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
