What is the Acid Used in Strecker Amino Acid Synthesis?

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Discussion Overview

The discussion centers on the acid used in the Strecker amino acid synthesis, exploring the roles of different reagents and mechanisms involved in the reaction. Participants examine the protonation of carbonyls and imines, the addition of cyanide, and the hydrolysis of intermediates.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the identity of the acid used in the Strecker synthesis, suggesting that HCN may not be strong enough to protonate carbonyls or imines.
  • Another participant proposes that H3O+ is the acid involved and outlines the reaction mechanism, including hydrolysis of the α-amino nitrile to form amino acids.
  • There is a discussion about the role of sulfuric acid and strong bases like NaOH in the reaction, with some participants asserting that both acid and base catalysis can be used.
  • Participants debate whether CN- or NH3 is the more kinetically favorable nucleophile for addition to carbonyls, with some suggesting that CN- is preferable.
  • One participant points out that while both CN and NH2- can add to carbonyls, one of them adds to the imine carbon instead.

Areas of Agreement / Disagreement

Participants express differing views on the identity of the acid used in the synthesis and the mechanisms of nucleophilic addition. There is no consensus on whether HCN is sufficient for protonation or whether CN- is the preferred nucleophile.

Contextual Notes

Some participants highlight the importance of equilibrium concentrations of CN- in solution and the conditions under which different nucleophiles add to carbonyls, indicating that the discussion involves complex mechanistic details that are not fully resolved.

sludger13
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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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