What is the Acid Used in Strecker Amino Acid Synthesis?

[sludger13]

In Stecker aminoacid synthesis, a cyanide is supplied in salt form that protonates at the beginning:
NH4Cl + NaCN = NH3 + HCN + NaCl
(Info source - http://www.organic-chemistry.org/namedreactions/strecker-synthesis.shtm)
That seems okay, because HCN is weak acid.

According to the source, the carbonyl and imine addition is then initiated by an acid. The acid is not mentioned. I don't believe HCN can protonate either carbonyl or imine due to its weakness. What is this acid?

I have learned that cyanide anion can only initiate its addition to carbonyl and proton from any acid cannot do that (that is from HCN (it's too weak) + from any stronger acid (then (CN-) is immediately protonated prior to additon)). Is this correct?

 

[AdityaDev]

It's $H_3O^+$.
$$RCHO + HCN + NH_3 --> RCH(NH_2)CN$$
Now the $\alpha$-amino nitrile on hydrolysis gives amino acid.
Note that the better term is hydrolysis.
Here the CN is hydrolyzed to COOH.
The reagents that can be used are sulphuric acid or strong bases like NaOH(acid catalysed and base catalysed respectively).

 

[sludger13]

The reagents that can be used are sulphuric acid or strong bases like NaOH(acid catalysed and base catalysed respectively).

Ok, that seems obvious. Still I do not understand this picture:

Whether it's acid catalysed, then HCN should add to carbonyl! Or does (CN-) in picture indicates its equilibrium concentration?

Simply put:
Also during (CN-) addition to carbonyl, base catalysation means there is higher equilibrium concentration of (CN-) in solution that adds straightly to carbonyl. Acid catalysation means acidic solution with such pH value that carbonyl is firstly protonised AND there is sufficient equilibrium concentration of CN- (HCN <= CN- + H+) in solution.

 

[AdityaDev]

Ok, that seems obvious. Still I do not understand this picture:

Whether it's acid catalysed, then HCN should add to carbonyl!.

In strecker synthesis CN- adds to carbonyl carbon.
That picture is not strecker synthesis. Moreover H+ is added in final step to hydrolyze the CN.
Everything in that picture is straight forward.

 

[sludger13]

In strecker synthesis CN- adds to carbonyl carbon.

Also addition of (CN-) (acid or base catalyzed) to carbonyl is kinetically preferable over (NH3) addition to carbonyl (acid or base catalyzed)?

 

[AdityaDev]

Also addition of (CN-) (acid or base catalyzed) to carbonyl is kinetically preferable over (NH3) addition to carbonyl (acid or base catalyzed)?

Both CN and NH2- adds to carbonyl carbon. Both are nucleophilic addition reactions.

 

[sludger13]

Both CN and NH2- adds to carbonyl carbon. Both are nucleophilic addition reactions.

No, one of them adds to imine carbon...

 

[AdityaDev]

I know that. I learned it like a shortcut from here:

And the actual mechanism is: