Chapter 5 Stereochemistry(????): Chiral Molecules(????)

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Chapter 5 Stereochemistry(????) Chiral
Molecules(????)

• 5.1 Isomerism Constitutional isomers and
  Stereoisomers (?????????)

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5.2 Enantiomers and chiral molecules

• A Chiral molecule is defined as one that is not
  superposable on its mirror image.
• The chiral molecule and its mirror image are
  enantiomers, and the relationship between the
  chiral molecule and its mirror image is defined
  as enantiomeric (????).
• The word chiral comes from the Greek word Cheir,
  meaning hand. That means, relationship between
  left hand and right hand.

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5.2 Chiral molecules (????)

• 1.(Chiral)?? ??????????????????

Left hand (??)?right hand(??)
??????
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Because models I and II are nonsuperposable
mirror images of each other, the molecules that
they represent are enantiomers.
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Problem 5.1 Classify the following objects as
to whether they are chiral (??) or achiral(???)

• (a) Screw ( b) plain spoon (c ) Fork
• (d) Cup (e) Foot (f)
  Ear
• (g) Shoe (h) Spiral staircase(???)

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Problem 5.2

• (a) If models are available, construct the
  2-butanols represented in Fig. 5.3 and
  demonstrate for yourself that they are not
  mutually superposable.
• (b) Make similar models of 2-propanol
  (CH3CHOHCH3). Are they superposable?
• (c ) Is 2-propanol chiral?
• (d) Would you expect to find enantiomeric forms
  of 2-propanol?

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one tetrahedral atom with four different groups
attached to carbon, it is called chiral
carbon(???)

• (a) CH3CHClCH3 (b) CH3CHBrCH2CH3
• (c ) CH3CHOHCHOHCH3 (d) (CH3)2CHOH
• (e) BrCHClI
• (f) CH3CH2CHClCH2CH3

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5.3 Historical origin of stereochemistry

• The following information was available to vant
  Hoff and le Bel.
• 1. Only one compound for CH3X is ever found
• 2. Only one compound for CH2X2 or CH3XY is
  ever found.
• 3. Two enantiomeric compounds for CHXYZ are
  found.

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5.4 Tests for Chirality Planes of Symmetry( ???)

• All molecules with a plane of symmetry are
  achiral
• All molecules with no plane of symmetry are chiral

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?????????????

• 1.????
•    ? ???s?????????????,?????????????,??????????
  (?)
• ?11,1-????

?2(E)-1,2-????
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Problem 5.7 Write three-dimensional formulas and
designate a plane of symmetry for all of the
achiral molecules in Problem 5.4.
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5.5 Nomenclature of enantiomers The (R-S) system
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Groups containing double or triple bonds
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Problem 5.11 Assign ( R) or (S) designations to
each of the following compounds
(3R) (3R)
(3R)
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Sample problem Consider the following pair of
structures and tell whether they represent
enantiomers or two molecules of the same compound
in different orientations.
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Problem 5.12 Tell whether the two structures in
each pair represent enantiomers or two molecules
of the same compound in different orientations.
(s) Enantiomers (R)
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( R ) Same
(R)
(2S)-Butanol Enantiomers
(2R)-Butanol
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???? R/S ???
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Problem Assign ( R) or (S) designations to each
of the following compounds
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5.6 Properties of enantiomers Optical Activity
(????)

• The molecules of enantiomers are not superposable
  one on the other, and on this basis alone, we
  have concluded that enantiomers are different
  compounds. Do enantiomers resemble constitutional
  isomers and diastereomers in having different
  melting and boing points? The answer is no.
  Enantiomers have identical melting and boiling
  points. Enantiomers have identical
  (refraction(???), solubilities(???), infrared
  spectra(????) and rates of reaction(????)

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Table 5.1 Physical properties of ( R )- and ( S
)-2-butanol
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5.6 A . Plane-polarized light (???)

• One easily observable way in which enantiomers
  differ is in their behavior toward plan-polarized
  light. When a beam of plane-polarized light
  passes throught an enantiomer, the plane of
  polarization rotates. Because of their effect on
  plane-polarized light, separate enantiomers are
  said to be optically active compounds.

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??????(The matter rotation for plane-polarized
light)

• ???????(plane-polarized light)????(rotation)
• The Characteristic of electric wave of light
  (?????)
• ? ???????????(vertical)
• ? ????????????????????
• Nicol??(Nicol polarizer)????????????????????
• ??????????????????????????(plane-polarized light)?

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• ??????????????????(??????(optically active
  compounds)
• ?????????(Levorotatory)
• ?????????- (Dextrorotatory)
• ????? (CH3CHOHCOOH) 3.8
• ???? (CH3CHOHCOOH) -3.8

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Fig. 5.8 The oscillating (??) electric and
magnetic fields of a beam of ordinary light in
one plane
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Fig. 5.9 Oscillation of the electrical field of
ordinary light occurs in all possible planes
Perpendicular (??)to the direction of
propagation (??).
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5.6B The polarimeter (??? ????)
The device that is used for measuring the effect
of plane-polarized light on optically active
compounds is a polarimeter
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Fig 5.11 The principal working parts of a
polarimeter and the measurement of optical
rotation
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Fig 5.11 a polarimeter
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5.6 C Specific Rotation (????)a

• The number of degrees that the plane of
  polarization is rotated as the light passes
  through a solution of an enantiomer depends on
  the number of chiral molecules that it
  encounters. This, of course, depends on the
  length of the tube and the concentration of the
  enantiomer. In order to place measured rotations
  on a standard basis, chemists calculate a
  quantity called the specific rotation a

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Specific Rotation (????a)
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The specific rotations of ( R )-2-butanol and ( S
)-2-butanol
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No necessary correlation exits between the ( R )
and ( S ) designation and the direction of
rotation of plane-polarized light
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5.7 the origin of optical activity
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5.7A Racemic forms (????)

• An equimolar mixture of two enantiomers is called
  a racemic form (???????????????????)

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Racemic forms (()????)

• An equimolar mixture of two enantiomers
• (???(levorotatory (-) )????(dextrorotatory ()
  )???????())
•   The characteristic of racemic forms (???????)
• 1)Chemical property is at same (??????)
•   2)(No rotation )?????
•   3)(Biological activity is different for
  enantiomers)?????????????????
• (????????????,????????)

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Meso-Compounds (????)

• The characteristic of meso-Compounds
• 1) ??????????(Achiral molecule)
• 2)(there is a plane of symmetry) ??????,??????(no
  rotation)
• 3)(No enantiomers) ??????

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5.7B Enantiomeric purity, optical purity, and
enantiomeric excess
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Enantiomeric excess (????????)(e.e)

• R-S
• e.e 100 R-S
• RS
• R???????
• S???????

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Problem 5.13 What relative molar proportions of
( S )- ()-2-butanol and ( R)- (-)-2-butanol
would give a specific rotation, a, equal to
6.76o ? (S)-()-2-butanol? And ( R )?
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Answer
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5.8 The synthesis of enantiomers

• The hydrogenation of ketone

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Fig 5.14. Shows why a racemic form of 2-butanol
is obtaned
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5.9 Molecules with more than one stereocenter
1) Compounds contain two different chiral carbon
(???????????????)
2n
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?and?or III and IV are enantiomers (???)?and? or
? are diasteromers(?????)(??????????) 2.The
charecteristic of diasteromers (?????????) 1)Phys
ical property , specific rotation is different
(???????????) 2)Chemical property is
similar(????,??????)
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??At the same two chiral carbon (??????????????)

• 1.???????

(2n-1)
?and? are enantiomers (?????)? and ? are at the
same compounds)?and ? are diastereomers?and?
are distereomers (????)? is a meso compound
(????)
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5.9A Meso Compounds (??????)

• A structures with two stereocenters will not
  always have four possible steroisomers. Sometimes
  there are only three (2n-1). This happens because
  some molecules with stereocenters are, overall,
  achiral

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2,3-Dibromobutane
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Problem 5.17 Write three-dimensional formulas
for all of the steroisomers of each of the
following compounds. In answer to parts (a)-(e)
label pairs of each enantiomers and meso
compounds.

• ( a) CH3CHClCHClCH3
• ( b) CH3CHBrCHClCH3
• ( c) CH3CHBrCHBrCH2Br
• ( d) CH3BrCHBrCHBrCH2Br
• ( e) CH3CHClCHClCHClCH3

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5.10 Naming compounds with more than one
stereocenter
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5.11 Fischer projection formulas (?????? )
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The rotation 180o in plane forms at the same
structure
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5.12 Stereoisomerism of cyclic compounds
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Problem 5.21 Write structural formulas for all
of the stereoisomers of 1,3-dimethylcyclopentane.
Label pairs of enantiomers and meso commpounds if
they exist.
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5.12A Cyclohexane derivatives
1) 1,4-Dimethylcyclohexanes
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Fig 5.17 cis-1,3-Dimethylcyclohexane
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Fig 5.18 trans-1,3-Dimethylcyclohexane
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Fig 5.19 trans-1,2-Dimethylcyclohexane
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Fig 5.20 cis-1,2-Dimethylcyclohexane
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Problem 5.22 Write formulas for all of the
isomers of each of the following. Designate pairs
of enantiomers and achiral compounds where they
exist.

• ( a) 1-Bromo-2-chlorocyclohexane
• ( b) 1-Bromo-3-chlorocyclohexane
• ( c) 1-Bromo-4-chlorocyclohexane

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( a) 1-Bromo-2-chlorocyclohexane
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trans-1-Bromo-2-chlorocyclohexane
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( c) 1-Bromo-4-chlorocyclohexane
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5.13 Relating configurations through reactions in
which no bonds to the stereocenter are broken

• If a reaction takes places in a way so that no
  bonds to the stereocenter are broken, the product
  will of necessity have the same general
  configuration of groups around the stereocenter
  as the reactant. Such a reaction is said to
  proceed with retention of configuration(????).

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Consider as an example the reaction that takes
place when (S)-(-)-2-methyl-1-butanol is heated
with concentrated hydrochloric acid
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Example---retention of configuration
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5.13A Relative and absolute configurations
(?????????)
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Tartaric acid (???)
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5.14 Separation of enantiomers (??????)
Resolution (??)

• How are enantiomers separated?
• Enantimores have identical solubilities in
  ordinary solvents. You couldnt do crystallzation
  for separations of racemic form.
• You have made diastereomers for each (because
  they have different melting points, different
  boing points, and different solubilities) and
  then resolution for each one

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????????????????????????????-----????(reso
lution)?
1.???(?????()Tartaric acid ???)
2. Biological ???
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5.15 Compounds with stereocenters other than
carbon

• Any tetrahedral atom with four different groups
  attached to it is a stereocenter.
• Listed here are general formulas of compounds
  whose molecules contain stereocenters other than
  carbon. Silicon and germanium are in the same
  group of the periodic table as carbon. They form
  tetrahedral compounds as carbon does.

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The molecules are chiral and the enantiomers can
be separated
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5.16 Chiral molecules that do not posses a
tetrahedral atom with four different
groups(?????????)
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Fig 5.21 Enantiomeric forms of
1,3-dichloroallene
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Diphenyl compounds(??????)-----Chiral molecules
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Chiral Nitrogen compounds
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Homeworks

• 5.26 5.28 5.30