Question

Question: Consider the compounds and ions with curved arrows drawn below. When the curved arrows give a second valid resonance structure, draw the resonance structure. When the curved arrows generate an invalid Lewis structure, explain why the structure is unacceptable.

a.

b.

c.

d.

Short answer

Answer

Compounds a and d do not give a valid Lewis structure.

b.

c.

Step-by-step solution

Step-by-Step solutionStep 1: Making resonance structures

The bonded and nonbonded pairs of electrons are moved in the direction of the curved arrows to give a new structure.

In doing the above changes, the number of bond pair and lone pair of electrons, the structure, and the connectivity of atoms must remain the same.

Checking the octet configuration

The octet of all the atoms other than hydrogen must be checked. If any atoms do not have a complete octet, then the new structure is not valid.

The new structure should have the same number of lone pairs of electrons as in the given structure.

Resonance structures of the given compounds

a. The structure obtained by following the given curved arrow contains a carbon center with an incomplete octet and a pentavalent carbon center. So, the structure formed is unstable and not a valid Lewis structure.

Invalid Lewis structure obtained

b. The structure obtained following the curved arrows has all the atoms with a complete octet, and all the carbon atoms are tetravalent. Thus, the new structure obtained is a valid Lewis structure.

New resonance structure of b

c. In the new structure, one of the bonds from the triple bond gets shifted as electron density on the nitrogen atom resulting in the generation of a positive charge on the carbon atom and a negative charge on the nitrogen atom.

Hence, the new Lewis structure is valid.

New resonance structure of c

d. In the new structure, one of the carbon atoms becomes pentavalent. Hence, this structure is not a valid Lewis structure.

Invalid Lewis structure obtained