Major and Minor Resonance Structures

Add yours
Resonance Made Easy! Finding the Most Stable Resonance Structure - Organic Chemistry

Tip: This isn't the place to ask a question because the teacher can't reply.

Key Questions

  • WHAT IS A MAJOR RESONANCE STRUCTURE?

    The most stable structural "snapshots" of a molecule's electron distribution is its major resonance structure. Let's suppose that we examine the resonance structures of urea, #"H"_2"NCONH"_2#.


    The curved arrows indicate the movement of electrons that lead to a new resonance contributor. We start at I and continuously move clockwise.

    FORMAL CHARGES & ELECTRONEGATIVITIES

    First, see how structures III and IV do not have minimized formal charges? Although there is net charge cancellation, nitrogen, being less electronegative than oxygen, tends less to pull electrons towards it; therefore, the major (most stable) resonance structure gives the most electron density to oxygen.

    That means I is the major resonance contributor. That automatically means III and IV are the minor resonance contributors (second-most stable).

    DEGENERACIES

    Also, notice how structures III and IV are actually identical; just reflect them along a vertical axis and they are the same. Those would be known as degenerate structures, i.e. structures that have the same energy as each other.

    #E_"III" = E_"IV"#

    OCTET RULE

    Finally, see how structure II leaves carbon without an octet? You may have learned that atoms prefer to satisfy the octet rule.

    Because of a lack of that here, structure II is the most minor resonance contributor (least stable) of these four.

    STABILITY & RESONANCE CONTRIBUTIONS

    In the end, we have this spectrum of stabilities and resonance contributions:

    I > [III = IV] > II

  • Minor resonance structures are all the resonance contributors that are higher in energy than the lowest-energy contributor.

    For example, we can draw three possible contributors for formamide, HCONH₂.

    Major

    We have to decide which of these is the lowest-energy form. That one will be the major contributor. All the others will be minor contributors.

    In order of importance, some rules that enable you to decide are:

    • Satisfy the octet rule.
    • Minimize charge separation.
    • Put negative charge on the more electronegative atom.

    Structures III and IV both obey the octet rule, but III has no charge separation. Structure V does not give the C atom an octet.

    III is the major contributor. So IV and V are minor contributors.

    IV is a more important contributor than V, because it obeys the octet rule.

    We could say that IV is a minor contributor and V is a very minor contributor.

  • We need to be careful of the cause/effect of this. It's not that certain resonance structures are stable because they occur most often, but that the resonance structures that represent the most stable state of a molecule occur most often.


    Molecules always strive for achieving the minimum energy, whether through electronic relaxations, electron delocalization, or other processes.

    MINIMUM ENERGY

    Minimum energy is analogous to not drinking too much coffee in the morning. If you drink too much coffee in the morning, you might get too hyper over the course of the day, and I don't think anyone really wants to be overly hyper. So, you drink the minimum amount of coffee so you can just stay awake.

    Similarly, molecules don't want to be overly excited/hyper, and instead want to achieve the minimum energy, or ground-state energy.

    #pi#-ELECTRON DELOCALIZATION

    http://mcat-review.org

    Delocalizing the electrons in a system with many #pi# electrons helps make that happen in molecules that we draw as resonance structures. The more room (orbitals) the electrons have available to move, the more distributed their kinetic energy can be, and in some sense, the less energy "buildup" there would be in select orbitals.

    You can also analogize electron delocalization with glasses of water. To achieve the smallest amount of water in multiple glasses, you should get the same amount in all glasses, not pour it all into one glass.

    So overall, that's why resonance structures that represent the most stable state of a molecule are the ones that occur most often.

Questions