Question

Which are pair of Resonating structures?

• A. $H-C \equiv \overset{(+)}{N}H$ & $H-\overset{(+)}{C}=\overline{N}H$
• B. $CH_3CH=CHCH_3$ & $CH_3CH_2CH=CH_2$
• C. The two species, $CH_3CH=CHCH_3$ (2-butene) and $CH_3CH_2CH=CH_2$ (1-butene), are structural isomers. They have different atomic connectivity and are different molecules, not resonance structures of the same molecule.
• D. The structures show a 6-membered ring with alternating double and single bonds and a negative charge on different carbon atoms. This represents the delocalization of a negative charge in a conjugated system, such as a phenyl carbanion ($C_6H_5^-$). The atomic connectivity is the same, and the electrons (negative charge and pi bonds) are redistributed, making them resonance structures.

Answer 1

Answer: (A), (D)

Options 1 and 4

Answer 2

Resonating structures are Lewis structures of a molecule or ion that differ only in the arrangement of electrons, particularly pi electrons and lone pairs. The connectivity of atoms must remain the same.

Option 1: $H-C \equiv N^+-H$ and $H-\overset{+}{C}=N-H$ (with a lone pair on N). These have the same atomic connectivity and differ in electron distribution.

Option 4: A 6-membered ring with alternating double bonds and a negative charge on different carbons. This shows delocalization of charge in a conjugated system.

Options 2 and 3 represent structural isomers, not resonance structures, as they have different atomic connectivity.