A PPT on stereospecific and stereoselective reaction. It is an important topics as per MSc Chemistry students. In this PPT we discuss defination and theory related to stereospecific and stereoselective reactions. Queries - What is stereoselective and stereospecific reaction What is stereoselective reactions What is stereospecific reaction What is enantioselective and diastereoselective reaction Different between stereospecific and stereoselective reactions Notes on stereospecific and stereoselective reactions

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GOVERNMENT P.G.
COLLEGE SEONI
TOPIC – STEREOSPECIFIC AND STEREOSELECTIVE REACTIONS
SUBMITTED TO -
Dr. D.P. PRAJAPATI
SUBMITTED BY –
VIKAS BALWANSHI
M.SC. CHEMISTRY (1ST SEM)

2. STEREOSPECIFIC AND
STEREOSELECTIVE REACTIONS
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3. ◉ Specific – Particular/ No choice
◉ Selective – Choice
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INTRODUCTION
STEREOSELECTIVE REACTIONS
A reaction or synthesis in which a particular stereoisomer reacts to give one
specific stereoisomer of the product is called Stereospecific Reactions
A Reaction or synthesis in which one stereoisomer is formed
predominantly or exclusively out of several stereoisomemic possibilities is
called a Stereoselective Reactions
STEREOSPECIFIC REACTIONS

4. A reaction or synthesis in which a particular stereoisomer
reacts to give one specific stereoisomer of the product is
called Stereospecific Reactions
STEREOSPECIFIC REACTIONS
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In a stereospecific reactions, a given isomer leads to one product
while another stereoisomer leads to the opposite product
A
B
C
D

5. ◉ A stereospecific gives a different stereoisomer of the product from each
stereoisomer of the starting material.
◉ Stereospecific reaction give a specific product from a certain reactant.
◉ The stereochemistry of product is completely determined by the
stereochemistry of the reactant.
◉ Stereospecificity is focused on the reactant and their stereochemistry.
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6. EXAMPLE - 1
Addition of bromine to cis-2-butene gives racemic 2,3-dibromobutane ,
while the trans isomer gives meso-2,3-dibromo butane
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This reaction is stereospecific because different stereoisomer gives different
stereoisomer.
cis-2-butene
trans-2-butene
Racemic mixture
Meso-2,3-dibromobutane
+

7. EXAMPLE - 2
The formation of epoxides (oxiranes) from alkenes on treatment with
peracids is stereospecific reaction. For example, cis-2-butene gives cis-2,3-
dimethyloxirane, while trans-2-butene gives trans-2,3-dimethyloxirane.
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Cis-2-butene cis-2,3-dimethyloxirane
(a meso compound)
Trans-2,3-dimethyloxirane
(Racemic mixture)
trans-2-butene
+

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EXAMPLE - 3
Diels-Alder reaction, e.g., 1,3-butadiene reacts with malic acid (a cis
isomer) to give cis-1,2,3,6- tetrahydropthalic acid, while fumaric acid(a
trans isomer) gives the trans isomer.
1,3-butadiene
1,3-butadiene
cis-1,2,3,6- tetrahydropthalic acid
trans-1,2,3,6- tetrahydropthalic acid

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STEREOSELECTIVE REACTIONS
A Reaction or synthesis in which one stereoisomer (or
one pair of enantiomers) is formed predominantly or
exclusively out of several stereoisomemic possibilities is
called a Stereoselective Reactions
In a stereoselective reaction a single starting material can give
two are more stereisomeric products but one of these in high
amounts and other is in low amount.
A B + C
MAJOR MINOR

10. ◉ The stereoelectronic requirement of the mechanism of a stereoselective
reactions offers alternative path so that reaction may proceed via a most
favorable path.
◉ The selectivity of the reaction pathway depends on the difference steric
effect and electronic effects. These effects leads to the formation of
different products.
◉ A stereoselective process will normally give multiple products.
◉ Stereoselectivity is focused on the product of their stereochemistry.
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11. EXAMPLE -
The conjugate addition of lithium dimethylcuprate to 4-methylcyclohexenone
is highly stereoselective.
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This reaction is highly stereoselective because the approach of the bulky
cuprate reagent occurs predominantly on the less sterically hindrance face of
the enone, i.e., away from the 4-methyl group. This, the trans isomer is the
major product.
trans (98%) cis (2%)
+

12. CLASSIFICATION OF STEREOSELECTIVE REACTIONS
Stereoselectivity can be further subdivided into two types -
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STEREOSELECTIVITY
ENANTIOSELECTIVITY DISTEREOSELECTIVITY

13. ENANTIOSELECTIVITY
◉ Reaction which involves preferential attack by a reagent on one of
the two enantiotopic group or faces are said to exhibit enantiotopic
or enantiofacial selectivity.
◉ Eantioselectivity is achieved by using chiral substrate, reagent,
catalyst or solvent.
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Enantioselectivity is define as the formation of one of
the two enantiomers predominantly or exclusively.

14. EXAMPLE -
Enzyme reduction of pyruvic acid in the presence of NADPH
(Nicotinamide adenine dinucleotide phosphate) gives S-(+)-Lactic acid as
a major product.
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(-) Lactic acid (+) Lactic acid
Pyruvic acid
Major product
Minor product
ENANTIOMERS

15. DIASTEREOSELECTIVITY
◉ Reaction which involves preferential attack by a reagent on one
of the two diastereotopic group or faces are said to exhibit
diastereotopic or distereofacial selectivity
◉ Diasteroselectivity is most commonly achieved through the
presence of steric hindrance.
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Diastereoselectivity is define as the formation of one of the
two or more diastereomer prodominantly or exclusively

16. EXAMPLE -
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The conversion of 2-norbornanone (I) into the diastereomeric alcohols (II)
and (III) is stereoselective because the attach of Grignard reagent from the less
hindered exo face is preferred tothe endo face.
Major product Minor product
DIASTEREOMERS

17. IMPORTANT NOTE
All the stereospecific reactions are stereoselective
but reverse is not true.
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From the definition of stereospecific reactions, it is
clear that if a reaction is carried out a compound which has no
stereoisomers, it cannot be stereospecific but at most it could
be stereoselective.

18. EXAMPLE - 1
Addition of bromine to methylacetylene result in preferential formation
of trans-1,2-dibromopropene.
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This reaction can only be stereoselective and not stereospecific because
substrate has no stereoisomers.

19. ◉ Lindler’s catalyst facilitates the addition of hydrogen to the alkyne to offer
the formation of cis alkene.
EXAMPLE - 2
Some reactions of alkenes are also stereoselective but not stereospecific
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◉ Reaction of an alkyne with a solution of an alkal metal (usually sodium)
in liquid ammonia gives a trans alkene.

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DIFFERENCE BETWEEN STEREOSPECIFIC AND STEREOSELECTIVE REACTIONS
◉ In stereospecific reactions a
particular stereoisomer reacts to
give one specific stereoisomer of
the product.
◉ In a stereoselective reaction a single
starting material can give two are
more stereisomeric products but
one of these in high amounts and
other is in low amount.
◉ Gives a specific product from a
certain reactant.
◉ Final product depends on the
stereochemistry of the reactant.
◉ Can result in multiple products.
◉ Selectivity of the reaction pathway
depends on differences in steric effect
and electronic effect.
STEREOSELECTIVE REACTIONS
STEREOSPECIFIC REACTIONS

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