Ch 5: Pushing Curly arrows

Chapter 5: Pushing Curly Arrows

Balancing Charges

This is a simple tool to help avoid / check for errors when using curly arrows.
In any individual mechanistic step, the overall charge of the reactents MUST EQUAL the overall charge of the products.
This is a simple check to undertake after each step to make sure that appropriate charges have been shown
The same principles apply to other applications of curly arrows such as deriving resonance structures.

Balance the charges ! The overall charge in any particular step of a mechanism (or scheme) should always be balanced with the previous step

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 (or scheme)
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:
- if an arrow takes e away from an atom, that atom will become more +ve
- if an arrow gives e to an atom, that atom will become more -ve

EXAMPLES

Before : The starting materials are neutral and positive, so we have an overall +ve charge in the starting materials.
After : The O atom in product is +ve, so the product is overall positive.

Arrow begins at a lone pair on the neutral O atom and goes to the +ve H atom forming a new O-H bond.
This means the neutral O in the reactents becomes +ve in the products and the +ve H gained e so it became neutral.

Before : The "starting material" has no formal charges so it is overall are neutral.
After : The O atom in the "product" is -ve, and the N in the "product" is +ve so the "product" is overall neutral.

The first arrow begins at the lone pair on the neutral N atom, and it goes to make a new bond to the C, but then the second arrow takes the e from a CO bond and gives them to the O.
This means the neutral N in the "starting material" becomes +ve in the "product", the C remains neutral, and the O becomes -ve.