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Drawing Resonance Structures


									CHEM 109A                                                                            CLAS
                             Drawing Resonance Structures
  Resonance structures (a.k.a. resonance contributors) are structures drawn to show
  the possible locations of delocalized electrons. Use a double headed arrow (↔)
  between structures to indicate that they are resonance contributors, and curved arrows
  to show how e-s “moved”. Resonance structures differ from each other ONLY in the
  location of the e-s and lone pairs (they must have same atoms connected in the same
  order. If the connectivity or orientation of the atoms changes then the structures are
  considered isomers which we will discuss in Ch 5). Resonance contributors are
  representations of what the molecule looks like at a given instance in time. The true
  molecular structure is a combination or hybrid of the most significant (read stable)
  resonance contributors.

  Drawing Resonance Strucutures/Contributors:
  See also Special Topic V: Drawing Resonance Contributors in Study Guide and
  Solutions Manual of text.
  1. Draw a Lewis structure (be sure to include all lone pairs)
  2. Use curved arrows to show how electrons (e-) move:
      Tail of a curved arrow shows where e- came from (multiple bond or lone pair,
  never a + charge or empty space).
      Head of curved arrow shows where e- go to (point between atoms to form a
  multiple bond or point to an atom – usually with a positive charge or fairly
  electronegative – to form lone pair, never into a – charge and only very rarely into
  empty space (EX. Diels-Alder reactions, radical reactions)).
      Two rules/commandments
              1. Do NOT break a single bond.
              2. Do NOT exceed an octet for second-row elements (C, N, O, F) – can
                  have less than an octet (use formal charge) but NOT more – or 12
                  electrons for a third-row element (P, S, etc).

              R        violates rule 2
                                                               violates rule 1
     N    N
     +        OK

  For example…
  1. For each set of drawings below, draw the curved arrow(s) that get from one
     structure to the next.


                  N+                     N                d.

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CHEM 109A                                                                                                                    CLAS
                                                Drawing Resonance Structures
                            O                                           O-

                  e.   -O                                   O

  2. For each of the compounds below, draw all resonance contributors.
                            O                                                                                   O

                  a.            O


                  c.            Cl

  Predicting Relative Stability of Resonance Structures/Contributors:
  Factors that INCREASE Stability               Factors that DECREASE Stability
      complete octets on all atoms                        incomplete octets
         no formal charge (FC)                       separated charges/large FC/
                                                   like charges on adjacent atoms
          more covalent bonds                           fewer covalent bonds
    negative FC on more EN atoms                   negative FC on less EN atoms
     positive FC on less EN atoms                  positive FC on more EN atoms

  Some examples comparing stabilities
                                                                             H 2C        N
                                                                                                  N-        X         H 2C      N       N

  HO              H               +HO                 H
                                     less stable
         O+                     O                                  O-

         C-                                                        C+
                                                                             H 2C       N         N                 H 3+ C    N        N-
  least stable              most stable
                                                                                    less stable                          less stable
          N       N    N-                            +N    N        N-
                  +                                        +
  H3 C                                        H 3C
                                                     less stable

  The more stable a resonance structure, the lower its energy and the more it
  contributes to the structure of the hybrid/actual molecule. The hybrid can be thought
  of as a weighted (based on stability) average of the resonance structures. In general,
  the more resonance structures you can draw for a particular compound, the more
  stable that compound – this is attributed to Resonance Stabilization Energy (RSE).
  RSE is defined at the difference in energy between the energy of the hybrid and the
  estimated energy of the lowest energy resonance contributor.
  RSE = Ehybrid-Elowest E contributor

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CHEM 109A                                                                          CLAS
                                        Drawing Resonance Structures
  EX. Which is more stable, methoxide ion or methanoate ion?

                                               O-        H       O-

  The methanoate ion is more stable b/c is has 2 resonance structures (& therefore the
  hybrid has RSE) whereas methoxide ion has only 1.
         O                    O-

     H           O-       H         O

  For example…
  3. Circle the most significant resonance contributor(s) in 1 & 2 above.

  Some examples of BAD resonance structures & why is each bad
  Atoms (nuclei) are not in the same positions – not the same connectivity between


  The two structures do NOT have the same number of unpaired electrons

  (                         this is an acceptable resonance structure, but it is not
  common to use these “no-bond” resonance structures)


             O                     O-

  These structures can NOT conform to any molecular geometry – too much strain

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