NextStep Chemistry

[5words]

The kinetics of the ninhydrin reaction were shown to be first order with respcct Io 1,2,3-indantrione and a-alpha amino acid, and second order overall. The reaction rate was shown to vary with solution pH. The initial reaction rate was greatest when the product of which two concentrations was maximized? (Note: R=C'H4O2.)

A. [RC=O+H] [H2NCHRCOO-]
B. [RC=O] [+H3NCHRCOO-]
C. [RC=O+H] [+H3NCHRCOOH]
D. [RC=O] [H2NCHRCOOH]

Here is the explanation, i am not understanding why RC=OH+ is a better resonance contributor than RC=O.

Choice A is correct. The reaction between 1,2,3-indantrione and the alpha amino begins with attack by the
nitrogen of the alpha amino acid's amino residue on a carbonyl carbon of 1,2,3-indantrione. The reaction requires an available lone pair on the amino nitrogen of the acid.
No lone pair is available in the protonated, quaternary amino nitrogen shown in choices B and C. The reaction rate will be maximized when the carbonyl oxygen of the carbonyl attacked in the reaction is activated to attack by protonated (choice A). Because of a morc favorable resonance contributor in the protonated versus the unprotonated carbonyl, the electrophilicity of the carbonyl carbon, and the reaction rate, are increased when compared to the carbonyl carbon electrophilicity and reaction rate of the unprotonated carbonyl-oxygen (choice D).

Thanks

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[aldol16]

It's poor wording by the test prep company. RC=O and RC=O-H+ are not resonance forms because one is protonated and the other is not. However, RC=O-H+ is more electrophilic than RC=O because of its resonance forms: C=O-H+ and +C-O-H. Since the second resonance form is a significant contributor, that carbonyl carbon is quite electrophilic. If you're unclear about this concept, take a look at Lewis acid catalysis of carbonyl-based reactions.

 

[5words]

It's poor wording by the test prep company. RC=O and RC=O-H+ are not resonance forms because one is protonated and the other is not. However, RC=O-H+ is more electrophilic than RC=O because of its resonance forms: C=O-H+ and +C-O-H. Since the second resonance form is a significant contributor, that carbonyl carbon is quite electrophilic. If you're unclear about this concept, take a look at Lewis acid catalysis of carbonyl-based reactions.

So aldol, i thought the carbonyl carbon was electrohpilic because the oxygen was so electronegative that it pulled electron towards itself, ok ..i see where you are going..

So question, it seems as if adding a Hydrogenn , increases the nuclear effective charge of the O (due to the positive charge) thereby causing the latter to pull electron towards itself even stronger. So, the hydrogen here isn't reducing the pulling force of O?.. anyways, i am understand their answer choice a lot better now.. Thanks and i will check "lewis acid catalysis of carbonyl-based reactions".

 

[aldol16]

Adding the proton creates a positive formal charge on the oxygen. Just do the formal charge yourself. You should end up with +1 on oxygen. This is better than a neutral formal charge. Therefore, if oxygen has a positive charge on it, that renders it even more electrophilic because it really really doesn't want a positive charge at all.