Chad's Quiz Acidity Question: I'm so freakin confused!

[toothhornet88]

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I thought that the more Donating groups you have the more stable the molecule is===> less acidic! and In my opinion I think that Anhydride>Formaldehyde>Aldehyde>Ketone

Also pKa of Formaldehyde is 13.3
pKa of aldehyde is 17
pKa of ketone is 20

So why is the answer A, I don't get it!

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[flin5845]

">
I thought that the more Donating groups you have the more stable the molecule is===> less acidic! and In my opinion I think that Anhydride>Formaldehyde>Aldehyde>Ketone

Also pKa of Formaldehyde is 13.3
pKa of aldehyde is 17
pKa of ketone is 20

So why is the answer A, I don't get it!

The carbons on a ketone or aldehyde can backdonate through hyperconjugation stabilizing the cation

 

[wizi]

For II and IV: Just by looking at it you can tell II is the most acidic one, and IV is the least acidic.

C A R DI O. We use R which is Resonance -> II: most acidic and IV: least acidic.

For I and III: just like you said, ketone's pKa > aldehyde's pKa -> aldehyde is more acidic.

 

[2PacClone23]

i too was staring at this question hardcore...


i think theyre asking about the acidity of the alpha hydrogen rather than the acidity of the whole compound....

 

[toothhornet88]

For II and IV: Just by looking at it you can tell II is the most acidic one, and IV is the least acidic.

C A R DI O. We use R which is Resonance -> II: most acidic and IV: least acidic.

For I and III: just like you said, ketone's pKa > aldehyde's pKa -> aldehyde is more acidic.

i too was staring at this question hardcore...


i think theyre asking about the acidity of the alpha hydrogen rather than the acidity of the whole compound....

Yeah Thank you guys very much! I totally forgot to use the CARDIO all the way! reasonance is key here

 

[amg09]

what is this CARDIO method?

 

[toothhornet88]

what is this CARDIO method?

CHAD talked about it in one of his videos. It's a way he uses to predict the basicity of a compound.

C-charge (more negative---> stronger conjugate base)
A-atom (where basic e- reside)
R- resonance (more reasonance ---> weaker conjugate base)
D- dipole (more electronegative atom---> weaker conjugate base)
I - induction (more closer the EWG the more stable it will make the compound----> weaker conjugate base)
O- orbital (sp3>sp2>sp)

So you draw out a conjugate bases of each acid and use the CARDIO method to rank from most basic to least basic. So the most basic conjugate base is going to be the least acidic acid and the least basic conjugate base will be the most acidic. That's CARDIO, very good mnemonic 🙂

 

[wired202808]

CHAD talked about it in one of his videos. It's a way he uses to predict the basicity of a compound.

C-charge (more negative---> stronger conjugate base)
A-atom (where basic e- reside)
R- resonance (more reasonance ---> weaker conjugate base)
D- dipole (more electronegative atom---> weaker conjugate base)
I - induction (more closer the EWG the more stable it will make the compound----> weaker conjugate base)
O- orbital (sp3>sp2>sp)

So you draw out a conjugate bases of each acid and use the CARDIO method to rank from most basic to least basic. So the most basic conjugate base is going to be the least acidic acid and the least basic conjugate base will be the most acidic. That's CARDIO, very good mnemonic 🙂

i came up with a quick shortcut
instead of ranking by conjugate bases and getting all confused and flustered. Heres what I wrote up
Base Acid
C - charge + charge
A Biggest Smallest
R Least Most
DI EDG EWG
O sp > sp2 > sp3 (then this is reversed for acids)

so when im being asking bout acidity, i just look for the smallest atom or the one that has most resonance or the one that has an EWG group.

Those 3 things usually work out the fastest. Try it out let me know how it works for you.

 

[toothhornet88]

i came up with a quick shortcut
instead of ranking by conjugate bases and getting all confused and flustered. Heres what I wrote up
Base Acid
C - charge + charge
A Biggest Smallest
R Least Most
DI EDG EWG
O sp > sp2 > sp3 (then this is reversed for acids)

so when im being asking bout acidity, i just look for the smallest atom or the one that has most resonance or the one that has an EWG group.

Those 3 things usually work out the fastest. Try it out let me know how it works for you.

I like the way you put that...however for A its supposed to be reverse (larger---> more stable, thus is a weaker base)
Also the more electronegative---> more stable, however when you are comparing two atoms from same row, size takes priority

 

[wired202808]

I like the way you put that...however for A its supposed to be reverse (larger---> more stable, thus is a weaker base)
Also the more electronegative---> more stable, however when you are comparing two atoms from same row, size takes priority

you're right man thanks for the correction. you know any place i can practice ranking acids and bases, i feel like destroyer does not cover enough of that.

 

[toothhornet88]

you're right man thanks for the correction. you know any place i can practice ranking acids and bases, i feel like destroyer does not cover enough of that.

Yeah I know what you mean!

Here is a link that I found on SDN that is really good. It has pdf files of questions/solutions

http://ocw.mit.edu/courses/chemistry/5-12-organic-chemistry-i-spring-2005/assignments/

the first problem set and exam 1 has acidity ranking questions