Chemistry Reference
In-Depth Information
Scheme 1.8 Application of arrow pushing to homolytic cleavage using single-barbed arrows.
Scheme 1.9 Application of arrow pushing to heterolytic cleavage using double-barbed arrows.
possibility that bonds can rearrange into lower energy configurations through concerted
mechanisms where bonds are simultaneously broken and formed. This third process, associ-
ated with pericyclic reactions, is illustrated in Scheme 1.6 using the Cope rearrangement
and does not involve free radicals or ions. Instead, it relies on the overlap of atomic orbitals,
thus allowing the transfer of electron density that drives the conversion from starting
material to product. Regardless, whether reactions rely on free radicals, ions, or concerted
mechanisms, all can be explained and
/
or predicted using the principles of arrow pushing.
Arrow pushing is a term used to define the process of using arrows to conceptually
move electrons in order to describe the mechanistic steps involved in the transition of
starting materials to products. An example of arrow pushing is illustrated in
Scheme 1.7 as applied to the Cope rearrangement introduced in Scheme 1.6. As the
Cope rearrangement proceeds through a concerted mechanism, the movement of electrons
is shown in a single step. As will become apparent, arrow pushing is broadly useful to
explain even very complex and multistep mechanisms. However, while arrow pushing
is useful to explain and describe diverse mechanistic types, it is important to note
that different types of arrows are used depending on the type of bond cleavage involved
in a given reaction. Specifically, when homolytic cleavage is involved in the reaction
mechanism, single-barbed arrows are used to signify movement of single electrons.
Alternatively, when heterolytic cleavage or concerted steps are involved in the reaction
mechanism, double-barbed arrows are used to signify movement of electron pairs.
Schemes 1.8 and 1.9 illustrate the use of appropriate arrows applied to homolytic cleavage
and heterolytic cleavage.
1.2 FUNCTIONAL GROUPS
Having presented the concept of arrow pushing in context of the steps that initiate chemical
reactions, some factors impacting the flow of electrons leading from starting materials to
products can now be explored.
As a rule, electrons will flow from atomic centers high in electron density to atomic
centers low in electron density. This dependence on polarity is similar to the way that
