The major product of the following chemical reaction is CH3CH2CN - (i) H3O+, Delta (ii) SOCl2 (iii)

Q: Why doesn't the nitrile reduce directly to an amine in the first step?

The reagent used in the first step is $H_3O^+$ with heat, which provides conditions for acidic hydrolysis, not reduction. To reduce a nitrile to an amine, you would need a reducing agent like $LiAlH_4$ or $H_2/Ni$.

Q: What if we only did partial hydrolysis of the nitrile?

Partial hydrolysis of a nitrile (often done with cold concentrated acid or alkaline hydrogen peroxide) yields an amide ($R-CONH_2$) instead of a carboxylic acid.

Q: Why is $SOCl_2$ preferred over $PCl_3$ or $PCl_5$ for making acyl chlorides?

Thionyl chloride ($SOCl_2$) is preferred because its byproducts, $SO_2$ and $HCl$, are both gases that easily escape the reaction mixture. This drives the reaction to completion and makes purifying the acyl chloride much simpler.

Q: What is the exact purpose of $BaSO_4$ in the final step?

Barium sulfate ($BaSO_4$) acts as a catalytic poison. It intentionally reduces the activity of the palladium catalyst so that the reduction stops at the aldehyde stage. Without it, the aldehyde would be further reduced to a primary alcohol.

Q: Could we use $LiAlH_4$ instead of the Rosenmund reduction to get the aldehyde?

No, $LiAlH_4$ is a very strong reducing agent and would reduce the acyl chloride completely to a primary alcohol. To stop at the aldehyde, you must use a poisoned catalyst like in the Rosenmund reduction, or a milder, bulky reducing agent like DIBAL-H.

Question

The major product of the following chemical reaction is
$CH_3CH_2CN \xrightarrow[	ext{(iii) } Pd/BaSO_4, H_2]{	ext{(i) } H_3O^+, \Delta 	ext{ (ii) } SOCl_2} ?$

Options:

(A) $CH_3CH_2CH_2OH$

(B) $(CH_3CH_2CO)_2O$

(C) $CH_3CH_2CH_3$

(D) $CH_3CH_2CHO$

Sigma Percentile · Accepted Answer

### Analyzing the Setup

Organic chemistry is often like a puzzle where each reagent acts as a specific tool to transform one functional group into another. In this problem, we are given a starting material, ethyl cyanide ($CH_3CH_2CN$), and a sequence of three distinct reagents. Our goal is to trace the journey of the molecule through each step to find the final major product.

### Step 1: Acidic Hydrolysis of Nitrile

The first reagent is hydronium ion ($H_3O^+$) accompanied by heat ($\Delta$). This is the classic condition for the acidic hydrolysis of a nitrile. When a nitrile is subjected to complete hydrolysis in an acidic medium, the carbon-nitrogen triple bond is completely broken. The nitrogen atom leaves as an ammonium ion, and the carbon atom is fully oxidized to a carboxylic acid group.

$$CH_3CH_2CN \xrightarrow[\Delta]{H_3O^+} CH_3CH_2COOH$$

So, our ethyl cyanide is transformed into **propanoic acid**.

### Step 2: Formation of Acyl Chloride

Next, the propanoic acid is treated with thionyl chloride ($SOCl_2$). Thionyl chloride is a highly favored reagent in organic synthesis for converting carboxylic acids into acyl chlorides. Why is it so popular? Because the byproducts of this reaction are sulfur dioxide ($SO_2$) and hydrogen chloride ($HCl$), both of which are gases that escape the reaction mixture, driving the reaction forward and making purification incredibly easy.

$$CH_3CH_2COOH \xrightarrow{SOCl_2} CH_3CH_2COCl$$

Through this step, propanoic acid becomes **propanoyl chloride**.

### Step 3: The Rosenmund Reduction

The final step involves treating the propanoyl chloride with hydrogen gas ($H_2$) in the presence of a palladium catalyst supported on barium sulfate ($Pd/BaSO_4$). This specific combination of reagents is the hallmark of the **Rosenmund reduction**.

Normally, palladium is a very strong hydrogenation catalyst that would reduce the acyl chloride all the way down to a primary alcohol. However, the barium sulfate acts as a catalytic poison. It intentionally decreases the activity of the palladium, stopping the reduction exactly at the aldehyde stage.

$$CH_3CH_2COCl \xrightarrow{Pd/BaSO_4, H_2} CH_3CH_2CHO$$

### Final Conclusion

Thanks to the selective poisoning by barium sulfate, the propanoyl chloride is reduced to **propanaldehyde** ($CH_3CH_2CHO$). Tracing the entire sequence, we successfully converted a nitrile into an aldehyde via a carboxylic acid and an acyl chloride intermediate. The correct option is indeed (d).

Animated Solution for Chemistry - Carbonyl Compounds: The major product of the following chemical reaction is $C H_{3} C H_{2} C N (i) H_{3} O^{+}, Δ (ii) S O C l_{2} (iii) P d / B a S O_{4}, H_{2} ?$

Select Answer:

Visualized Solution

$R e a c t i o n S e q u e n ce O v er v i e w$

$S t e p 1 : H y d r o l y s i so f N i t r i l e$

$S t e p 2 : F or ma t i o n o f A cy l C h l or i d e$

$S t e p 3 : R ose nm u n d R e d u c t i o n$

$F ina l P r o d u c t$

The Sigma Insight: Preparation of Aldehydes

Analyzing the Setup

Step 1

Step 2

Step 3

Final Conclusion