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Topic: Determine acidity between compounds  (Read 1803 times)

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Offline owlpower

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Determine acidity between compounds
« on: November 24, 2018, 10:29:37 AM »
Question: How to arrange the molecules in increasing order of Bronsted acidity?

I know that mineral acid will have the greatest acidity, thus VI is most acidic.

This is followed by carboxylic acids, I and II. II has an electron donating group, which makes it less stable than I.

So I know the ranking will be
X,X,X,II,I,VI.

However, between III, IV and V, I have trouble determining the acidity between them.
IV will be anti-aromatic when it loses H+, which makes it really unstable (hence weak acid).
Not very sure about esters, but the only way it can be an acid is to donate H+ from the Me group.
For alkynes, I know it's rather acidic too (compared to alkanes and alkenes at least).

My guess would be that the ranking is IV,III,V,II,I,VI.
Any thoughts?

Offline Babcock_Hall

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Re: Determine acidity between compounds
« Reply #1 on: November 24, 2018, 01:21:05 PM »
Your thinking about IV is incorrect.  I would also think about resonance delocalization and which acids might see an effect from this.

Offline owlpower

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Re: Determine acidity between compounds
« Reply #2 on: November 24, 2018, 10:40:48 PM »
IV is resonance stabilised. Ester has electron donating Me group which decreases stability. So I guess III,V,IV,II,I,Vi

Offline Babcock_Hall

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Re: Determine acidity between compounds
« Reply #3 on: November 26, 2018, 08:02:44 PM »
IV is also aromatic, not antiaromatic.  The reason why ethyne has a lower pKa than an alkane is very different from the reason why cyclopentadiene has a lower pKa, and I think it is difficult to compare the two without looking up the pKa values for that reason.  I don't see obvious problems with your order.

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