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Understanding Electron Redistribution and Bond Formation in Organic Chemistry, Slides of Chemistry

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.

What you will learn

  • How does the inductive effect influence electron distribution in molecules?
  • What is the role of curly arrows in expressing electron pair movement during chemical reactions?
  • What are electron-rich and electron-poor centers in organic chemistry?

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2021/2022

Uploaded on 09/12/2022

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NOTES ON ARROW PUSHING (CURLY ARR OWS)
These notes are intended to be used as a supplemen t to your 1s t year lectures notes. Some important
concepts are underlined, others which are treated only briefly here have been italicised ; for more
detailed coverage reference should be made to P. Sykes “A Primer to Mechanism in Organic
Chemistry”.
Introduction
During reaction between organic molecules a number of covalent bonds are broken and formed. Since
bonding is associated with electrons, the process of reaction may be considered as a redistribution of
electrons between the molecules. The organic chemist uses ‘arrow-pushing’ to express the ways in
which such electron redistribution may occur, i.e. h e/she attempts to d efine possible mechanisms for
the reactions. Subsequently the organic chemist tries to decide which of the many mechanistic
possibilities is the most likely but this may require consideration other than simple ‘arrow-pushing’.
Before attempting to ‘arrow-push’, however, the organic chemist must have an idea of the electron
distribution in the reactant molecules and how this distribution will influence reactivity.
Electron Distribut ion in Molecu les
Certain atoms in mo lecules may be considered as being relatively electron rich or electron poor and
these atoms define Nucleophilic (nucleus seeking) or Electrophilic (electron seeking) centres
respectively. Cons ider first that par t of the Periodic Table which is of most concern to the organic
chemist:
H
Li Be B C N O F
Na Mg Al Si P S Cl
K Br
I
1. Elements to the right of those in Group IV bear lone pairs of elec trons in their neutral
compounds. These elements, therefore, represent electron rich centres in neutral molecules.
Conversely, elements (except H) to the left bear low-lying, empty p-orbitals in their neutral
compounds and these define electron poor centres.
2. If 1. were the only criterion of electron ‘density’ then H and C (or Si) should be neither
electron rich nor poor in their neutral compounds. However, they are made so by virtue of a
property of the elements known as Electronegativity. This property determines to which atom
bonding electrons will be most attracted and this resultant Bond Polarisation will define which
atom is relatively electron deficient or rich:
X more electronega tive than C or H X more electroposi tive than C or H
R N R
R
R P R
R
R O R R S R R Hal
R
B
R
R
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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

  1. Elements to the right of those in Group IV bear lone pairs of electrons in their neutral compounds. These elements, therefore, represent electron rich centres in neutral molecules. Conversely, elements (except H) to the left bear low compounds and these define electron poor centres. -lying, empty p-orbitals in their neutral
  2. If 1. were the only criterion of electron ‘density’ then H and C (or Si) should be neither electron rich nor poor in their neutral compounds. However, they are made so b property of the elements known as bonding electrons will be most attracted and this resultant Electronegativity. This property determines to which atom Bond Polarisation will define whichy virtue of a X more electronegative than C or H^ atom is relatively electron deficient or rich: X more electropositive than C or H

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

  1. Atoms which are charged by virtue of dative covalent bondi rich centres but induce a relative deficiency or excess in adjacent atoms:ng are neither electron poor nor
  2. Atoms which are charged by virtue of removal of an adjacent atom with or without the bonding pair of electrons are electron poor or rich respec Cl+ (^) R 2 Ntively:-
  3. Isolated less tightly bound to the atoms than are those in π-bonds represent relatively rich electron centres because electrons in^ R^3 C+^ σ-orbitals. AI- π-bond is, therefore, an example π-orbitals are of a nucleophilic centre not associated with one particular atom.
  4. In group may be transferred to other sites in the molecule by means of electrons. The molecule is then considered as a hybrid of a number of structur contributing to a different extent to the final form. This is the essence of conjugated systems a relative excess or deficiency associated with one particular atom or Delocalisation Valence Bond es each of Theory. The resultant polarisation caused by this delocalisation is known as the important not to confuse this with the Inductiv transmitted by σ-electrons and the former by πe Effect (section 2). Essentially the latter is-electrons. Resonance Effect. It is

C!+^ X!-^ C!-^ X!+

H C!+!^ + XX!!--^ H!-^ X!+

C!+^ X!-

A B A B A B

C!+^ N+H 3 Cl-^ H!-^ B-H^3 Na+

O O O

  1. Since each electron is associated with one negative charge, any redistribution of electron result in the development of full charges whereas redistribution of single electrons will not. In order to understand this point constituent atoms and subsequent bond fission. Each atom donated one electron to the formation consider covalent bond formation, A-B, from the electrons of its pairs will of this bond and, considering the bond as a separate 2-electron entity, we may represent this as: The movement of this bonding electron atom and results in 2 reverse is then: - + 1+ =1- charge on the other. The arrow pushing for this p pair onto either a or B leaves a single + charge on the onerocess and its However, in free radical process to the other and, therefore, no charge development. The arrow pushing for this process and its reverse is: es there is no net transfer of an extra electron from one atom
  2. Now we need to decide whether a particular atom in any species should bear a charg do this, first replace the groups bound to that atom by one electron each i.e. by that atom's share in the covalent bond electrons. Then count the total number of electrons surrounding the atom, including lone pairs, and compare it to the number which the atom has in its atomic state. Ae. In order to deficiency of one electron corresponds to 1+ charge, 2 electrons to 2+, etc., and conversely, an excess of 1 electron corresponds to 1 encountered in organic chemistry: - charge etc. Below are listed some of the common species

A B A B

A B A^ B

A B A^ B

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

  1. Another consequence of the Octet Rule is that any nucleophilic attack on a 1 molecule which already has a share of 8 electrons must resu which become associated with another atom i.e. bond formation and bond fission accompany each other during reaction in such cases: lt in displacement of a pair of electronsst^ row element in a
  2. When writing a mechanistic equation electroneutrality must be maintained i (including counterions) on both sides of the equation must balance out. If, as is often the case, the counterion is omitted, then each side of the mechanistic equation must maintain the excess of the same charge e.g.: .e. all charges same
  3. Where an ar position of the head or tail of the arrow is obvious. However, in the formation of a new bond the terminus of the arrow head is sometimes difficult to determine. In these cases i to draw the formulae so that atoms which will become bonded in the product are close androw indicates movement of a pair of electrons onto or away from an atom thet is helpful then draw a dotted line between these atoms. The terminus of the arrow head is on the dotted line since this represents approximately the final position of the new bond e.g. :

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+

R R 34 NC++^ CCll--

R 3 O+^ Cl-

R^ R 43 NC--CCll

R 3 O-Cl

A B Nu A A B Nu^ A^ B-

Nu-

Nu-

B-

(Na+) Br-^ R OSO 2 Me Br R + MeSO 2 O- (Na+)

  1. Some reactions (pericyclic reactions) do not involve any nucleophilic, electrophil centres and are associated with a cyclic redistribution of electrons. Such reactions are best dealt with by Molecular Orbital Theory (2nd (^) year) rather than in Valence Bond terms.ic or radical