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Inductive or Resonance? (pKa discussion)
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Nora Roy
Inductive or Resonance? (pKa discussion)
Thank you mjc123 for the comment and spotting the mistake in my drawing of mesomeric structures. However, it has always confounded me on why the inductive effect can impact the acidity of the carboxylic acid with the similar impact as the mesomeric effect–– by decreasing the pKa value by 0.1. I would have expected the decrease to be even more marginal than one that is attributed to mesomeric effect. Could someone kindly direct me in the right direction and give my some insight on the interpretation. Thanks!!
Thank you mjc123 for the comment and spotting the mistake in my drawing of mesomeric structures. However, it has always confounded me on why the inductive effect can impact the acidity of the carboxylic acid with the similar impact as the mesomeric effect–– by decreasing the pKa value by 0.1. I would have expected the decrease to be even more marginal than one that is attributed to mesomeric effect. Could someone kindly direct me in the right direction and give my some insight on the interpretation. Thanks!!
No, that is impossible. You can tell something is wrong because the charges don't add up - the total charge is zero for structures 1, 2 and 3, +2 for structure 4 and +1 for structure 5. Going from 3 to 4 you are pushing electrons onto an O- - there is no room for that. 4 should have a double bond between C1 and C2, and a 2- on the carbonyl O (assuming that's possible, which it isn't). 4 to 5, you are taking electrons away from an O+ - is that likely? 5 should have (in addition to the corrections for 4) a double bond between C3 and C4, and a 2+ on the hydroxyl O (not C3). Structures 4 and 5 are impossible (as resonance structures of m-hydroxybenzoic acid), so m-OH does not mesomerically affect the acidity of the acid. You need to revise the rules for forming resonance structures.
No, that is impossible. You can tell something is wrong because the charges don't add up - the total charge is zero for structures 1, 2 and 3, +2 for structure 4 and +1 for structure 5. Going from 3 to 4 you are pushing electrons onto an O- - there is no room for that. 4 should have a double bond between C1 and C2, and a 2- on the carbonyl O (assuming that's possible, which it isn't). 4 to 5, you are taking electrons away from an O+ - is that likely? 5 should have (in addition to the corrections for 4) a double bond between C3 and C4, and a 2+ on the hydroxyl O (not C3). Structures 4 and 5 are impossible (as resonance structures of m-hydroxybenzoic acid), so m-OH does not mesomerically affect the acidity of the acid. You need to revise the rules for forming resonance structures.
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You are comparing which compounds , I got confused , please elaborate little .
You are comparing which compounds , I got confused , please elaborate little .
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Going from 3 to 4 you are pushing electrons onto an O- - there is no room for that. 4 should have a double bond between C1 and C2, and a 2- on the carbonyl O (assuming that's possible, which it isn't). 4 to 5, you are taking electrons away from an O+ - is that likely? 5 should have (in addition to the corrections for 4) a double bond between C3 and C4, and a 2+ on the hydroxyl O (not C3). Structures 4 and 5 are impossible (as resonance structures of m-hydroxybenzoic acid), so m-OH does not mesomerically affect the acidity of the acid.
You need to revise the rules for forming resonance structures.
Going from 3 to 4 you are pushing electrons onto an O- - there is no room for that. 4 should have a double bond between C1 and C2, and a 2- on the carbonyl O (assuming that's possible, which it isn't). 4 to 5, you are taking electrons away from an O+ - is that likely? 5 should have (in addition to the corrections for 4) a double bond between C3 and C4, and a 2+ on the hydroxyl O (not C3). Structures 4 and 5 are impossible (as resonance structures of m-hydroxybenzoic acid), so m-OH does not mesomerically affect the acidity of the acid.
You need to revise the rules for forming resonance structures.
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