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An introduction to arrow pushing, a method used by organic chemists to express the ways in which electron redistribution occurs during chemical reactions. It covers the concept of electron-rich and electron-poor centers, the inductive effect, and the mechanics of arrow pushing. It also discusses the use of curly arrows to denote electron pair movement and the importance of considering other factors such as stereoelectronic effects and steric hindrance.
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These notes are intended to be used as a supplement to your 1 concepts are underlined, others which are treated only briefly here have been italicised; for more NOTES ON ARROW PUSHING (CURLY ARROWS) st^ year lectures notes. Some important detailed coverage Chemistry”. Introduction reference should be made to P. Sykes “A Primer to Mechanism in Organic During reaction between organic molecules a number of covalent bonds are broken and formed. Since bonding is associated with electrons, the process of reaction electrons between the molecules. The organic chemist uses ‘arrow which such electron redistribution may occur, i.e. he/she attempts to define possible mechanisms for may be considered as a redistribution of-pushing’ to express the ways in the reactions. possibilities is the most likely but this may require consideration other than simple ‘arrow Before attempting to ‘arrow distribution in the reactant molecules and how this distribution will influence reactivity. Subsequently the organic chemist tries to decide which of the many mechanistic-push’, however, the organic chemist must have an idea of the electron-pushing’. Electron Distribution in Molecules Certain atoms in molecules may be considered as being relatively electron rich or electron poor and th respectively. Consider first that part of the Periodic Table which is of most concern to the organicese atoms define Nucleophilic (nucleus seeking) or Electrophilic (electron seeking) centres chemist: H Li Be B C N O F Na K Mg Al Si P S ClBr I
R N R R R P R R R O R R S R R Hal (^) R R B R
This polarisation is known as the represented either as above or as: Inductive Effect of a particular atom and is usually
C!+^ N+H 3 Cl-^ H!-^ B-H^3 Na+
Note that for some species lone pairs are omitted from the commonly used notation; their presence must not be forgotten. Note also the difference between electrons around carbon (a carbenium ion or cabocation) whereas the last two both have 8 +CR 3 and +NR 4 or +OR 3 species. The first has a total of 6 electrons around the central atom (ammonium, oxonium ions). As a consequence of the Rule octet (nucleophilic attack) whereas the last t the first species will be attacked by species bearing an available lone pair two will not suffer such attack: o complete the Octet
RR 33 CC CC RR 33 CC-+^ carbanion RRN^2 N^ NN neRut^2 rNal- nitrene R R 32 CC CC nneeuuttrraall rcaadrbiceanl (^) e RRO 4 N ON RRO 4 - N+ Cl Cl-^ R 3 O O R 3 O+
A B Nu A A B Nu^ A^ B-