Chapter 2.7, Problem 2.38P

Interpretation Introduction

Interpretation:

Resonance structure has to be drawn for the given compound.

Concept Introduction:

Resonance is a way of representing a structure which has the same connectivity but the electrons are distributed differently in the structures.  In resonance structure drawing, the electrons are considered as clouds.  These electron clouds very often spread across a large region of a molecule.  Curved arrows are the best way to represent the resonance structures.  With the help of curved arrows, the movement of electron can be shown.  Every curved arrow comprises of a head and tail.  Tail shows the place from where electrons are coming and head shown the place where electrons are going.  It is not the electrons migrating as such.  We treat the electrons to migrate to draw all resonance structures.  For drawing curved arrows, two important commandments has to be always followed.  They are,

• Thou shall not break a single bond.
• Thou shall not exceed an octet for second-row elements ($\text{C,}\text{N,}\text{O,}\text{F}$).

Resonance structures should have the same connectivity.  Hence, breaking of a single bond is not at all allowed.

Second-row elements should never exceed the octet.  This is because they have only four orbitals in their valence shell.  The sum of bonds and the lone pairs for second row elements should never exceed the number four.

Three conversions can be carried out for drawing resonance structure and they are,

    Convert lone pair of electrons into pi bond obeying both commandments

    Convert pi bonds into lone pair of electrons obeying both commandments

    Convert pi bonds into pi bonds obeying both commandments

Resonance structure can also be drawn by recognizing patterns.  There are five patterns that can be recognized to draw resonance structures.  They are,

• Lone pair present next to pi bond
• Lone pair present next to ${\text{C}}^{\text{+}}$
• pi bond present next to ${\text{C}}^{\text{+}}$
• pi bond present between to atoms, out of which one atom is electronegative
• pi bonds present all the way around the ring