Why are orbitals ranked last in CARDIO? (DAT question of the day question)

[510586]

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Hi everyone. In Chad's videos, orbitals is ranked last in the cardio rule. However, in today's DAT bootcamp the 1-propyne was ranked as more acidic than the benzene, which has more resonance structures. Can someone explain the correct order and if there are more exceptions?

http://datquestionoftheday.com/category/organic-chemistry/
http://datquestionoftheday.com/wp-content/uploads/2013/07/dat77.png

 

[FeralisExtremum]

In the conjugate base of benzene: the benzene ring is aromatic but the lone pair is orthogonal to the pi system of the aromatic ring and can't participate in resonance, so the benzene anion isn't actually particularly stabilized.

 

[DAT Destroyer]

Hi everyone. In Chad's videos, orbitals is ranked last in the cardio rule. However, in today's DAT bootcamp the 1-propyne was ranked as more acidic than the benzene, which has more resonance structures. Can someone explain the correct order and if there are more exceptions?

http://datquestionoftheday.com/category/organic-chemistry/
http://datquestionoftheday.com/wp-content/uploads/2013/07/dat77.png

This is a dangerous game to play. Resonance usually trumps all other factors when it comes to acidity. However, when it comes to carbons that have different hybridizations orbital considerations are usually the more important factor. An sp orbital has electrons closer to the nucleus and renders a more stable anion.The more stable the anion, the more acidic the compound. I suggest you get a reputable book such as David Klein or Wade who have done an excellent job and gives many examples of how resonance and orbitals act to stabilize an anion and make a molecule acidic.

Dr. Romano

 

[510586]

Alright thank you I will add this to my notes. I'm sure after doing a few more chad quizzes and some destroyer problems any other special cases will be found. Thank you for your help Dr. Romano!

 

[510586]

The anion formed from removing a proton from benzene isn't aromatic anymore, so the benzene anion isn't actually particularly stabilized.

But isn't the acid itself more stable/acidic because of resonance? Do we always have to compare the conjugate to do these problems?

 

[FeralisExtremum]

But isn't the acid itself more stable/acidic because of resonance? Do we always have to compare the conjugate to do these problems?

When you're evaluating acidity, you look at the stability of the conjugate base, and not the stability of the acid itself.

 

[DAT Destroyer]

When you're evaluating acidity, you look at the stability of the conjugate base, and not the stability of the acid itself.

Well stated Mr Feralis!!

Dr. Romano

 

[510586]

When you're evaluating acidity, you look at the stability of the conjugate base, and not the stability of the acid itself.

Hmm looks like the person who taught me this in undergrad just taught me it as a trick to answer basic exams questions better I guess. But if there was carbon resonance, this orbital rule would win out? I'm having a hard time thinking of a carbon only resonance structure as a conjugate base without there being charges.

 

[BYU4you]

Vice versely when you look at BASE strengths, the most stable (before protenation) is the weakest

 

[510586]

So it's not like the most stable compound is the most acidic? Like benzene is quite stable but we would ignore it we only care about the conjugate base of it (before protonation).

 

[BYU4you]

So it's not like the most stable compound is the most acidic? Like benzene is quite stable but we would ignore it we only care about the conjugate base of it (before protonation).

For acidity
The best example is an alcohol vs a carboxylic acid, Deprotenatr it first then look at the stability of what you have left.

You have an O- from the alcohol and you have an O- from the carboxylic acid, however the carboxylates - on the O can resonate with the double bond so that conjugate base is more stable than just the O- conjugate base of the alcohol. Hence carboxylic acid is more acidic.

 

[510586]

Got it thank you BYU!