Question on acidity of benzene rings VS alkanes

[Xdentallos]

So my question is, on a benzene ring with a COOH when we add an electron withdrawing group (NO3 for example) the benzene gets more acidic.

However when we add an electron donation group (like F) to an alkane like propyl-OH it makes it more acidic? but i though electrons withdrawing groups make it more acidic? or does this switch when you go from a benzene ring to an alkane? Thanks for your time guys!

They also referred to F as a withdrawing group but when I look at the groups for benzene ring activation and deactivating F id under activation electron DONOR

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[DAT Destroyer]

So my question is, on a benzene ring with a COOH when we add an electron withdrawing group (NO3 for example) the benzene gets more acidic.

However when we add an electron donation group (like F) to an alkane like propyl-OH it makes it more acidic? but i though electrons withdrawing groups make it more acidic? or does this switch when you go from a benzene ring to an alkane? Thanks for your time guys!

They also referred to F as a withdrawing group but when I look at the groups for benzene ring activation and deactivating F id under activation electron DONOR

What a perspicacious student you are !!!! You have asked probably one of the best question I have EVER seen in this forum. Let us first look at Fluorine . On a benzene ring......it donates electron density a bit.....it also withdraws electron density a bit...... Calculations have shown that fluorobenzene is about as activated as a benzene ring alone. Also a fluorine can overlap with a carbon atom in the resonance hybrid...a carbocation that we call an Arenium ion....to give some stability to this carbocation intermediate. Why ? The carbon and Fluorine orbital are of very similar size... they both use 2p orbitals. Iodine is much larger, thus resonance is poorer in iodobenzene because carbon uses a 2p orbital, Iodine uses a 5 p orbital. Thus Iodobenzene is less reactive than fluorobenzene. Now lets place the Fluorine atom on a compound such as acetic acid. We see an INCREASE in acidity hence a DECREASE in pKa value....why ? The Fluorine gives ZERO resonance,,,,,,and only an inductive effect, the ANION IS STABILIZED !!!!! In the benzene compounds,,,,,,we have a CARBOCATION......and it is destabilized by electron withdrawing groups. Thus I hope you can see my point. Fluorine somewhat hurts you in a benzene reaction since a carbocation is generated and NOT stabilized,,,,,,but in an acidic compound,,,,,, we deal with an ANION being stabilized and not a carbocation. I hope this helps. Your question was very very exciting !!!!

Dr. Romano

 

[Xdentallos]

Thank you Dr. Romano! Yes I was going through your DAT destroyer book and noticed that trend so I decided to ask. So electron donating groups in the benzene ring hurts the acidity but in an acidic compound it helps acidity, and this is because of the type of charge that is being stabilized. If that is correct then I believe I understood!

so the same is true for electron withdrawing when added to a benzene ring it helps acidity and when added to a acidic compound it hurts acidity?

 

[DAT Destroyer]

Thank you Dr. Romano! Yes I was going through your DAT destroyer book and noticed that trend so I decided to ask. So electron donating groups in the benzene ring hurts the acidity but in an acidic compound it helps acidity, and this is because of the type of charge that is being stabilized. If that is correct then I believe I understood!

so the same is true for electron withdrawing when added to a benzene ring it helps acidity and when added to a acidic compound it hurts acidity?

Almost correct.......Electron withdrawing groups enhances acidity......but on a benzene ring.....these groups will be DEACTIVATING and slow you down !!!!!