most acidic hydrogen OCHEM question help

[thefifthbeatle]

i know the most acidic hydrogen is the one(s) on the alpha carbon of a carbonyl

I have encountered ones such as
----O----O
----II----II
CH3CHCH2CHCCH3

The ones in middle are the most acidic--All the questions I have seen always have two carbonyls, with the H's in the middle

BUT, if it were only one carbonyl:

----O
----II
CH3CHCH2CH2CH3



Are these five all the same??? Does the propyl group affect the acidity vs the methyl group?

Thanks

PS. I have taken the DAT and know friends that have---they guarantee ask you this type of question

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[poc91nc]

Draw the carbanions with the respective alpha hydrogens removed. Then ask yourself which is more stable. One is more stable than the other...they're not all equal

 

[jdmsamurai]

carboanions are most stable as

meth > 1 > 2 > 3

therefore the most acidic should be the CH3 and not the CH2

correct me if i am wrong

 

[PChemGrad]

----O
----II
CH3CHCH2CH2CH3

The Hydrogen colored red will be the most acidic, followed by the hydrogen in blue will be the second most acidic.

this is due to the resonance structures possible. after abstraction of a hydrogen the remaining negative charge back form a double bond to the carbonyl carbon causing a single bond to the oxygen.
hydrogens bonded to a alph carbon in a carbonyl will always be the most acidic.

 

[jdmsamurai]

----O
----II
CH3CHCH2CH2CH3

The Hydrogen colored red will be the most acidic, followed by the hydrogen in blue will be the second most acidic.

this is due to the resonance structures possible. after abstraction of a hydrogen the remaining negative charge back form a double bond to the carbonyl carbon causing a single bond to the oxygen.
hydrogens bonded to a alph carbon in a carbonyl will always be the most acidic.

i get your reasoning...but both of them can form double bonds to the carbonyl carbon...but what makes the red one more acidic though?...and isnt both the red and blue alpha carbons?

 

[poc91nc]

carboanions are most stable as

meth > 1 > 2 > 3

therefore the most acidic should be the CH3 and not the CH2

correct me if i am wrong

You're right, therefore C1 will be most acidic. Because of the negative charge...carbanion on C3 will be destabilized due to the inductive effect

 

[jdmsamurai]

You're right, therefore C1 will be most acidic. Because of the negative charge...carbanion on C3 will be destabilized due to the inductive effect

so therefore the "blue" one should be the most acidic...and not the red...

 

[poc91nc]

Yes

 

[Mia305]

carboanions are most stable as

meth > 1 > 2 > 3

therefore the most acidic should be the CH3 and not the CH2

correct me if i am wrong

A side question...If that is the order of carbanion stability then why is a carbanion with three phenyl groups attached more stable than a methyl carbanion... according to Kaplan this is due to all the resonance structures that the first one can form. but based on that order, the methyl carbanion would be more stable

 

[jdmsamurai]

resonance > methyl

always

 

[Mia305]

so even though the order is meth > 1 > 2 > 3, a tertiary carbanion that can form multiple resonance structures will be more stable than a methyl carbanion?

 

[phungy]

I just noticed that the carbonyl has 5 bonds

 

[thefifthbeatle]

Hahahahhahah oops!!!!! I was more concerned with writing the chain and spacing it correctly!!! I will NOT mess things up like that on the test!

 

[jeninny44]

Kryptonite.

 

[supraman]

Could have wrote it like this:

CH3(C=O)CHCH2CH2CH3

 

[blueyes34]

With the way I was taught...one of the hydrogens on C1 would be more acidic because it is more readily available for attack due to less steric hinderance. Is this right?