Chad's O chem quiz question. !!

[john216]

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Rank the following in order of increasing acidity.


The answer is IV < I < III < II


I don't understand why I is less acidic than III

so I has two carbons that are donating groups where as III has only one Carbon and one Hydrogen.
Since donating groups donate electrons to partial positive charge on Carbonyl carbon, it makes less reactive, meaning more stable conjugate base = more acidic.

so I should be more acidic because its got two Carbons
AM I wrong?

 

[LazyP]

Basically you are trying to find which hydrogen is the most acidic at the alpha carbon. Chad goes over this, look at the alpha carbon and then group on the other side. You know that the alpha carbon, when deprotonated, is stabilized by the carbonyl group in both cases...So now the question remains is which is more stable. Would a methyl or H on the right side next to the carbonyl make it more stable? Remember methyl is electron donating, so it will make the molecule more reactive, thus less acidic. Hope that helps!

 

[FeralisExtremum]

Agreed with LazyP...the methyl groups are donating, which makes the conjugate base stronger, and the acid weaker. III has two methyl groups, so it has even more electron donation than I does. IV has no electron donating group and is stabilized by the negative charge of the conjugate base being on an sp2 carbon.

 

[john216]

Thanks,
I don't get where this concept comes from but

I remember that in general,
When molecule is acidic, it's easier to break bonds so react faster
which is opposite of what you said "so it will make the molecule more reactive, thus less acidic"

Do you guys recognize where am I getting this from?
and is this wrong?

 

[LazyP]

In this case, the more reactive means more basic. In an acid, yes the more basic the faster it reacts, however while in base, the more acidic (less reactive its conjugate base) the faster it reacts. It all depends on the conditions. In this case, it asks for ranking acids. By definition, a bronsted lowry acid is one which gives up protons. The more acidic, the more stable (or less reactive) its cognate base, the quicker it will give up the proton (the vice versa is also true).

What you say is true but depends on the conditions. In this case when I said "more reactive", means the conjugate base of the acid was more reactive when you deprotonate the alpha H. Thus weaker the acid.

 

[FeralisExtremum]

john216, I think you're mixing up kinetics with acidity/basicity. Don't approach problems like these in terms of how fast they react, because that can be complicated by the conditions. Unless conditions are specified (here they are not), simply evaluate the strength of the acid/conjugate base and go from there. Remember:

Stronger conjugate base --> weaker acid.
Weaker conjugate base --> stronger acid.

 

[LazyP]

john216, I think you're mixing up kinetics with acidity/basicity. Don't approach problems like these in terms of how fast they react, because that can be complicated by the conditions. Unless conditions are specified (here they are not), simply evaluate the strength of the acid/conjugate base and go from there. Remember:

Stronger conjugate base --> weaker acid.
Weaker conjugate base --> stronger acid.

agreed.👍