DAT DESTROYER Organic Chem #7

[TeethRCool]

Solution says do Aldol condensation when in dilute acid or base... the problem is in dilute sodium hydroxide so shouldn't the answer have an alkene?
Thanks

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[Euphoria36]

when you see aldehydes with a base like NaOH think Aldol Condensation!


in the dilute base the propanal reacts with itself, so you have a base catalyzed condensation of *two* propanals. basically, the two propanals fuse together to form one compound

In the first step, the base strips away one of the alpha hydrogens from one of the propanals and forms a *temporary* alkene. But, in doing so the oxygen looses its double bond with the C b/c C can only hold 4 bonds. This is unstable for the oxygen so it rearranges to reform the double bond on the oxygen, leaving lone pairs on the alpha carbon. The deficient alpha carbon then attacks the second propanal. it attacks the carbonyl carbon of the second propanal. From there, just rearrange the molecule a little bit and you have the 3-hydroxy-2-methylpentanal - the oxygen is protonated to form the hydroxy group

it's a little hard to explain w/o figures, but if you have the kaplan notes/bluebook Aldol Condensation is explained on pg 360.

hope this helped!

 

[TeethRCool]

Thanks for the explanation but my question was after forming 3-hydroxy-2-methylpentanal, doesn't this undergo condensation where u lose an H and OH to form an alkene since it is in conditions favoring dehydration.

 

[lifeisgood]

Thanks for the explanation but my question was after forming 3-hydroxy-2-methylpentanal, doesn't this undergo condensation where u lose an H and OH to form an alkene since it is in conditions favoring dehydration.

i'm not positive but i think you need heat to form the alpha-beta unsaturated product...

 

[Manyak222]

yes, you need heat. check out his road map number 4 bottom right corner.