another orgo question!!

[JustwantDDS]

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Why is the answer : IV<I<III<II

Isnt IV > III because it doesnt have any electron donating groups?😕

 

[JustwantDDS]

Question is :Rank the following in order of increasing acidity.

 

[mh0000]

formaldehyde is not acidic

 

[aliss4]

Here you are looking for ALPHA hydrogen acidity. Formaldehyde doesn't have any alpha hydrogen. (All hydrogens are on carbonyl carbon).

Between ketone and aldehyde: They both have alpha hydrogen but different in the nature of groups attached to the carbonyl carbon. H atoms DON'T WITHDRAW or DONATE, basically they have no electronic effect. Alkyl groups are weakly electron donating, they tend to destabilize the anion. Therefore aldehyde alpha hydrogen is a better acid than ketone alpha hydrogen.

 

[JustwantDDS]

so formaldehyde is the weakest because it has no alpha hydrogens?

 

[Golfguy]

so formaldehyde is the weakest because it has no alpha hydrogens?

Yes, the hydrogens of an aldehyde have a higher pKa's, thus aren't acidic at all, and aren't alpha hydrogens.

Aldehydes are generally more reactive then ketones as the alkyl groups help to stabilize the partial positive charge on the carbonyl carbon.

 

[UBCStudent128]

A way I like to do is to pretend to deprotonate one hydrogen from all of the answers. The molecule that is most stable with the -ve charge after deprotonation will be the more acidic molecule.

IV will have a secondary carbocation which is very unstable, therefore it is the least acidic.

II will have two carbonyl groups providing extra resonance.

I ranks over III since it is a ketone vs aldehyde ( you should memorize pKa's for these) Or you can think of hyperconjugation ketone has over aldehyde with the extra methyl group

 

[needzmoar]

so formaldehyde is the weakest because it has no alpha hydrogens?

It's the weakest because it doesn't have an acidic hydrogen at all. None. Why is it that alpha hydrogens are acidic? They're acidic because the conjugate base is resonance stabilized. Now lets have a look at the 'conjugate base' of formaldehyde.

View attachment needzmoar.gif​

Hope it helps.

 

[JustwantDDS]

thanks guys

@UBC student
but doesnt stability mean weaker?

stronger the acid, the weaker the conj. base right? and the weaker base is most stable?

 

[Bis-GMA111]

^ yup. more stability=weak base=strong acid