Finding Resonance Structures

Finding Resonance Structures Made Easy! - Part 1 - Organic Chemistry

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1 of 2 videos by Frank W.

Key Questions

Resonance structures show different areas of reactivity from which different reactions can occur dependent on the conditions the substrate is in.

Explanation:

Resonance structures show the electron movement as it 'swings' between one overall electron layout and the next (can be 2 or more different resonance structures depending on the electronegativity areas of the compound).

Typically the charge is shown as a shared distribution when no breakdown of the resonance structures is given. Because this is a generalisation and not fully representative of how certain reactions take place it can then be hard to see how and why a compound reacts the way it does. By showing the various structures of resonance you're able to see where electrons are 'hanging out' and therefore where the compound will react with another compound.

So some would argue you should always show the resonance structures as part of your workings so you become familiar with a compounds areas of reactivity (which is what chemistry is all about!).

They usually just are.

Explanation:

Electrons are delocalized within the molecule and through this process the molecule gains extra stability.

In fact, elements that could be unstable at first are made stable by their resonance structures. For example, Ozone with both of its opposite charges creates a neutral molecule and through resonance it becomes a stable molecule.

Because it helps to explain and rationalize experiment; if you like it explains reality.

Explanation:

Ozone is a bent molecule. That's the experimental fact. A Lewis structure of $O = \stackrel{+}{O} - {O}^{-}$ in which there are 3 regions of electron-density around the central atom explains this geometry. It also explains the trigonal planar geometry of $C {O}_{3}^{2 -}$. The benzene molecule, ${C}_{6} {H}_{6}$, crops up persistently in organic chemistry. It is reactive under certain circusmtances, but not as reactive and not reactive in the same way as an olefin or an alkyne.

Our ideas of resonance can help rationalize the observed reactivity. The idea of $6$ $\pi$ electrons delocalized around a 6-membered ring can be extended to inorganic chemistry. Borazine, ${B}_{3} {N}_{3} {H}_{3}$, a benzene analogue, is isostructural and isoelectronic with benzene, and has similar aromatic chemistry.

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