Why only amino (-NH2) group in the amino acid show color reaction? Why not COOH?

28 minutes ago, Syed Qizar Afzaal said:

Why only amino (-NH2) group in the amino acid show color reaction? Why not COOH?

What reaction are you talking about? 

29 minutes ago, exchemist said:

What reaction are you talking about? 

I guess so reaction with Ninhydrin..

https://en.wikipedia.org/wiki/Ninhydrin

"It is used to detect ammonia and amines. Upon reaction with these amines, ninhydrin gets converted into deep blue or purple derivatives, which are called Ruhemann's purple. Ninhydrin is most commonly used to detect fingerprints, as the terminal amines of lysine residues in peptides and proteins sloughed off in fingerprints react with ninhydrin."

 

 

@Syed Qizar Afzaal

The color of a chemical compound and atoms depend on the energies at which electrons are excited (transit from higher energy level to lower and perhaps multiple times). If you emit white light toward a substance that reflects it, you get white light back. If you emit white light toward a substance that absorbs it entirely, you get black color. But if some electrons absorb photons only at certain energies/frequencies/wavelengths, they disappear and you can't see them back.

15 minutes ago, Sensei said:

I guess so reaction with Ninhydrin..

https://en.wikipedia.org/wiki/Ninhydrin

"It is used to detect ammonia and amines. Upon reaction with these amines, ninhydrin gets converted into deep blue or purple derivatives, which are called Ruhemann's purple. Ninhydrin is most commonly used to detect fingerprints, as the terminal amines of lysine residues in peptides and proteins sloughed off in fingerprints react with ninhydrin."

 

Ah yes, probably. Let's see if our poster can take part in a conversation about this. 

It's my fault, I should have elaborated the question.

For identifying amino acids, one approach is to see the colors they impart with different reagents.

All the color reactions which I have studied (Biuret Test, Ninhydrin Test, Xanthoproteic Test, Millon's Reaction, Sakaguchi Reaction) are based on Amino group (-NH2).

Why aren't we using Carboxylic Acid (-COOH) group for the color reactions?

47 minutes ago, Syed Qizar Afzaal said:

It's my fault, I should have elaborated the question.

For identifying amino acids, one approach is to see the colors they impart with different reagents.

All the color reactions which I have studied (Biuret Test, Ninhydrin Test, Xanthoproteic Test, Millon's Reaction, Sakaguchi Reaction) are based on Amino group (-NH2).

Why aren't we using Carboxylic Acid (-COOH) group for the color reactions?

Hmm. I suspect that will be to do with the tendency of amino acids to exist as zwitterions. I don't think you can rely on an acid/base colour indicator to work, because of the basicity of the amine end.  So you would have to find reactions that bind carboxylate specifically and thereby produce a colour change. Perhaps something involving a transition metal ion, for which RCOO- could be a coordination ligand?  

If I recall correctly I once used bromocresol green to detect compounds containing a carboxylic acid group on a TLC plate.  That is probably different from the sort of reaction that you were thinking about.