stereospecificity and STEREOSELECTIVE.pptx

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

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

2. STEREOSPECIFIC AND STEREOSELECTIVE REACTIONS 2

3. Specific – Particular/ No choice Selective – Choice 3 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 4 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. 5

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 6 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. 7 Cis-2-butene cis-2,3-dimethyloxirane (a meso compound) Trans-2,3-dimethyloxirane (Racemic mixture) trans-2-butene +

8. 8 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

9. 9 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. 10

11. EXAMPLE - The conjugate addition of lithium dimethylcuprate to 4-methylcyclohexenone is highly stereoselective . 11 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 - 12 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. 13 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. 14 (-) 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. 15 Diastereoselectivity is define as the formation of one of the two or more diastereomer prodominantly or exclusively

16. EXAMPLE - 16 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. 17 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. 18 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 19 Reaction of an alkyne with a solution of an alkal metal (usually sodium) in liquid ammonia gives a trans alkene.

20. 20 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

21. 21 THANK YOU