A few questions about aromatic rings and electron donating/withdrawing groups.

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Astra

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  1. How do electron donating and electron withdrawing groups affect stability of the ring?

  2. How can you tell if a substituent is electron withdrawing or electron donating?

  3. Are there trends on the periodic table that allow you to tell if something is electron donating or electron withdrawing?

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1) You can't really have some absolute standard of measurement for "stability" of a ring. You could measure it by seeing whether EWG or EDG activate or de-activate a ring for a certain purpose, e.g. electrophilic aromatic substitution or nucleophilic aromatic substitution. EWG and EDG have differing consequences depending on what property you're interested in. In general, EWG will withdraw electron density from the ring, making it more electron poor. EDG will donate electron density into a ring, making it more electron rich. So say you have a phenyl ligand and you want to deactivate the metal center towards a certain redox reaction by making it more basic. You would attach a EDG to the ring to increase the electron density that's being pushed towards the metal.

2) While there are rules, it takes a bit of practice and experience. Strongly electronegative substituents are generally electron-withdrawing. So for example, think CF3, cyanides, acyl chlorides, etc. Electron-donating groups include ethers, hydroxides, pi systems, and amines. Generally, these tend to be fairly basic (e.g. hydroxide or an amine). This is a good chart that summarizes some properties of common substituents: http://www.mhhe.com/physsci/chemist...ics/carey04oc/ref/ch12substituenteffects.html

3) Electronegativity will tell part of the story but you also must consider pi effects. For instance, you would expect an ether or hydroxide to be electron-withdrawing by electronegativity alone, but there are lone pairs on the oxygen that can donate into the ring and this effect outweighs the electronegativity factor. You wouldn't know that unless you've done experiments on it and that's where experience comes in but the chart I cited above is pretty useful.
 
Thanks aldol!

The complexity makes sense because in a passage, they gave the stability values for each molecule. I was just wondering if it was possible to know the effects on our own, but it looks like there are multiple factors to take into account
 
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