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9. Stereochemistry. Based on McMurry’s Organic Chemistry , 6 th edition. Stereochemistry. Some objects are not the same as their mirror images (technically, they have no plane of symmetry) A right-hand glove is different than a left-hand glove (See Figure 9.1)

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9 stereochemistry

9. Stereochemistry

Based on

McMurry’s Organic Chemistry, 6th edition

stereochemistry
Stereochemistry
  • Some objects are not the same as their mirror images (technically, they have no plane of symmetry)
    • A right-hand glove is different than a left-hand glove (See Figure 9.1)
    • The property is commonly called “handedness”
  • Organic molecules (including many drugs) have handedness that results from substitution patterns on sp3 hybridized carbon
enantiomers mirror images
Enantiomers – Mirror Images
  • Molecules exist as three-dimensional objects
  • Some molecules are the same as their mirror image
  • Some molecules are different than their mirror image
    • These are stereoisomers called enantiomers
    • Called chiral molecules
    • Lack a plane of symmetry
plane of symmetry
Plane of Symmetry
  • The plane has the same thing on both sides for the flask
  • There is no mirror plane for a hand
examples of enantiomers
Examples of Enantiomers
  • Molecules that have one carbonwith 4 different substituents have a nonsuperimposable mirror image – chiral molecule - pair of enantiomers
  • But there are chiral molecules that lack this feature
mirror image forms of lactic acid
Mirror-image Forms of Lactic Acid
  • When H andOH substituents match up, COOHand CH3don’t
  • when COOH and CH3coincide, Hand OH don’t
9 2 the reason for handedness chirality
9.2 The Reason for Handedness: Chirality
  • Molecules that are not superimposable with their mirror images are chiral (have handedness)
  • A plane of symmetry divides an entire molecule into two pieces that are exact mirror images
  • A molecule with a plane of symmetry is the same as its mirror image and is said to be achiral (See Figure 9.4 for examples)
chirality
Chirality
  • If an object has a plane of symmetry it is necessarily the same as its mirror image
  • The lack of a plane of symmetry is called “handedness”, chirality
  • Hands, gloves are prime examples of chiral object
    • They have a “left” and a “right” version
chirality centers
Chirality Centers
  • A point in a molecule where four different groups (or atoms) are attached to carbon is called a chirality center
  • There are two nonsuperimposable ways that 4 different different groups (or atoms) can be attached to one carbon atom
    • If two groups are the same, then there is only one way
  • A chiral molecule usually has at least one chirality center
chirality centers in chiral molecules
Chirality Centers in Chiral Molecules
  • Groups are considered “different” if there is any structural variation (if the groups could not be superimposed if detached, they are different)
  • In cyclic molecules, we compare by following in each direction in a ring
slide16

Interchanging any two substituents

on a chirality center converts it to its

mirror image.

Only sp3 hybridized C atoms can

be chirality centers.

slide17

A pair of enantiomers have identical

physical properties except for the

direction in which they rotate

the plane of polarized light.

A single enantiomer is said to be optically active.

9 3 optical activity
9.3 Optical Activity
  • Light restricted to pass through a plane is plane-polarized
  • Plane-polarized light that passes through solutions of achiral compounds remains in that plane
  • Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically active
  • Phenomenon discovered by Biot in the early 19th century
a simple polarimeter
A Simple Polarimeter
  • Measures extent of rotation of plane polarized light
  • Operator lines up polarizing analyzer and measures angle between incoming and outgoing light
optical activity
Optical Activity
  • Light passes through a plane polarizer
  • Plane polarized light is rotated in solutions of optically active compounds
  • Measured with polarimeter
  • Rotation, in degrees, is []
  • Clockwise rotation is called dextrorotatory
  • Anti-clockwise is levorotatory
measurement of optical rotation
Measurement of Optical Rotation
  • A polarimeter measures the rotation of plane-polarized that has passed through a solution
  • The source passes through a polarizer and then is detected at a second polarizer
  • The angle between the entrance and exit planes is the optical rotation.
specific rotation
Specific Rotation
  • To have a basis for comparison, define specific rotation, []D for an optically active compound
  • []D = observed rotation/(pathlength x concentration) = /(l x C) = degrees/(dm x g/mL)
  • Specific rotation is that observed for 1 g/mL in solution in cell with a 10 cm path using light from sodium metal vapor (589 nanometers)
  • See Table 9.1 for examples
specific rotation and molecules
Specific Rotation and Molecules
  • Characteristic property of a compound that is optically active – the compound must be chiral
  • The specific rotation of the enantiomer is equal in magnitude but opposite in sign
9 4 pasteur s discovery of enantiomers 1849
9.4 Pasteur’s Discovery of Enantiomers (1849)
  • Louis Pasteur discovered that sodium ammonium salts of tartaric acid crystallize into right handed and left handed forms
  • The optical rotations of equal concentrations of these forms have opposite optical rotations
  • The solutions contain mirror image isomers, called enantiomers and they crystallized in distinctly different shapes – such an event is rare
relative 3 dimensionl structure
Relative 3-Dimensionl Structure
  • The original method was a correlation system, classifying related molecules into “families” focused on carbohydrates
    • Correlate to D- and L-glyceraldehyde
    • D-erythrose is the mirror image of L-erythrose
  • This does not apply in general
9 5 sequence rules for specification of configuration
9.5 Sequence Rules for Specification of Configuration
  • A general method applies to the configuration at each chirality center (instead of to the the whole molecule)
  • The configuration is specified by the relative positions of all the groups with respect to each other at the chirality center
  • The groups are ranked in an established priority sequence and compared
  • The relationship of the groups in priority order in space determines the label applied to the configuration, according to a rule
sequence rules iupac
Sequence Rules (IUPAC)
  • Assign each group priority according to the Cahn-Ingold-Prelog scheme With the lowest priority group pointing away, look at remaining 3 groups in a plane
  • Clockwise is designated R (from Latin for “right”)
  • Counterclockwise is designated S (from Latin word for “left”)
r configuration at chirality center
R-Configuration at Chirality Center
  • Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn
examples of applying sequence rules
Examples of Applying Sequence Rules
  • If lowest priority is back, clockwise is R and counterclockwise is S
    • R = Rectus
    • S = Sinister
9 6 diastereomers

2S,3S

2R,3R

2R,3S

2S,3R

9.6 Diastereomers
  • Molecules with more than one chirality center have mirror image stereoisomers that are enantiomers
  • In addition they can have stereoisomeric forms that are not mirror images, called diastereomers
  • See Figure 9-10
9 7 meso compounds
9.7 Meso Compounds
  • Tartaric acid has two chirality centers and two diastereomeric forms
  • One form is chiral and the other is achiral, but both have two chirality centers
  • An achiral compound with chirality centers is called a meso compound – it has a plane of symmetry
  • The two structures on the right in the figure are identical so the compound (2R, 3S) is achiral
9 8 molecules with more than two chirality centers
9.8 Molecules with More Than Two Chirality Centers
  • Molecules can have very many chirality centers
  • Each point has two possible permanent arrangements (R or S), generating two possible stereoisomers
  • So the number of possible stereoisomers with n chirality centers is 2n

Cholesterol has eight chirality centers

Morphine has five (9.17)

9 9 physical properties of stereoisomers
9.9 Physical Properties of Stereoisomers
  • Enantiomeric molecules differ in the direction in which they rotate plane polarized but their other common physical properties are the same
  • Daistereomers have a complete set of different common physical properties
9 10 racemic mixtures and their resolution
9.10 Racemic Mixtures and Their Resolution
  • A 50:50 mixture of two chiral compounds that are mirror images does not rotate light – called a racemic mixture (named for “racemic acid” that was the double salt of (+) and (-) tartaric acid
  • The pure compounds need to be separated ot resolved from the mixture (called a racemate)
  • To separate components of a racemate (reversibly) we make a derivative of each with a chiral substance that is free of its enantiomer (resolving agent)
  • This gives diastereomers that are separated by their differing solubility
  • The resolving agent is then removed
constitutional isomers
Constitutional Isomers
  • Different order of connections gives different carbon backbone and/or different functional groups
stereoisomers
Stereoisomers
  • Same connections, different spatial arrangement of atoms
    • Enantiomers (nonsuperimposable mirror images)
    • Diastereomers (all other stereoisomers)
      • Includes cis, trans and configurational
9 12 stereochemistry of reactions addition of hbr to alkenes
9.12 Stereochemistry of Reactions: Addition of HBr to Alkenes
  • Many reactions can produce new chirality centers in compounds. ( 0 1, 1 2, etc.)
  • What is the stereochemistry of the chiral product?
  • What relative amounts of stereoisomers form?
  • Example addition of HBr to 1-butene
achiral intermediate gives racemic product
Achiral Intermediate Gives Racemic Product
  • Addition via carbocation
  • Top and bottom are equally accessible
9 13 stereochemistry of reactions addition of br 2 to alkenes
9.13 Stereochemistry of Reactions: Addition of Br2 to Alkenes
  • Stereospecific
    • Forms racemic mixture
  • Bromonium ion leads to trans addition
addition of bromine to trans 2 butene
Addition of Bromine to Trans 2-Butene
  • Gives meso product (both are the same)
slide51

Reaction between two optically inactive (achiral) partners that produces a product with a center of chirality, always

leads to an optically inactive

product – either racemic or meso.

If you start out with optically inactive reactants, you will end up with optically inactive products.

slide52

Problems 9.20 and 9.21

Draw detailed mechanism and

consider stereochemistry of

addition of Br2 to cis and trans

2-hexene

9 14 stereochemistry of reactions addition of hbr to a chiral alkene
9.14 Stereochemistry of Reactions: Addition of HBr to a Chiral Alkene
  • Gives diastereomers in unequal amounts.
  • Facial approaches are different in energy
  • Product is optically active
slide55

As a general rule, reaction of a chiral reactant with an achiral reactant which introduces a new center of chirality, leads to unequal amounts of the distereometric products. If the chiral reactant is optically active theproduct will also be optically active.

slide56

Problem 9.22 HBr + racemic(+) 4-methyl-1-hexene

Problem 9.23 What products are

formed from reaction of HBr

with 4-methylcyclopentene

slide57

Chapter 9 Homework

27-30, 32-34, 36-43, 45, 46,

48-50, 52, 53, 57, 59, 65-67, 73

Owl for Chapt 9 Nov 18

Test on Chapter 9 Nov 20

9 15 chirality at atoms other than carbon
9.15 Chirality at Atoms Other Than Carbon
  • Trivalent nitrogen is tetrahedral
  • Does not form a chirality center since it rapidly flips
  • Also applies to phosphorus but it flips more slowly
9 16 chirality in nature
9.16 Chirality in Nature
  • Stereoisomers are readily distinguished by chiral receptors in nature
  • Properties of drugs depend on stereochemistry
  • Think of biological recognition as equivalent to 3-point interaction
  • See Figure 9-19
9 17 prochirality
9.17 Prochirality
  • A molecule that is achiral but that can become chiral by a single alteration is a prochiral molecule
prochiral distinctions faces
Prochiral distinctions: faces
  • Planar faces that can become tetrahedral are different from the top or bottom
  • A center at the planar face at a carbon atom is designated re if the three groups in priority sequence are clockwise, and si if they are counterclockwise
prochiral distinctions paired atoms or groups
Prochiral distinctions, paired atoms or groups
  • An sp3 carbon with two groups that are the same is a prochirality center
  • The two identical groups are distinguished by considering either and seeing if it was increased in priority in comparison with the other
  • If the center becomes R the group is pro-R and pro-S if the center becomes S
prochiral distinctions in nature
Prochiral Distinctions in Nature
  • Biological reactions often involve making distinctions between prochiral faces or or groups
  • Chiral entities (such as enzymes) can always make such a distinction
  • Examples: addition of water to fumarate and oxidation of ethanol