Ch 1 : Pushing Curly Arrows

Chapter 1: Structure Determines Properties

Pushing Electrons and Curly Arrows

Mastering curly arrows
A few simple lessons (1, 2, 3, 4)
Questions

"Curly arrows" or "curved arrows" are how organic chemists communicate and "think" about mechanisms.
The ability use curly arrows is probably the single most important skill or tool for simplifying organic chemistry.
Understanding how to use curly arrows allows you to appreciate how organic chemistry works since the arrows show how reactions proceed and this helps remove the need to memorise reactions.

Curly arrows show how the electrons and therefore how the bonds are reorganised. Reorganising bonds implies a reaction has taken place.

Curly arrows should "talk to you" ! Looking at a set of curly arrows literally tells you all the bonding changes, both breaking and forming that happen in a particular step of a reaction sequence.

Study Tip:
Many students struggle with organic chemistry because they never master curly arrows and so miss out on the important information they are trying to tell you.

Steps to mastering curly arrows

There are two types of curly arrows:


used to show the motion of pairs of electrons (more common)		used to show the motion of single of electrons (i.e. radical reactions)

We will focus on the more common arrows here :

Remember that a bond is due to a pair of electrons between atoms (HINT : think of the Lewis structures)
Curly arrows always flow from electron rich to electron poor (i.e. nucleophile to electrophile)

Curly arrows must start from either lone pairs or bonds (σ or π bonds)
Therefore, it's a good idea to draw in all the lone pairs as two dots e.g. or especially on oxygen or nitrogen atoms
If the curly arrow starts from a lone pair then the formal charge on that source atom will become more +ve (since electrons have been removed)
If the curly arrow starts from a bond then electrons are being taken out of that bond, so the bond is broken

Curly arrows must end either between a pair of atoms or on an atom
If the curly arrow ends between a pair of atoms, a new bond is formed
If the curly arrow ends on an atom then the formal charge on that destination atom will become more -ve (since electrons have been added)

Balance the charges ! The overall charge in any particular step of a mechanism should always be balanced.

i.e. Σ(charges on reactents) = Σ(charges on products)

This is because the total number of electrons must be constant
Take the time to check this after every step of a mechanism.
This requires that you pay attention to the formal charges at each atom to do this.
The curly arrows themselves can also tell you this

EXAMPLE

Arrow begins at a lone pair on the O atom and goes to the H atom forming a new O-H bond.
The starting materials are neutral and positive, so we have an overall +ve charge in the starting materials.
The O atom in product is +ve, it gave away the electrons to form the O-H bond. Products are overall positive.

Remember to obey the rules of valence ! (i.e. no more that 8 valence electrons for C, N, O, F)
Curly arrows in a single step flow, they should be drawn in order so they flow and they don't "oppose each other" (i.e. converge or diverge)

Don't compress several mechanistic steps into a single diagram since this makes it difficult to find and correct errors. Draw out each step.

Study Tip:

REMEMBER

Understanding the location of electrons and being able to draw the curly arrows that depict the mechanisms by which a reaction occurs is one of the most critical tools for learning organic chemistry since they allow you to appreciate what controls reactions, how reactions proceed and highlight the similarities between seemingly unrelated reactions.

In a correctly drawn MECHANISM, curly arrows should be used to show ALL the BONDING changes that occur.