What is the Acid Used in Strecker Amino Acid Synthesis?

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SUMMARY

The discussion centers on the acid used in Strecker amino acid synthesis, specifically questioning the role of HCN as a weak acid in the reaction mechanism. Participants clarify that while HCN is involved, it cannot effectively protonate carbonyls or imines due to its weak acidity. Instead, they identify that stronger acids, such as sulfuric acid, or strong bases like NaOH, are necessary for the acid-catalyzed and base-catalyzed pathways, respectively. The final product of the synthesis is an α-amino nitrile that hydrolyzes to form an amino acid.

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  • Understanding of Strecker amino acid synthesis
  • Knowledge of acid-base chemistry, particularly regarding weak and strong acids
  • Familiarity with nucleophilic addition reactions involving carbonyl compounds
  • Basic grasp of hydrolysis reactions and their mechanisms
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  • Study the mechanism of Strecker amino acid synthesis in detail
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  • Investigate the differences between acid-catalyzed and base-catalyzed reactions
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Chemistry students, organic chemists, and researchers involved in amino acid synthesis and reaction mechanisms will benefit from this discussion.

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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?
 
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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).
 
AdityaDev said:
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:
Streck2.gif

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.
 
sludger13 said:
Ok, that seems obvious. Still I do not understand this picture:
Streck2.gif

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.
 
AdityaDev said:
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)?
 
sludger13 said:
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.
 
AdityaDev said:
Both CN and NH2- adds to carbonyl carbon. Both are nucleophilic addition reactions.
No, one of them adds to imine carbon...
 
I know that. I learned it like a shortcut from here:
WP_20150105_23_30_39_Pro.jpg

And the actual mechanism is:
WP_20150105_23_32_56_Pro.jpg
 

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