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CONSTITUTIONAL ISOMERS
Isomers with a differentorder of attachment ofthe atoms in their molecules
STEREOISOMERSIsomers with the same orderof attachment, but a differentconfiguration (3D arrangement)of groups on one or more of the atoms
ISOMERSDifferent compoundswith the same molecular formula
cis/trans ISOMERS
ENANTIOMERSStereoisomers whose molecules are non-superimposible mirrorimages of each other
DIASTEREOMERSStereoisomers whose molecules are not mirror images of each other
Each isomer could have stereoisomers
double bond or ring
both can applywith a ring
TYPES OF ISOMERISM(geometric)
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Relationship of Constitutional and Stereoisomers
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Relationships of Stereoisomers
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FISCHER PROJECTIONS
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في الكيمياء ) وخاصة (, يعنى والحيوية الكيمياء العضوية
مسقط فيشر تمثيل ثنائي األبعاد ثالثي األبعاد للجزيء العضوي
كمسقط. يتم رسم جميع الروابط كخطوط أفقية أو عمودية.
رأسيا, الكربونويتم تمثيل سلسلة
وتمثيل ذرات الكربون بمركز الخطوط المتقاطعة. كما يتم تمثيل ذرة
في األعلى. وقد C1الكربون األولى تم تقديم مسقط فيشر بواسطة
.إيميل فيشر
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6
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• , الروابط كل فيشر مسقط فىلو ) كما لألمام إسقاطها يتم األفقية
أو الصفحة من خارجة كانت , تدوير(. يمكن ال ذلك وعلى الشاشة
فيشر في 90°( × 2n+1)مسقط , حيث الشاشة أو الصفحة مستوىلبعضه متناسب الروابط إتجاه أن
, الجزيء محور يتغير ولذلك البعضمقابله إلى .الضوئي
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CHO
HOCH2 HOH
OHH
CHO
CH2OHH
CH2OH
CHO
OH
EVOLUTION OF THEFISCHER PROJECTION
Substituents willstick out towardyou like prongs
Fischer Projection
Main chain bendsaway from you
“Sawhorse” Projection
Orient themain chainvertically withthe mostoxidized groupat the top.
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ORIENTATION OFTHE MAIN CHAINAND THE SUBSTITUENTSIN A FISCHERPROJECTION
continuation of the main chain
CH3
OHH
OHH
OHH
CH3
OH
OH
OH
H
H
H
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H HCH3 CH3
Cl Br
CH3
CH3
H
H
Cl
Br
Cl
CH3
Br
CH3
main chain in red
orient main chain vertically
convert toFischerProjection
rotate 90o
Br
Cl
CH3
H
H
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DETERMINATION OF R / S CONFIGURATIONIN FISCHER PROJECTIONS
المطلقة الفراغية الهيئة تحديد (R وS ) فيشر مسقط خالل من
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H
CH2OH
CHO
OH
PLACE THE PRIORITY=4 GROUP IN ONE OF THE VERTICALPOSITIONS, THEN LOOK AT THE OTHER THREE
H
OH
OHC CH2OH1
2
3
4
1
2 3
4
R
alternatively:
H
CH2OH
CHO
OH1
2
3
4 HOCH2 CHO
OH2
1
4
3
H
R
#4 at top position
#4 at bottom position
BOTH IN BACKSAME RESULT
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H
CH2OH
CHO
OH 1
2
3
4
FOR THE MENTALLY AGILEWHY BOTHER INTERCHANGING? JUST REVERSE YOUR RESULT!
H comingtoward you
Same moleculeas on previousslide. S reverse R
Same resultas before.
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CH3 C
CH2OH
CH3
CH2 CH CH
CH3
CH3
OH*
*
22 = 4 stereoisomers
R RR SS RS S
CH3
OH
CHCH3 CH3
***
23 = 8 stereoisomers
R R RR R SR S RS R R
R S SS R SS S RS S S
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Link to: Drawing Enantiomers and Diastereomers Tutorial and Exercises
Tutorials Topic List Stereochemistry Practice Problems
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Stereospecificityand
stereoselectivity
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stereospecificity is the property
of a chemical reaction that
yields different stereoisomeric
reaction products from two
stereoisomeric reactants
depending on the reaction
conditions
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stereoselectivity is the property of
a chemical reaction that yields an
unequal mixture of stereoisomers
from a single
reactant
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مقارنــة بـين
stereoselectivity
describes a reaction where the mechanism allows for the formation of both products, but the formation of one of the products is
favored
Stereospecificity
describes a reaction that necessarily yields a given stereoisomer because of the
mechanism of the reaction
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بين مقارنة
stereoselectivity
yields an unequal mixture of stereoisomers from a single reactant.
Stereospecificity
yields different stereoisomeric reaction products from two stereoisomeric
reactants depending on the reaction conditions
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بين مقارنة
stereoselectivity
may be partial, where the formation of one stereoisomer is favoured over the other, or
it may be total where only one stereoisomer is formed
Stereospecificity
always yields 100% of a particular stereoisomer
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بين مقارنة
stereoselectivity
An example of partial stereoselectivity is dehydrohalogenation of 2-iodo-butane
which yields 80% trans-2-butene and 20% cis-2-butene
Stereospecificity
For example, the addition of dibromocarbene to cis and trans-2-butene
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CONFIGURATION
ABSOLUTE CONFIGURATION ( R / S )
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CONFIGURATION
The three dimensional arrangement of the groups attached to an atom
Stereoisomers differ in the configuration at one ormore of their atoms.
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2
CONFIGURATION - relates to the three dimensionalsense of attachment for groups attached to a chiralatom or group of atoms (i.e., attached to a stereocenter).
clockwise counterclockwise
(rectus) (sinister)
view with substituentof lowestpriority inback
1 2
4
3
C C
1
4
3
R S
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Enantiomers are assigned a CONFIGURATIONusing the same priority rules we developed for E/Z stereoisomers.
1. Higher atomic number has higher priority.
2. If priority cannot be decided based on the first atom attached move to the next atom, following the path having the highest prioity atom.
3. Expand multiple bonds by replicating the atoms attached to each end of the bond.
SPECIFICATION OF CONFIGURATION
SEQUENCE RULECAHN-INGOLD-PRELOG
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I
C
BrCl
F
I
C
ClBrF
1
2
3
4
R S
Bromochlorofluoroiodomethane
1
32
4
Enantiomers
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How Many Stereoisomers Are Possible?
maximum number of stereoisomers = 2n,
where n = number of stereocenters(sterogenic carbons)
sometimes fewerthan this numberwill exist
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IMPORTANCE OF STEREOCHEMISTRY
# AN ACHIRAL REAGENT REACTS IDENTICALLY WITH BOTH ENANTIOMERS.
# AN ACHIRAL SOCK FITS ON EITHER FOOT
# A CHIRAL REAGENT REACTS DIFFERENTLY WITH EACH ENANTIOMERS.
# A CHIRAL SHOE FITS ON ONLY ONE FOOT.
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RECEPTORS ARE PROTEINS THAT BIND PARTICULAR MOLECULES.
BECAUSE A RECEPTOR WILL RECOGNISE ONLY ONE OF A PAIR OF ENANTIOMERS, ENANTIOMERS CAN HAVE DIFFERENT PHYSIOLOGICAL
PROPERTIES.
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RECEPTORS LOCATED ON THE EXTERIOR OF NERVE CELLS IN THE NOSE ARE ABLE TO PERCIEVE AND DIFFERENTIATE THE ESTIMATED 10000 SMELLS TO WHICH THEY ARE EXPOSED.
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(R-)-(-)CARVONE IS FOUND IN SPEARMINT OIL AND )S(-)+(- CARVONE IS FOUND IN
CARAWAY SEED OIL .
EACH HAS A DIFFERENT SMELL AS IT FITS INTO DIFFERENT RECEPTORS.
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CH3O
C
CH2H3C
(R)-(-) CARVONE
CH3O
C
CH2H3C
(S)-(+) CARVONE
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(S)-(-)-KETAMINE IS FOUR TIMES MORE POTENT THAN (R)-(-)-KETAMINE AND HAS LESS SIDE EFFECTS.
THE ACTIVE INGREDIENT OF IBUPROFEN IS THE (S)-(+)-ISOMER.
OCONHCH3
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THE STEREOCHEMISTRY OF REACTIONS ARE VERY IMPORTANT.
A REGIOSELECTIVE REACTION FORMS MORE OF ONE CONSTITUTIONAL ISOMER THAN THE OTHER.
A B + C MORE ‘B’ IS FORMED THAN ‘C’, WHERE B AND C ARE CONSTITUTIONAL ISOMERS.
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A STEREOSELECTIVE REACTION FORMS MORE OF ONE STEREOISOMER THAN THE OTHER.
A B + C MORE ‘B’ IS FORMED THAN ‘C’, WHERE B AND C ARE STEREOISOMERS.
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IN A STEREOSPECIFIC REACTION, EACH STEREOISOMERIC REACTANT FORMS A DIFFERENT STEREOISOMERIC PRODUCT OR A DIFFERENT SET OF STEREOISOMERIC PRODUCTS.
A B
C D
‘A’ AND ‘C’ ARE STEREOISOMERS
‘B’ AND ‘D’ ARE STEREOISOMERS
A STEREOSPECIFIC REACTION IS ALSO STEREOSELECTIVE. A STEREOSELECTIVE REACTION IS NOT NECESSARILY STEREOSPECIFIC
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ALL THE REACTIONS THAT OCCUR IN BIOLOGICAL SYSTEMS ARE CATALYSED BY ENZYMES.
AN ENZYME CATALYSED REACTION FORMS ONLY ONE STEREOISOMER BECAUSE AN ENZYME POSSESSES A CHIRAL BINDING SITE.
THEY ARE COMPLETELY STEREOSELECTIVE.
THE ENZYME FUMARASE, WHICH CATALYSES THE ADDITION OF WATER TO FUMARATE, FORMS ONLY )S(-MALATE.
COO-
-OOCH2COH
HC C
H
-OOC
COO-
H
+ H2Ofumarase
fumarate (S)-malate
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ENZYME CATALYSIS IS STEREOSPECIFIC. AN ENZYME CATALYSES THE REACTION OF ONLY ONE STEREOISOMER.
FUMARASE CATALYSE THE ADDITION OF WATER TO FUMARATE BUT NOT TO MALEATE
C C
-OOC
H
COO-
H
+ H2Ofumarase
maleate
no reaction
AN ENZYME CAN DIFFERENTIATE STEREOISOMERS BECAUSE OF THE CHIRAL BINDING SITE
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