Q56. - Organic Chemistry

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.

Step-by-Step Solution

Answer

Answer

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

 b.

c.

Step-by-Step solution Step 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.

Step 2: 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. 

Step 3: 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