Question

Identify correct method of preparation of acetaldehyde from reaction of cyanide (a) \(\mathrm{Me}-\mathrm{C} \equiv \mathrm{N} \frac{\text { (i) DIBAL }}{\text { (ii) } \mathrm{H}_{3} \mathrm{O}^{+}}\) (b) \(\mathrm{Me}-\mathrm{C} \equiv \mathrm{N} \frac{\text { (i) } \mathrm{SnCl}_{2}+\mathrm{HCl}}{\text { (ii) } \mathrm{H}_{3} \mathrm{O}^{+}}\) (c) \(\mathrm{Me}-\mathrm{C} \equiv \mathrm{N} \frac{\text { (i) Conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\text { (ii) dill. } \mathrm{NaOH}}\) (d) \(\mathrm{Me}-\mathrm{C} \equiv \mathrm{N} \frac{\text { (i) } \mathrm{Pd} / \mathrm{BaSo}_{4} / \mathrm{H}_{2}}{\text { (ii) } \mathrm{H}_{3} \mathrm{O}^{\oplus}}\)

Short answer

The correct method is (a), using DIBAL-H followed by hydrolysis.

Step-by-step solution

Review the Mechanisms

To determine the correct method of preparing acetaldehyde from a cyanide, review the chemical mechanisms associated with each option. We are looking for the reduction of a nitrile group to form an aldehyde.

Examine Option (a)

In option (a), the reaction involves \( \text{Me-C} \equiv \text{N} \) treated with DIBAL-H followed by hydrolysis with \( \text{H}_3\text{O}^+ \). DIBAL-H is known to reduce nitriles to aldehydes, making this a viable method to produce acetaldehyde.

Evaluate Option (b)

This option uses \( \text{SnCl}_2 + \text{HCl} \) followed by \( \text{H}_3\text{O}^+ \). This method generally reduces nitriles to amines, not aldehydes, thus not suitable for preparing acetaldehyde.

Consider Option (c)

Option (c) involves \( \text{Conc. } \text{H}_2\text{SO}_4 \) and then \( \text{dil. NaOH} \), which typically dehydrates or converts nitriles into acids or amides, rather than aldehydes.

Assess Option (d)

This option uses \( \text{Pd/BaSO}_4/\text{H}_2\) followed by \( \text{H}_3\text{O}^+ \). This setup is known as the Rosenmund reduction, traditionally used for acid chlorides to aldehydes; however, it is not applicable to nitriles.

Conclusion

After analyzing the mechanisms of each option, the reduction of nitriles to aldehydes is best achieved using DIBAL-H as in option (a). This option uses DIBAL followed by hydrolysis to successfully produce acetaldehyde from cyanide.

Key concepts

cyanide reduction

Cyanide reduction is an important process in organic chemistry, especially when converting nitriles into more functional molecules. When we talk about cyanide reduction, particularly in the context of preparing acetaldehyde, the nitrile (represented as \( R-C \equiv N \)) is reduced to form an aldehyde (\( R-CHO \)). In our case, the nitrile is \( \text{CH}_3-C\equiv N \), which needs reduction to \( \text{CH}_3-CHO \), or acetaldehyde.

Common reducing agents for this purpose include DIBAL-H (Diisobutylaluminium hydride), which we find in our example as option (a). This reagent is specially known for selectively reducing nitriles to aldehydes when used in controlled conditions.

To sum up, the goal of cyanide reduction here is the successful transformation of a nitrile group into an aldehyde, making it critical for producing acetaldehyde from cyanide through reduction.

DIBAL-H reduction

DIBAL-H is an abbreviation for Diisobutylaluminium hydride, a powerful reagent used in organic chemistry for reduction purposes. It is particularly famous for reducing nitriles to aldehydes. This makes it very useful when a chemist desires controlled reduction without fully reducing down to an amine.

**How DIBAL-H Works**
- DIBAL-H is a milder reducing agent compared to others like lithium aluminum hydride (LiAlH\(_4\)). This is crucial as it stops the reduction at the aldehyde stage.
- It selectively reduces nitrile groups when the reaction mixture is kept cold (often around -78°C).
- DIBAL-H is used in conjunction with hydrolysis (addition of \( \text{H}_3\text{O}^+ \)) to convert the intermediate formed into the final aldehyde product.

In the preparation of acetaldehyde from \( \text{CH}_3-C\equiv N \) using DIBAL-H, the reduction step involves the DIBAL-H imparting a hydride ion \( \text{H}^- \) to form an iminium ion, which hydrolyzes to yield the desired aldehyde product upon the addition of water.

nitrile to aldehyde

Converting a nitrile to an aldehyde involves partial reduction, often requiring precise conditions and reagents. This transformation makes possible the synthesis of aldehydes without stepping down to further reduced forms like alcohols or amines.

In our scenario, the nitrile \( \text{CH}_3-C\equiv N \) needs to be transformed into \( \text{CH}_3-CHO \), acetaldehyde. Specific reagents are necessary, like DIBAL-H, due to its ability to halt reduction at the aldehyde stage.

**Steps in the Transformation**

• Initial Reduction: DIBAL-H inserts a hydride, forming a complex intermediate with the nitrile.
• Selective Reduction: Controlled parameters ensure it reacts only once, producing an iminium ion rather than further reducing to an amine.
• Hydrolysis Stage: Water is added to convert the iminium to the final aldehyde product through the addition of \( \text{H}_3\text{O}^+ \).

This controlled process is key to converting nitrile directly into aldehydes efficiently.

organic reaction mechanisms

Understanding organic reaction mechanisms is essential in predicting the outcome of chemical reactions. They describe how atoms and molecules behave during the processes. It gives insight into pathways taken by electrons and the breaking and forming of bonds.

For the preparation of acetaldehyde from a nitrile using DIBAL-H, understanding the mechanism helps to grasp why this reagent is suitable.

**Basic Steps in the Mechanism**

• **Initiation**: The DIBAL-H complexes with the nitrile, offering a hydride ion \( \text{H}^- \).
• **Intermediate Stage**: The reaction halts at the iminium ion stage with careful temperature control and stoichiometry preventing further reaction.
• **Completion**: Hydrolysis transforms the intermediate to an aldehyde, driven by the chemistry of the iminium ion reacting with water \( \text{H}_3\text{O}^+ \).

As with many reactions, understanding these mechanisms allows chemists to design specific reactions that target desired functional groups while avoiding unwanted product formations.