Amino acid + HCl reaction and mechanism

[kateman]

I was interested in knowing if my thoughts on this are right or if anyone else has any other thoughts on the reaction of any amino acid and concentrated HCl solution with water as the solvent;

As an example of an amino acid such as cystine with an NH2 bonded to a carbon; I was thinking that the water would dissociate and react at the site where nitrogen is attached to the carbon - the hydroxyl ion would be transffered to the carbon while the NH2 anion would react with the HCl and then also react with the hydrogen ion to create NH4Cl.

Would this reaction even result in NH4Cl? I could find nothing on the net to suggest it would and when I conduct the reaction myself I can't distinguish NH4Cl from HCl by smell alone [unless NH4Cl is being created as a dissolved solid and not evolved as a gas]

This has been playing in my head for months, any clarity would be really appreciated! :)

 

[Borek]

Ammonium chloride is an ionic salt so no chances of getting it airborne over a solution.

An obvious product is just a protonated amino acid, whether anything else happens I have no idea.

 

[DDTea]

Big problem with your proposed mechanism: you will *never* find a hydroxyl ion in a solution of concentrated HCl (or any acidic solution). You will find Hydronium (H₃O⁺), which is an extremely poor nucleophile, and you will find water (H₂O). The former is your "acid" in solution and the latter is your "base." In basic solutions, you will find Hydroxide (HO^-) and water. Hydroxide is your base and water is your acid, in this case. In either case, you will never find protons floating around in solution; although it's OK to draw mechanisms with protons to simplify things, you should always be aware that they are, in fact, hydronium ions.

Now that that is cleared up, let's move on. Basically, it depends entirely on how strong the acid is and how concentrated it is. Many reactions can happen on amino acids and their chemistry can get rather complicated and counter intuitive! But let's assume that this is a *really* strong acid. First of all, you're going to protonate that -NH₂ to form -NH₃⁺. That's likely to be the *only* thing happens, by the way. Unless you really force the issue and do an elimination or nucleophilic substitution somehow, and I won't venture a guess how to because my background with amino acid chemistry is very weak, you will not isolate NH₄Cl. More likely, you will have the protonated amine group coordinated with the chloride ion.

This is kind of a neat topic, really. Have you heard of electrophoresis? It takes advantage of the behavior of amino acids in varying pH's to separate them by their isoelectric points. I've only done capillary electrophoresis *once* in a classroom lab, but it's a pretty powerful technique.

By the way, NH₄Cl smells like ammonia, if you're wondering.

 

[Borek]

Big problem with your proposed mechanism: you will *never* find a hydroxyl ion in a solution of concentrated HCl (or any acidic solution).

I know it is trivial, but *never* is an exaggeration, and unjustified one:

pOH = 14 - pH

 

[cis-ddp]

First, some terminology: the amino acid is "cysteine." "Cystine" is two cysteines held together by a disulfide linkage via their side chains.
If I remember correctly, HCl is utilized to hydrolyze the peptide bonds during primary structure characterization. This operates through a standard acid-catalyzed hydrolysis of an amide mechanism, so the free amino acid should not be affected (beyond protonation of COO-, NH2, and S-). The only amino acids whose side chains ARE affected by acid hydrolysis are W and H (I believe the nitrogenous aromatic rings become protonated causing a chain reaction ending with almost complete destruction of the ring).

 

[DDTea]

I know it is trivial, but *never* is an exaggeration, and unjustified one:

pOH = 14 - pH

This is a big source of confuson that I've seen in sophomore organic chemistry students and it does lead to trouble with mechanisms. I say that to help students write sensible reaction mechanisms.

Yes, it's an exaggeration but not much of one and it's a fair one to make. You're right: there would still be *some* -OH present in solution due to the natural dissociation of water into H⁺ and -OH, but the lone electron pair would quickly find a proton to bond to. This would create a steady-state equilibrium. However, proton transfer occurs so much faster than any other reaction in organic chemistry: it's on the order of 10^-14 s, about as fast as a bond vibration. Bearing in mind that a dissociating water molecule transfers a proton directly to an adjacent water molecule, thus creating a cascade of protons, it is nonsensical to show -OH participating in a meaningful way in a reaction that involves concentrated HCl. For a nucleophilic substitution, the kinetics are several orders of magnitude slower than for proton-transfer. Of course, that level of detail is rarely necessary when describing hypothetical reactions, so I prefer to use my simple "rule" :)