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An excerpt from a chemistry textbook by d.m. Collard, focusing on chapter 5 of topic 4: stereochemistry. The three-dimensional structure of organic molecules, recognizing isomers, chirality, and designating configurations. It also includes problems for practice.
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Isomers Compounds with same molecular formula, but different structures
Constitutional Isomers Different connectivity
Stereoisomers Same connectivity, different three dimensional arrangement
Enantiomers Non-superposable mirror images
Diastereomers Stereoisomers which are not enantiomers, includes geometic isomers
S:5.1-
An object (molecule) which has a non-superposable* mirror image is chiral (the opposite of chiral is âachiralâ).
Prob 5.30,31, 35a,b,f
S:5. 5.5-5.
Problem: Which of the following are identical to the first structure?
3
OH
Problem: Which of the following are identical to the first structure?
Problem: Which of the following are identical to the first structure?
Stereocenters are designated as having either R - or S -configurationsâŠ.
Clockwise = R
Counterclockwise = S
e.g. , F
Br H Cl
D = deuterium = 2 H; D > H
If the atoms joined to the chiral center are identical, you must consider the set of three atoms connected to them:
A double bond is considered to be two bonds to the next atom:
Orient the molecule so that substituents oriented up and down are pointing away from you and then apply a steamroller.
Converting a Fischer projection back into a 3D tetrahedral representation.
C http://www.r-r-a.org.uk/aandp.html
D (^) AB
CH (^3)
CH 2 CH (^3)
HO H
S:5. Prob: 5.35k-l
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Problem : Which of the following represent the S -enantiomer of 2-phenyl-1- ethanol, which ones are R?
Problem : Which of the following are identical to the first structure? [You should be able to work out how Fischer projections can be redrawn without inverting the configuration]
COOH
CH 3
H OH
H
OH
H 3 C COOH
COOH
CH 3
HO H
COOH
CH 3
H OH
CH 3
COOH
HO H
OH
H
HOOC
r otate by 90 o^ rotate by 180 o^ r otate by 270 o
swap hor izontal substit uent s swap vertical substituents
CH 3
COOH
OH
H 3 C H
rotate set of subst ituents
Most biomolecules are chiral and only exist as one enantiomer in nature.
Amino acids Carbohydrates Nucleotides (in proteins) (sugars) (in DNA) e.g. , valine e.g. , glucose e.g. , cytosine
Problem: What is the configuration of each stereocenter (chiral carbon atom) in each of the above compounds?
HO O
OH H
H H H H
N
N
NH 2
O HO
O
H
H
HO
H
HO
H (^) OH
H
OH
H 2 N
O
OH
HCH(CH 3 ) (^2)
S:5.1; 5.4; 5.
Enantiomers have identical physical and chemical properties in the absence of other chiral molecules, except for their influence on plane- polarized light.
Prob:5.42; 43
S:5.8-5.
(1 dm = 10 cm = 10 -1^ m)
polarized light. Generally the sodium D line is used for the light source and the experiment is done at room temperature, 25 °C.
The specific rotation is then noted as
The specific rotation of an optical pure chiral compound is a âpropertyâ (like melting point or boiling point)
purityâ. Only the excess of one enantiomer over the other gives rise to a rotation.
25 D
The rotation of enantiomers are equal but opposite. If the [α] of the R -
enantiomer of compound A is +100°, the [α] of the S -enantiomer is _____
The [α] of a equal mixture of the R - and S -enantiomers (racemic mixture)
is _____
The % excess amount of one enanatiomer over the other is called the enantiomeric excess (ee). e.g. , 100% R , 0% S : ee = ____ 50% R , 50% S : ee = ____ 70% R , 30% S : ee = ____
Problem : The [α] of the R -enantiomer of compound A is +100°. The [α] of a certain mixture of R - and S -enantiomers of compound A is -50°. What is the ee of this mixture? What is the % R -enantiomer in this mixture?
Most syntheses are not enantiospecific: if a reaction forms a stereocenter, it will proceed to give a racemic mixture of two enantiomers. achiral achiral product racemic starting materials â or â starting materials racemic mixture
Chiral catalysts (such as enzymes) can be used to induce chirality
Chiral starting materials can be transformed to:
Molecules with the same stereochemistry ( e.g. , R â R , âretention of stereochemistryâ) Molecules with the opposite stereochemistry ( i.e. , R â S , âinversionâ) Racemic mixture ( i.e. , R â [ R/S or (d,l ) or (±)] , âracemizationâ)
S:5.10; 5.
Prob: 5.33,34, 35c-e,
S:5.12-
For Fischer projections of compounds with more than one stereocenter,
remember that each represents and consider each
stereocenter one at a time.
Problem: Complete the Fischer projection on the right to represent the molecule on the left. Consider the configuration of the two carbon atoms independent of one another. Provide a complete name for this compound (including stereochemistry!) Is this compound chiral?
CH 3
CH 3
C C CH (^3)
H 3 C H
BrH
Br
Problem: Draw all the stereoisomers of CH 3 CH(OH)CH(OH)COOH:
If the sets of substituents on stereogenic centers are different there will be 2 n stereoisomers.
Stereoisomers which are not mirror images of each other are called diastereomers. Diastereomers have different properties from each other (e.g., solubility, mp, bp, polarity). They can be separated by recrystallization, distillation or chromatography.
CH 3
COOH
CH (^3)
COOH
CH 3
COOH
CH (^3)
COOH C C COOH
H 3 C H
HOH
OH
If the sets of substituents on stereogenic centers are identical there will be fewer than 2 n^ stereoisomers.
Compounds with stereogenic centers which are not chiral are called meso compounds.
Meso compounds possess a point or plane of symmetry (in at least one conformation)
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CH 3
CH 3
CH (^3)
Br H
CH (^3) H Br
CH 3
CH 3
CH 3
CH 3 C C CH (^3)
H 3 C H
Br H
Br
Enantiomers have identical physical properties. Racemic mixtures of carboxylic acids can be treated with a pure enantiomer of an amine to form a pair of diastereomeric ammonium carboxylates which has different properties general reaction:
e.g. ,
R' N R O
O H (^) +
H
H R O R'^ N
O H
H
H
Ph O
H
Me H
O H O
Ph
O
H Me
Ph NH 2
Me
H
( S )- A ( R )- A
( R )- B
( R )- B ( R )- B
S:5.
The pair of diastereomeric ammonium carboxylates can usually be separated by recrystallization (with hard work and luck) and the two acids (and the amine) recovered.
e.g. ,
Ph O
H
Me H
O
H O
Ph
O
H Me
Ph (^) NH 2
Me
H
( S )- A
( R )- B
( R )- B ( R )- B ( S )- A + salt salt
( R )- B ( S )- A salt
( R )- B
salt
HCl (^) HCl
is recovered for reuse
separate by crystallization
( R )- A
( R )- A
( R )- A
RCOO-^ R'NH 3 +^ + HCl RCOOH + R'NH 3 +^ Cl-
acid-base chemistry to convert diastereomeric ammonuim carboxylate to carboxylic acid:
e.g., trans- and cis-1,2-dimethylcyclobutane
e.g., trans-1,2-dimethylcyclohexane
Prob: 5.35h,j,o,p; 37-
CH (^3)
CH (^3)
H CH^3 3 C
CH 3
CH 3
S:5.
