Beginning with sodium acetylide (NaCCH), propose a three-step synthesis of hexanal.
The correct answer and explanation is :
To synthesize hexanal starting from sodium acetylide (NaCCH), a three-step synthesis could be designed as follows:
Step 1: Addition of Sodium Acetylide to Formaldehyde
The first step involves the nucleophilic addition of sodium acetylide (NaCCH) to formaldehyde (CH₂O). Sodium acetylide is a strong nucleophile, with the acetylide ion (C≡C⁻) attacking the electrophilic carbonyl carbon of formaldehyde.
Reaction: NaCCH+CH2O→CH3C(OH)CH2 \text{NaCCH} + \text{CH}_2\text{O} \rightarrow \text{CH}_3\text{C}(\text{OH})\text{C}H_2
In this reaction, sodium acetylide reacts with formaldehyde to form 1-propynol (CH₃C(OH)CH₂), which is an alcohol with a terminal alkyne group.
Step 2: Reduction of 1-Propynol to 1-Propanol
The second step involves the reduction of the alkyne in 1-propynol to a saturated alcohol. This can be achieved by catalytic hydrogenation or by using a reducing agent such as Lindlar’s catalyst, which specifically reduces alkynes to cis-alkenes.
Reaction: CH3C(OH)CH2→H2Lindlar’s catalystCH3CH2OH\text{CH}_3\text{C}(\text{OH})\text{C}H_2 \xrightarrow[\text{H}_2]{\text{Lindlar’s catalyst}} \text{CH}_3\text{CH}_2\text{OH}
The reaction reduces the triple bond to a single bond, resulting in 1-propanol (CH₃CH₂OH), an alcohol.
Step 3: Oxidation of 1-Propanol to Hexanal
In the final step, 1-propanol undergoes oxidation to form hexanal (C₆H₁₂O). This can be accomplished using an oxidizing agent like PCC (pyridinium chlorochromate) or any mild oxidizing agent that selectively oxidizes alcohols to aldehydes without further oxidation to acids.
Reaction: CH3CH2OH→PCCCH3CH2CH2CHO\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{PCC}} \text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}
This oxidation reaction results in the formation of hexanal (CH₃CH₂CH₂CHO), which is an aldehyde.
Explanation:
In this three-step synthesis:
- Sodium acetylide acts as a nucleophile, attacking the carbonyl carbon of formaldehyde to form a 1-propynol intermediate.
- The alkyne functional group in 1-propynol is reduced to a saturated alcohol, yielding 1-propanol.
- Finally, the alcohol is oxidized to an aldehyde, yielding hexanal.
This synthesis route is efficient, utilizing relatively simple reagents and common reactions to achieve the desired product.