Chapter 5: Pushing Curly Arrows

Steps to mastering curly arrows

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

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)
known as "double headed arrow"		known as "fish hook arrow"

We will focus on the more common arrows here to begin with because (1) they are more common and (2) they are easier to master (they look and are less complex). Remember that a bond is due to a pair of electrons between atoms  (HINT : think of the Lewis structures)

Always remember that curly arrows show what the electrons are doing (NOT the atoms or molecules !)

Double headed 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

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 (usually) "oppose each other" (i.e. converge or diverge)

• Avoid the temptation to compress several mechanistic steps into a single diagram since this makes it difficult to find and correct errors.  Draw out each reaction step.

© Dr. Ian Hunt, Department of Chemistry