C/P Section Bank, Q29

[mariposas905]

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Why is the answer Compound 2b instead of 2a? Don't they have the same set of double bonds (10 each)?

 

[pretty_positron]

2b has more conjugated double bonds which result in colored compounds. This is more important than whether they're aromatic or not.

 

[mariposas905]

2b has more conjugated double bonds which result in colored compounds. This is more important than whether they're aromatic or not.

Okay, so more conjugation means more delocalization of electrons?

 

[pretty_positron]

Okay, so more conjugation means more delocalization of electrons?

Yeah, conjugation gives you a pi bond every other carbon so it's very easy for electrons to move from HOMO to LUMO orbitals, reducing the energy difference between these two. This results in longer wavelength which happens to be in the visible spectrum.

For uncolored compounds, they still of course give off photons but just in shorter frequencies that we can't see.