Question

Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation.
a) $C_6{H}_{5}OH$
b) $C_6{H}_{5}N{O}_2$
c) $C{H}_{3}CH=CH-CHO$
d) $C_6{H}_5-CHO$
e) $C_6{H}_5-\stackrel{+}{C}{H}_2$
f) $C{H}_{3}CH=CH\stackrel{+}{C}{H}_2$

Answer 1

Resonance structure:
Resonance structures are sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule.

a) Resonance structures for $C_6{H}_{5}OH$
The lone pair of electron starts shifting to oxygen - carbon bond and form a double bond character and $\pi$ electron of $C-C$ bond shift to next $C-C$ single bond.


b) Resonance structures for $C_6{H}_{5}N{O}_2$
Here the electrons of $N-O$ bonds shift to oxygen atom (more electronegative) and then the $\pi$ electron of carbon-carbon double bonds starts delocalising towards the $N$ atom.

c) Resonance structures for $C{H}_{3}CH=CH-CHO$
Here the $\pi$ electrons of $C-O$ bond shifts to oxygen atom (more electronegative) and then the $\pi$ electron of carbon-carbon double bonds starts shifting towards the next $C-C$ bond, introducing a partial double bond character.

d) Resonance structures for $C_6{H}_5-CHO$
Here the $\pi$ electron of carbon-oxygen double bond starts shifting towards the electronegative oxygen atom and the $\pi$-electrons of $C-C$ double bonds shifting towards carbonyl group (as shown in figure).

e) Resonance structures for $C_6{H}_5-\stackrel{+}{C}{H}_2$
The $\pi$ - electrons of $C-C$ double bond shift towards the bond to minimise the deficiency of electron density and the rest of the $\pi$ - electrons follow the same process

f) Resonance structure for $C{H}_{3}CH=CH\stackrel{+}{C}{H}_2$
The $\pi$-electrons of $C-C$ double bond shift towards the $C-C^{+}$ bond to minimise the deficiency of electron density at $C{H}_2^{+}$. So, in this way only one resonant is possible.