This past Thursday fulfilled its promise of group problems. The majority of them, however, were something you would see in Sophomore Organic Chemistry. Sure, they were some upper end stuff, but Sophomoric none-the-less. It did provide an opportunity to council younger students on the dangers of saying "never".
The two problems shown here each have an abnormal leaving group. The first scheme can be written a number of different ways, but it is generally agreed that the final step is the displacement of a hydroxide group. Normally, you would never see it, but in basic enough conditions you can work some magic.
The second reaction (presented here to work out on your own) shows a methyl as the leaving group, again, a big no-no. Yet, through the magic of Iodoform (hint-hint) reactions, it can pop right off. Have fun!
-Woodward
It's not really true that the reaction will only work "under basic enough conditions"... in fact, condensations of this type (Knoevenagel, if you wanna play the named reactions game) are usually done with fairly weak bases. This is because it's vitally important not to deprotonate the alcohol that you're trying to eliminate. It's also worth noting that the elimination of water is greatly assisted by the two esters, which massively acidify the proton being removed (as an aside, an E1cB mechanism would also be reasonable). You would not be able to remove water under these conditions if those two esters were just protons (i.e. from 1-phenylethanol, http://www.sigmaaldrich.com/catalog/product/aldrich/p13800?lang=en®ion=US). This is a special case, just like your iodoform reaction or the apparent loss of H- in the chichibabin reaction.
ReplyDeleteThere is no magic!