Question

Using your reaction roadmap as a guide, show how to convert 3-hexyne into propanal. All of the carbon atoms of the target molecule must be derived from the starting material as efficiently as possible. Show all intermediate molecules synthesized along the way.

Short answer

Answer: The most efficient route to convert 3-hexyne into propanal involves three steps: 1) Convert the alkyne to an aldehyde using Lindlar's catalyst and hydrogen gas (H2), yielding 3-hexenal; 2) Perform an oxidative cleavage reaction using ozone (O3) followed by a reduction with Me2S to shorten the carbon chain, resulting in the formation of Propanal and Acetaldehyde; 3) Isolate Propanal from the mixture by utilizing conventional purification techniques such as distillation based on their different boiling points.

Step-by-step solution

Step 1. Convert Alkyne to Aldehyde

To begin the conversion of 3-hexyne to propanal, we should first convert the 3-hexyne (alkyne) functional group to an aldehyde. This can be done using a partial reduction with Lindlar's catalyst and hydrogen gas (H2). The product of this step will be 3-hexenal, an aldehyde. 3-hexyne + Lindlar's catalyst + H2 -> 3-hexenal

Step 2. Chain Shortening

After converting the alkyne to an aldehyde, we now have 3-hexenal as our intermediate molecule. We need to shorten the carbon chain from 6 to 3 carbons. This can be achieved by an oxidative cleavage reaction using ozone (O3) followed by a reduction using a mild reducing agent like Me2S. 3-hexenal + O3 -> Ozonide Ozonide + Me2S -> Propanal + Acetaldehyde

Step 3. Isolation of Propanal

After the oxidative cleavage and reduction steps, we have formed the target molecule Propanal along with a side product Acetaldehyde. We can separate the two products through conventional purification techniques such as distillation, taking advantage of the different boiling points of the two aldehydes. Now you have the final product propanal, which was efficiently derived from the starting material 3-hexyne.