Question

In the following reaction the cyanoacetate is first treated with methoxide and then slowly added to the aldehyde. Draw the structure of the major product. The methoxide ion will react with the most acidic hydrogen available.

Answer 1

Concepts and reason

Condensation reaction: The reaction in which two small compounds react together and form larger molecules. During condensation reaction, elimination of small molecules such as water, methanol or $HCl$ will take place.

Knoevenagel condensation: It is nucleophilic addition of an ${\rm{\alpha }}$ -hydrogen compound to the carbonyl carbon, followed by deprotonation. The obtained product is ${\rm{\alpha ,\beta }}$ - unsaturated ketone. The ${\rm{\alpha }}$ -hydrogen is the hydrogen of the carbon adjacent to the functional group.

${\bf{\alpha }}\,{\bf{ - }}\,{\bf{Hydrogen}}$ : The hydrogen bonded to the carbon(s) that is/are adjacent to a functional group, is termed as the ${\rm{\alpha - carbon}}$ , hydrogen atom bonded to such carbon atom is called as ${\rm{\alpha }}\,{\rm{ - }}\,{\rm{hydrogen}}$ . In case of electron withdrawing group ${\rm{\alpha }}\,{\rm{ - }}\,{\rm{hydrogen}}$ are acidic in nature, due to the partial positive charge created in ${\rm{\alpha - carbon}}$ atom.

Fundamentals

Representative example Knoevenagel condensation

When cynoacetate is first treated with methoxide, then slowly it is added to the aldehyde. The product will be formed as follows:

The mechanism involved for the formation of product is given below:

Elimination of water molecule:

Ans:

When cynoacetate reacts with methoxy ion, then slowly it is added to aldehyde and the product that will be formed as follows: