Sunday, October 14, 2012

Curly Arrows

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.

  1. Identify which is the nucleophile and while atom is the electrophilic centre.
  2. Decide what is the driving force of the mechanism in other words where are the electrons being pushed from.
  3. Draw the molecules in such a way that you can show the bonds being formed and broken.
  4. Curly arrows should always flow in one direction and never meet head on.
  5. Mark charges on all reactants and intermediates.
  6. If you make a bond to an uncharged C,H,N or O, a bond must be broken or you will break the octet rule.
    Showing an impossible arrow push as the Carbon breaks the octet rule
    Electrophilic addition.  The first step could be considered a pull from the empty orbital of the electrophile.

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