Can anyone tell me where I am doing it wrong for the following reaction? 
In the first step, Br should be attached to the alpha carbon.
In second step, the bromide is replaced by cyanide.
I am able to get the third product that is final compound.
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$\begingroup$ Heating a carboxylic acid often leads to decarboxylation. So decarboxylate your product from step 2 (well actually the product formed following acidification in step 3!) $\endgroup$– LeeserJul 21, 2016 at 12:26
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$\begingroup$ @Leeser can you tell me the products formed $\endgroup$– user32086Jul 21, 2016 at 12:31
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2 Answers
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Step 1: $\alpha$-bromination $$\ce{CH3-COOH + Br2/P -> Br-CH2-COOH}$$
Step 2: Nucleophilic Substitution by Sodium Cyanide $$\ce{Br-CH2-COOH + NaCN -> NaBr + NC-CH2-COOH}$$
Step 3: Acid-catalyzed Conversion of Nitrile to Amide or Acid
(1) First hydration... $$\ce{NC-CH2-COOH + H2O -> H2N-CO-CH2-COOH}$$ (2) Further heating... $$\ce{H2N-CO-CH2-COOH -> HOOC-CH2-COOH + NH4+}$$
Special Notes
Be VERY CAREFUL with Step 2 as you are potentially forming $\ce{HCN}$ which could get everyone in the lab killed if you're not careful.
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$\begingroup$ But @sixtytrees has made different products $\endgroup$– user32086Jul 21, 2016 at 12:41
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3$\begingroup$ @user123733 No, he only went step further with decarboxylation, without any good reason. $\endgroup$– MithoronJul 21, 2016 at 13:43
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There are two possibilities. You will have a two step reaction
NC-CH2COOH -> HOOC-CH2-COOH This is malonic acid. It can be further decarboxylated to
HOOC-CH2-COOH + H$^+$ + heat -> CO2 + CH3COOH
the final product is acetic acid.
