same boiling point, melting point, and density.• Same refractive index.• Rotate the plane of polarized light in the same
magnitude, but in opposite directions.• Different interaction with other chiral molecules:
– Active site of enzymes is selective for a specific enantiomer.
– Taste buds and scent receptors are also chiral. Enantiomers may have different smells.
![Page 19: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/19.jpg)
Chapter 5 19
Optical Activity
• Enantiomers rotate the plane of polarized light in opposite directions, but same number of degrees.
![Page 20: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/20.jpg)
Chapter 5 20
Polarimeter
Clockwise Clockwise
Dextrorotatory (+)
CounterclockwiseCounterclockwise
Levorotatory (-)
Not related to (R) and (S)
![Page 21: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/21.jpg)
Chapter 5 21
Specific Rotation Observed rotation depends on the length
of the cell and concentration, as well as the strength of optical activity, temperature, and wavelength of light.
[] = (observed) c l
Where (observed) is the rotation observed in the polarimeter, c is concentration in g/mL and l is length of sample cell in decimeters.
![Page 22: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/22.jpg)
Chapter 5 22
When one of the enantiomers of 2-butanol is placed in a polarimeter, the observed rotation is 4.05° counterclockwise. The solution was made by diluting 6 g of 2-butanol to a total of 40 mL, and the solution was placed into a 200-mm polarimeter tube for the measurement. Determine the specific rotation for this enantiomer of 2-butanol.
Since it is levorotatory, this must be (–)-2-butanol The concentration is 6 g per 40 mL = 0.15 g/ml, and the path length is 200 mm = 2 dm. The specific rotation is
[]D25 =
– 4.05°(0.15)(2)
= –13.5°
Solved Problem 2
Solution
![Page 23: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/23.jpg)
Chapter 5 23
Biological Discrimination
![Page 24: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/24.jpg)
Chapter 5 24
Racemic Mixtures
• Equal quantities of d- and l- enantiomers.• Notation: (d,l) or ()• No optical activity.• The mixture may have different boiling point (b. p.) and melting point
(m. p.) from the enantiomers!
![Page 25: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/25.jpg)
Chapter 5 25
Racemic Products
If optically inactive reagents combine to form a chiral molecule, a racemic mixture is formed.
![Page 26: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/26.jpg)
Chapter 5 26
Optical Purity
• Optical purity (o.p.) is sometimes called enantiomeric excess (e.e.).
• One enantiomer is present in greater amounts.
observed rotation
rotation of pure enantiomerX 100o.p. =
![Page 27: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/27.jpg)
Chapter 5 27
Calculate % Composition
The specific rotation of (S)-2-iodobutane is +15.90. Determine the % composition of a mixture of (R)- and (S)-2-iodobutane if the specific rotation of the mixture is -3.18.
3.18
15.90X 100o.p. = = 20%
2l = 120% l = 60% d = 40%
Sign is from the enantiomer in excess: levorotatory.
![Page 28: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/28.jpg)
Chapter 5 28
Chirality of Conformers
• If equilibrium exists between two chiral conformers, the molecule is not chiral.
• Judge chirality by looking at the most symmetrical conformer.
• Cyclohexane can be considered to be planar, on average.
![Page 29: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/29.jpg)
Chapter 5 29
Chirality of Conformational Isomers
The two chair conformations of cis-1,2-dibromocyclohexane are nonsuperimposable, but the interconversion is fast and the molecules are in equilibrium. Any sample would be racemic and, as such, optically inactive.
![Page 30: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/30.jpg)
Chapter 5 30
Nonmobile Conformers
• The planar conformation of the biphenyl derivative is too sterically crowded. The compound has no rotation around the central C—C bond and thus it is conformationally locked.
• The staggered conformations are chiral: They are nonsuperimposable mirror images.
![Page 31: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/31.jpg)
Chapter 5 31
Allenes
• Some allenes are chiral even though they do not have a chiral carbon.
• Central carbon is sp hybridized.
• To be chiral, the groups at the end carbons must have different groups.
![Page 32: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/32.jpg)
Chapter 5 32
2,3-Pentadiene Is Chiral
![Page 33: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/33.jpg)
Chapter 5 33
Fischer Projections
• Flat representation of a 3-D molecule.• A chiral carbon is at the intersection of
horizontal and vertical lines.• Horizontal lines are forward, out-of-plane.• Vertical lines are behind the plane.
![Page 34: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/34.jpg)
Chapter 5 34
Fischer Projections (Continued)
![Page 35: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/35.jpg)
Chapter 5 35
Fischer Rules
• Carbon chain is on the vertical line.
• Highest oxidized carbon is at top.
• Rotation of 180 in plane doesn’t change molecule.
• Do not rotate 90!
![Page 36: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/36.jpg)
Chapter 5 36
180° Rotation
• A rotation of 180° is allowed because it will not change the configuration.
![Page 37: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/37.jpg)
Chapter 5 37
90° Rotation
• A 90° rotation will change the orientation of the horizontal and vertical groups.
• Do not rotate a Fischer projection 90°.
![Page 38: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/38.jpg)
Chapter 5 38
Fischer Mirror Images
• Fisher projections are easy to draw and make it easier to find enantiomers and internal mirror planes when the molecule has 2 or more chiral centers.
CH3
H Cl
Cl H
CH3
![Page 39: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/39.jpg)
Chapter 5 39
Fischer (R) and (S)
• Lowest priority (usually H) comes forward, so assignment rules are backwards!
• Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R).
• Example:(S)
(S)
CH3
H Cl
Cl H
CH3
![Page 40: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/40.jpg)
Chapter 5 40
Diastereomers
• Molecules with two or more chiral carbons.• Stereoisomers that are not mirror images.
![Page 41: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/41.jpg)
Chapter 5 41
Alkenes
• Cis-trans isomers are not mirror images, so these are diastereomers.
![Page 42: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/42.jpg)
Chapter 5 42
Two or More Chiral Carbons• When compounds have two or more chiral
centers they have enantiomers, diastereomers, or meso isomers.
• Enantiomers have opposite configurations at each corresponding chiral carbon.
• Diastereomers have some matching, some opposite configurations.
• Meso compounds have internal mirror planes.• Maximum number of isomers is 2n, where n =
the number of chiral carbons.
![Page 43: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/43.jpg)
Chapter 543
Are the structures connected the same?Are the structures connected the same?
yesyes nono
Are they mirror images?Are they mirror images? Constitutional IsomersConstitutional Isomers
yesyes nono
EnantiomersEnantiomersAll chiral centers will All chiral centers will
be opposite between them.be opposite between them.
Is there a plane of symmetry?Is there a plane of symmetry?
yesyes nono
DiastereomersDiastereomersMesoMesosuperimposablesuperimposable
Comparing Structures
![Page 44: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/44.jpg)
Chapter 5 44
• Meso compounds have a plane of symmetry.• If one image was rotated 180°, then it could be
superimposed on the other image.• Meso compounds are achiral even though they have
chiral centers.
Meso Compounds
![Page 45: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/45.jpg)
Chapter 5 45
Number of Stereoisomers
The 2n rule will not apply to compounds that may have a plane of symmetry. 2,3-dibromobutane has only 3 stereoisomers: (±) diastereomer and the meso diastereomer.
![Page 46: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/46.jpg)
Chapter 5 46
Properties of Diastereomers
• Diastereomers have different physical properties, so they can be easily separated.
• Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated.
• Enantiomers are difficult to separate.• Convert enantiomers into diastereomers to be
able to separate them.
![Page 47: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/47.jpg)
Chapter 5 47
Resolution of Enantiomers
React the racemic mixture with a pure chiral compound, such as tartaric acid, to form diastereomers, then separate them.
![Page 48: 05 - Stereochemistry - Wade 7th](https://reader033.vdocument.in/reader033/viewer/2022061417/554e9135b4c90573338b4dcc/html5/thumbnails/48.jpg)
Chapter 5 48
ChromatographicResolution of Enantiomers