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.

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)
• Remember that the arrows show the motion of the electrons and not atoms or molecules.
• 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)

• Avoid compressing several mechanistic steps into a single diagram since this makes it difficult to find and correct errors.  Draw out each step separately, this tends to keep it simple and clear.

Note: How do you know how much to include in a "step" ? The most common mistake students tend to make is that they merge several steps in to a single step.

This seemingly simple question is actually not easy to answer. The following factors should be considered:

• The mechanism needs to reflect the kinetics (i.e. concerted or stepwise, unimolecular or bimolecular)
• Termolecular events are very very rare, so don't normally draw mechanisms that show them
• Steps need to produce stable intermediates
• What do you know about the process ? Use that knowledge (e.g. SN1 or SN2, nature of the leaving group)
• Do the arrows flow ? If not, separate the steps

© Dr. Ian Hunt, Department of Chemistry, University of Calgary