Students often complain there is a lot
to remember in organic chemistry especially the hundreds of different
mechanisms. I reply in disagreement, I think its the lack of
understanding the curly arrows and perhaps the lack of pattern
recognition that makes people think there is a lot to memorize. I
think if you really understand your arrows you should be able to
predict mechanisms. In this post I aim to make you understand curly
arrows and the definitions that often accompany them.
Curly arrows are used by organic
chemists to show the movement of electrons, in basic conditions this
is referred to as electron pushing. Some people like to use the term
electron pulling when in acid conditions since its the pull of the
positive charge of the acid which drives the reaction forward.
Remember a covalent bond is basically two electrons being shared
between two atoms. Curly arrows are used to indicate bonds being
formed and bonds being broken. I like to make the analogy of 2
people. One person is holding a ball with a tail attached to it,
this person throws the ball to the other person but keeps hold of the
tail. The path of the ball from one person to the other is the curly
area, the tail between the two people is the bond.
Electrons always have to flow from an
area where there is more electrons (HOMO, highest occupied molecular
orbital) to an area which may be lacking in electrons (LUMO, lowest
unoccupied molecular orbital). When we use a curly arrow we are
indicating the movement of two electrons from the tail to the head of
the arrow. If we only want to show the movement of one electron we
use a fish hook as in free radical reactions.
Bonds being made
A new bond may be formed when none
bonded electrons move towards a positive center. You may see the
area the electrons are moving from referred to as a nucleophile. A
simple definition of a nucleophile is an electron donor (Lewis Base)
and is described a nucleus loving. While the area where the
electrons are accepted is called the electrophile (Lewis acid) and is
described as electron loving.
The tail is on the end where the none
bonded electrons are and the head points to where the new bond
attaches to.
Bonds being broken.
The arrows tail begins in the middle of
the bond being broken to indicate that’s the bond being broken.
The head of the arrow goes to the element which accepts the electrons
from the broken bond.
It should be noted that over all in a
reaction charge is conserved.
General rules for arrow pushing.
- Identify which is the nucleophile and while atom is the electrophilic centre.
- Decide what is the driving force of the mechanism in other words where are the electrons being pushed from.
- Draw the molecules in such a way that you can show the bonds being formed and broken.
- Curly arrows should always flow in one direction and never meet head on.
- Mark charges on all reactants and intermediates.
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