Learn- More. Weblinks. PAPER 5: Organic Chemistry- II MODULE 23 : SE2 Reactions

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2 Suggested Readings Text book: Organic Chemistry Author : C. H. Bamford and C. F. H. Tipper Text Book: Organic Chemistry Authors : Jonathan Clayden, Nick Greeves and Stuart Warren Glossary D Deuteration: Deuteration is introduction of deuterium ( 2 H) in a compound. Generally, deuteration is done to determine kinetic isotopic labelling to understand reaction mechanism. E Electrophile: These are electron deficient species that form a covalent bond with a reaction partner (the nucleophile) by accepting both electrons for the bond from the nucleophile. I Inductive effect: The electron cloud in a σ-bond between two atoms with different electronegativities is not uniform and is slightly displaced towards the more electronegative of the two atoms. This

3 causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge (δ ) and the other atom has a slight positive charge (δ+). This experimentally observable transmission of charge through a chain of atoms in a molecule is called Inductive effect. All inductive effects are permanent polarisations in the ground state of a molecule and therefore are manifested in its physical properties such as dipole moment. K Kinetic isotopic labelling: Kinetic Isotope Effects (KIEs) is a method used to determine reaction mechanisms by determining rate limiting steps and transition states. In a KIE experiment an atom is replaced by its isotope and the change in rate of the reaction is measured. A very common isotope substitution is when hydrogen is replaced by deuterium. O Organometallic compounds: Organometallic compounds are chemical compounds which contain at least one bond between a carbon atom of an organic compound and a metal. Such compounds are important for electrophilic substitution reactions and are widely used for catalysis. Organomercurials: Organo mercury compounds refer to the group of organometallic compounds that contain at least one mercury atom. Typically the Hg C bond is stable towards air and moisture but sensitive to light. Due to easy method of preparation and chiral purity of Hg-C bond, organomercurials are excellent candidates to study electrophilic substitution at aliphatic carbon atom. S Sneen s ion-pair mechanism: According to Sneen ion-pair mechanism, all S N 1 and S N 2 reactions can be described by one basic mechanism which is based on the ion-pair mechanism. In the mechanism, the substrate first ionizes to an intermediate ion pair that is then converted to products. RX k1 R + X - k2 Products This mechanism gives satisfactory explanation for racemic and retention of stereochemistry in products of Nucleophilic substitution reactions. Steric course of reaction: The steric course of a chemical reaction is defined by the stereochemical relationship between reactants and products. With tetrahedral substitution only two possibilities of retention or inversion exist. However, which one of the two predominates is determined by bulk of the substrate and other reaction conditions.

4 Biography Sir Christopher Kelk Ingold Image Sir Christopher Kelk Ingold (28 October December 1970) was a British chemist from Leeds in London. His groundbreaking work in the 1920s and 1930s on reaction mechanisms and the electronic structure of organic compounds lead to the introduction of many important reaction mechanisms and terminologies such as nucleophile, electrophile, inductive and resonance effects. His pioneering work shaped the reaction mechanism S N 1, S N 2, S E 1 and S E 2 that are still the backbone of reaction mechanisms in Organic Chemistry. He also co-defined the Cahn-Ingold- Prelog priority rules to assign stereochemistry to organic molecules. Sir Ingold authored and coauthored 443 papers. He received the Longstaff Medal of the Royal Society of Chemistry in 1951, the Royal Medal of the Royal Society in 1952, and was knighted in The chemistry department of University College London is now housed in Sir Christopher Kelk Ingold the Sir Christopher Ingold building, opened in Source: Did You Know? Description Image Kinetic isotope effect (KIE) refers to the change in the rate of a chemical reaction upon substitution of an atom in the reactants with one of its isotopic element. In secondary kinetic isotope effects the rate changes due to isotopic substitutions at a site other than the bond breaking site are considered to determine rate of a reaction. These come in three forms: α, β, and γ effects. Secondary kinetic isotope effects tend to be much smaller than primary kinetic isotope effects; however, since these effects can be calculated and measured to very high precision, secondary kinetic isotope effects are still very useful for elucidating reaction mechanisms. Deuterated substrate Source: netic_isotope_effects

5 References 1. Charmane HB, Hughes D, Ingold C, Mechanism of electrophilic substitution at a saturated carbon atom. Part I. Scope and plan. Optical activity from a single asymmetric carbon atom bearing a metal. J. Chem. Soc. 1959, Buckle MJC, Fleming I, Gil S. Accurate determination of the extent to which an S E 2 reaction of an allylsilane is anti. Tcnahedron Lett.1992, 33, Hughes ED. Mechanism and kinetics of substitution at a saturated carbon atom. Trans. Faraday Soc. 1941, 37, Hughes ED, Volger HC Mechanism of electrophilic substitution at a saturated carbon atom. Part VI. The pattern of steric effects among alkyl groups and mechanisms in one-alkyl mercury-exchange, J. Chem. Soc. 1961,