Question

Suppose you were asked to carry out a crossed-aldol condensation reaction between 4-methylbenzaldehyde and 3,3-dimethylbutanal modelled after an experiment performed with benzaldehyde and acetone, a. what product would you expect to make? Write the equation showing the detailed structures of reactants and products? b. propose a detailed mechanism for this reaction? c. what unwanted by-product might be expected in this experiment? Write the relevant equations with detailed structures? What experimental strategy might you attempt to minimize the formation of this by-product. d. how would your major product react with 2,4-dinitrophenylhydrazine? write an equation to depict the relevant details and describe what is observed visually. e. how would your major product react with molecular bromine in carbon tetrachloride? describe what is observed visually.

Answer 1

The reaction between  4-methylbenzaldehyde and 3,3-dimethylbutanal modelled after an experiment performed with benzaldehyde and acetone is shown below along with the mechanism answering both question a) and b)

The possible side product is the self condesation of 3,3-dimethylbutanal

We can minimize this reaction by carrying out the reaction by slow addition of the 3,3-dimethylbutanal so that concentration of 3,3-dimethylbutanal is low in the medium or by taking a large excess of to start with.

Reaction of the major product with 2,4-dinitrophenylhydrazine

Visibly you will get an orange or yellow precipitate from a carbon-oxygen double bond in an aldehyde or ketone.

The final compound has a structure which in the presence of H⁺ or OH^- will get converted to a enol. Enols are isomers of aldehydes or ketones in which one alpha hydrogen has been removed and replaced on the oxygen atom of the carbonyl group. The resulting molecule has both a C=C (-ene) and an –OH (-ol) group, so it is referred to as an enol.

These enols will react with molecular bromine in CCl4 to give bromine addition product followed by loss of bromine colour