Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Big_Joe on April 14, 2009, 07:19:57 AM
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When a carbonyl reacts with hydrogen cyanide the reaction is started by a CN- ion reacting with the electron deficient carbon adjacent to the electronegative oxygen, but if the molecule is reacting with HCN where does the original CN- ion come from? I know that the ions are created in the reaction but where does the first one come from?
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Consider: if you were trying to make an acetal, you can't just add a ketone and an alcohol... the alcohol is not a good enough nucleophile to attack the carbonyl straight out. You'd need to activate the carbonyl first, and to do so we add catalytic acid.
This is the same way. HCN is not a good enough nucleophile to attack the carbonyl... it needs to be activated.
What activates the carbonyl, and what is the byproduct of that activation?
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Ah yes of course a catalyst is needed. It seemed as though my textbook suggested that it could react without the presence of a catalyst. Is the catalyst Ni/H2?
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nope, those are reductants.
Hint: the common name for HCN is prussic acid (http://en.wikipedia.org/wiki/Prussic_acid)
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Thanks! So it just needs to be dilute?
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This is the same way. HCN is not a good enough nucleophile to attack the carbonyl... it needs to be activated.
I used to think as you do, but CN- is a MUCH better nucleophile than water or an alcohol. Cyanide will attack a carbonyl without activation. The acid serves another purpose.
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I was reading the question as reacting the carbonyl with ONLY HCN, with NO CN- around. Yes, if KCN is included in the rxn conditions, then HCN does something else... If the only reagent is HCN, then HCN activates the carbonyl AND creates the first molecule of CN-.