Electron Withdrawing/Donating

[moose45]

"Electron donation and withdrawal helps to explain many of the reactions in MCAT organic chemistry. For instance, placing an electron withdrawing group on the alcohol increases its acidity by reducing the negative charge on the conjugate base."

I read this in a book and have a question based on it. If the negative charge is reduced, then wouldn't the oxygen be less reactive and how is that related to the alcohol increasing in acidity?

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[G1SG2]

"Electron donation and withdrawal helps to explain many of the reactions in MCAT organic chemistry. For instance, placing an electron withdrawing group on the alcohol increases its acidity by reducing the negative charge on the conjugate base."

I read this in a book and have a question based on it. If the negative charge is reduced, then wouldn't the oxygen be less reactive and how is that related to the alcohol increasing in acidity?

Yes, the oxygen would be less reactive. Think about conjugate acid/base pairs. The weaker the conjugate base, the stronger the acid. The stronger the conjugate base, the weaker the acid. In this case, if you're reducing the negative charge, you're weakening the conjugate base, thus increasing the acidity of the acid.

 

[moose45]

I got that part. What I am asking is how do you know what makes a molecule more reactive?

What I have read is that by adding electron withdrawing groups, you are adding more negative charge to the molecule, making the negative charge on oxygen less stable and so does that mean its more reactive and thus wants to react with some partial positive charge somewhere else???

 

[G1SG2]

What I have read is that by adding electron withdrawing groups, you are adding more negative charge to the molecule, making the negative charge on oxygen less stable and so does that mean its more reactive and thus wants to react with some partial positive charge somewhere else???

No, you do not add more negative charge to the molecule by adding electron withdrawing groups, you decrease the negative charge on the oxygen (electron withdrawing groups withdraw electrons/negative charge). If you add more electron donating groups, you increase the negative charge on the oxygen, making it more unstable. For example, a tertiary alcohol would be weaker than a primary alcohol because it has more electron donating alkyl groups on it. The more electron donating groups, the more negative charge on the conjugate base, the stronger the conjugate base, the weaker the acid. So, I suppose in order to make the oxygen more reactive to a partial positive charge, you would have to increase its negative charge, which can be accomplished by adding electron donating groups, not withdrawing.

 

[moose45]

oh man, i made a mistake, i meant donating not withdrawing so everything you said makes total sense. Let me wrap it up and agree or disagree with me if you may:

By adding electron donating groups to an alcohol, like CH3, you are adding negative charge to the oxygen..which increases basicity and decreases the acidity and with that the hydrogen atom is off, the oxygen is less stable and more reactive wanting to attach to a positive charge somewhere.

By adding electron withdrawing groups to an alcohol, like F, you are decreasing the negative charge on the oxygen..which leaves the oxygen more stable and it has increase in acidity and decrease in basicity.

 

[sleepy425]

By adding electron donating groups to an alcohol, like CH3, you are adding negative charge to the oxygen..which increases basicity and decreases the acidity and with that the hydrogen atom is off, the oxygen is less stable and more reactive wanting to attach to a positive charge somewhere.

I just wanted to point something out to be thorough. I hear this all the time and I think it is extremely important to clear it up. In an acid/base reaction (Bronsted), we are not dealing with gain or loss of a hydrogen atom. We are dealing with gain or loss of a proton, or "cationic hydrogen." A hydrogen atom is one proton, one electron, so if you lose a hydrogen atom from an alcohol, you make a neutral oxygen radical. This is not stable, and oxygen radicals are stabilized by the opposite things that stabilize an oxyanion (negatively charged oxygen).

When you lose just a proton, you don't lose that electron, and you're left with an oxyanion that is stabilized by whatever diminishes the effective negative charge on the oxygen (so electron withdrawing groups).

My point is that I hear the word hydrogen used to describe protons, hydrogen atoms, and hydrides, when in reality the word only describes hydrogen atoms. It is important to use the right word, because it can lead to a lot of confusion otherwise.

 

[G1SG2]

oh man, i made a mistake, i meant donating not withdrawing so everything you said makes total sense. Let me wrap it up and agree or disagree with me if you may:

By adding electron donating groups to an alcohol, like CH3, you are adding negative charge to the oxygen..which increases basicity and decreases the acidity and with that the hydrogen atom is off, the oxygen is less stable and more reactive wanting to attach to a positive charge somewhere.

By adding electron withdrawing groups to an alcohol, like F, you are decreasing the negative charge on the oxygen..which leaves the oxygen more stable and it has increase in acidity and decrease in basicity.

Yes, pretty much!