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$$\ce{CH2=CH-CH=O ->[CH3SH][(1,4 addition)] Product (P_1) ->[(i) NaCN][(ii) NH3\\(iii) H3O+] Q_1}$$

Correct statement regarding $\ce{Q_1}?$

  1. $\ce{Q_1}$ is essential amino acid
  2. $\ce{Q_1}$ is acidic amino acid
  3. $\ce{Q_1}$ is nonessential amino acid
  4. $\ce{Q_1}$ on Lassign test gives prussian blue colour with $\ce{FeSO4}$ and $\ce{FeCl3}$ solution

By 1,4 addition (as mentioned in question) carbon-3 (taking carbon as carbon-1 of aldehyde group) will have $\ce{SCH3}$ added to it and oxygen must have $\ce{H}$ added to it.

After $\text{P}_1$ is formed, it can again tautomerize from an alcohol to an aldehyde.

The $\ce{^-CN}$ can now attack the aldehyde. Looking at the answer we can ay that the product must be an amino acid and so the $\ce{NH3}$ group and $\ce{^-CN}$ must be present on same carbon because then the $\ce{^-CN}$ will hydrolyze to give a $\ce{-COOH}$ group.

But how is $\ce{NH3}$ supposed to attack a carbon having $\ce{^-CN}$ and a hydroxyl group. Can we make the alcohol a leaving group somehow?

Or is the number of carbons in chain 1 less than what it should be? What should be the correct answer and approach for this problem?

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  • $\begingroup$ Did you consider tautomerisation that could take place? $\endgroup$ – Safdar Jul 19 '20 at 18:55
  • $\begingroup$ @safdar yes considered it after you mentioned and edited the question with a bit more attempt. $\endgroup$ – the.eleventh.letter Jul 19 '20 at 19:11
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The starting compound is an organic compound known as acrolein. $\ce{CH2=CH-CH=O}$

Now your first reaction would be: $$\ce{CH2=CH-CH=O ->[CH3SH][1,4 addition]CH3-S-CH2-CH=CH-OH} $$

Now, this compound tautomerizes to form methional $$\ce{CH3-S-CH2-CH=CH-OH <-> CH3-S-CH2-CH2-CH=O}$$

Now, the second step in your reaction is a named reaction known as Strecker Amino Acid Synthesis. This converts an methional (in this case) into methionine. enter image description here

Now, onto the question at hand, it asks about certain properties of methionine. Methionine is an essential amino acid. So A is correct. Since methionine doesn't have any extra acid or base groups, it is neutral.

As for the Lassaigne Test, if the compound had only nitrogen, it would have shown a prussian blue color. However since there is sulphur as well, it shows a violet colouration. So, the answer is A.

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  • $\begingroup$ However in strecker amino acid synthesis reaction they have added NH3 in first step and -CN in the next. And its given vice versa in the question. So will we still get the same product? Btw thank you for the explanation $\endgroup$ – the.eleventh.letter Jul 20 '20 at 2:47
  • $\begingroup$ Cyanide adds to the imine of the aldehyde. Ammonia doesn't displace hydroxyl in a cyanohydrin but ammonia will reverse the formation of the cyanohydrin. $\endgroup$ – user55119 Jul 20 '20 at 3:18
  • $\begingroup$ @user55119 So can I sum this up as after we add cyanide Methional adds up the cyanide to it's aldehyde group , however upon addition of NH3 , some percentage of reaction goes back and then Streaker Amino Synthesis is followed. $\endgroup$ – the.eleventh.letter Jul 20 '20 at 7:41
  • $\begingroup$ No. The ammonia condenses with the thioaldehyde with loss of water to form an imine (Schiff's base). Then the cyanide adds to the imine. $\endgroup$ – user55119 Jul 20 '20 at 11:22

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