1. why does the mixed aldo condensations producing benzalacetophenone is the mai
ID: 489219 • Letter: 1
Question
1. why does the mixed aldo condensations producing benzalacetophenone is the main reaction in experimet claisen schmidt reaction and not both other possible reaction, acetophenone condensation and cannizzaro benzaldehyde reaction?
2. propose a reason to explain the statment that the trans-benzalacetophenone isomer is the major product in aldol condensation.
3. explain why the main product from bromine addition to the trans- acetofenone is erythro-dibromide.
4. write a mechanisme for the addition of aniline to benzalacetophenone.
TQ.
Explanation / Answer
Q1: The condensation between aldehydes and ketones which having alpha-hydrogen led to alpha, beta unsaturted compounds called as aldol condensation. If it happens between two different carbonyl compounds called as crossed (mixed) aldol which is nothing but claisen schmidt reaction.
To get benzalacetophenone we need benzaldehyde and acetphenone, this is an example for claisen schmidt reaction. But if you take two moles of acetophenone, the product is different from benzalacetophenone, and it is not claisen scmidt reaction since both the molecules are acetophenone only.
The cannizaro reaction is between aliphatic or aromatic aldehydes which doesnt have alpha hydrogen atom and here products are correspoding alochol and acid salts. If benzaldehyde is the starting material, this doesnt have alpha hydrogen and the products are benzyl alcohol and benoic acid salt. The major different between aldol condensation and cannizaro is alpha hydrogen, then products are entire different.
Q2: In alodlol reaction the mechanism follows alodol formation then elimination of water led to alpha, beta unsatured derivatives. Generally elimination reactions are more facile when the two atoms are opposite to each other and it requires less energy. In case of benzalacetophenone formation, the hydroxy next carbon contains two hydrogen atoms. But one is exactly oppiste and other is towards hydroxy. As we discussed elimination more passionate with opposite atoms, here hydroxy eliminates with opposite hydrogen and led to trans product. So in this reaction trans benzalacetophenone is major product.
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