Question

Using your roadmap as a guide, show how to convert 1-propanol into 2 -hexyne. You must use 1-propanol as the source of all carbon atoms in the target molecule. Show all reagents needed and all molecules synthesized along the way.

Short answer

Question: Outline the steps to convert 1-propanol into 2-hexyne using a series of chemical reactions. Answer: To convert 1-propanol into 2-hexyne, follow these steps: 1. Convert 1-propanol to 1-bromopropane using HBr. 2. Create a Grignard reagent (propyl magnesium bromide) by reacting 1-bromopropane with magnesium in dry ether. 3. Elongate the carbon chain by reacting propyl magnesium bromide with propanal to form a six-carbon alcohol. 4. Convert the secondary alcohol to an alkyl halide (2-bromohexane) using PBr3. 5. Perform an elimination reaction with a strong base (NaNH2) to form the desired alkene, 2-hexyne.

Step-by-step solution

Convert 1-propanol to 1-bromopropane

To transform 1-propanol into 2-hexyne, we need to first convert the starting alcohol into a better leaving group. Treat 1-propanol with a strong acid like HBr, and the hydroxyl group gets protonated, making it a better leaving group. This will be followed by the nucleophilic attack of bromide ions on the carbon, leading to the formation of 1-bromopropane. Reagents: HBr Reaction: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OH} + \text{HBr} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{Br} + \text{H}_{2}\text{O}\)

Perform Grignard reaction to elongate the carbon chain

To form a Grignard reagent, react 1-bromopropane with magnesium in the presence of dry ether. This creates the Grignard reagent, propyl magnesium bromide. Reagents: Mg, dry ether Reaction: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{Br} + \text{Mg} \xrightarrow{\text{dry ether}} \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{MgBr}\)

Elongate the carbon chain with an aldehyde

To further elongate the carbon chain, the propyl magnesium bromide can be reacted with a three-carbon aldehyde like propanal. This will create a six-carbon alcohol. Reagents: Propanal (\(\text{CH}_{3}\text{CH}_{2}\text{C}=\text{O}\text{H}\)) Reaction: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{MgBr} + \text{CH}_{3}\text{CH}_{2}\text{C}=\text{O}\text{H} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{O}\text{H} + \text{MgBr}_{2}\)

Convert alcohol to alkyl halide

To convert the secondary alcohol to an alkyl halide, treat the hexanol with PBr3. This will replace the hydroxyl group with a bromide, resulting in 2-bromohexane. Reagents: PBr3 Reaction: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{O}\text{H} + \text{PBr}_{3} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{Br} + \text{P}\text{O}\text{Br}_{2} + \text{HBr}\)

Perform Elimination reaction to form 2-hexyne

Finally, perform an elimination reaction using a strong base like sodium amide (NaNH2) to create the desired alkene, 2-hexyne. Reagents: NaNH2 Reaction: \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{Br} + 2 \text{NaNH}_{2} \rightarrow \text{CH}_{3}\text{C}\equiv\text{C}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3} + 2 \text{NaBr} + 2 \text{NH}_{3}\)