Question

Two students are given the starting material benzoic acid and are asked to prepare benzaldehyde. The first student starts by refluxing her sample of benzoic acid in thionyl chloride in the fume hood. Upon completion of the reaction, she evaporates the thionyl chloride to isolate compound A. She treats compound A with a stoichiometric amount of lithium tri-tert-butoxyaluminum hydride at –78 °C in diethyl ether, producing compound B. Adding water, she isolates her product, compound C. The second student takes a different route. She treats benzoic acid with an excess of lithium aluminum hydride (LAH) in diethyl ether, followed by careful addition of ethyl acetate to remove any unreacted LAH. She adds water and isolates her product, compound D. Draw the structure for compound A, compound C, and compound D below.

Answer 1

Answer:

Compound A: Benzoyl chloride

Compound B: Benzaldehyde - (tBuO)₃Al complex

Compound C: Benzaldehyde

Compound D: Benzyl alcohol

Explanation:

The lithium tri-tert-butoxyaluminum hydride that the first student used is a milder reagent than LAH and will stop reacting at the aldehyde.

The LAH that the second student used is much more reactive and will continue to reduce the benzoic acid as far as possible, going all the way to the alcohol.

See the attachment for the reaction steps.