Ozonolysis of an organic compound A produces acetone and propionaldehyde in equimolar mixture. Ident

Q: Why do we get an aldehyde and not a carboxylic acid?

This is an example of reductive ozonolysis (typically using $Zn/H_2O$ or $(CH_3)_2S$). If oxidative ozonolysis were performed (using $H_2O_2$), the aldehyde would be further oxidized to a carboxylic acid.

Q: How do I know which way to number the carbon chain for IUPAC naming?

You must select the longest continuous carbon chain containing the double bond. Then, number the chain from the end that gives the double bond the lowest possible locant.

Q: What is the structure of acetone?

Acetone is the simplest ketone, with the IUPAC name propan-2-one. Its structure is $CH_3-CO-CH_3$.

Q: What is the structure of propionaldehyde?

Propionaldehyde is a three-carbon aldehyde, with the IUPAC name propanal. Its structure is $CH_3-CH_2-CHO$.

Q: Can this trick be used for all ozonolysis problems?

Yes! Whenever you are given the products of ozonolysis and asked to find the reactant, placing the carbonyl groups face-to-face and replacing the oxygens with a double bond will always give you the original alkene.

Question

Ozonolysis of an organic compound $A$ produces acetone and propionaldehyde in equimolar mixture. Identify $A$ from the following compounds.

Options:

(A) 2-methyl-1-pentene

(B) 1-pentene

(C) 2-pentene

(D) 2-methyl-2-pentene

Sigma Percentile · Accepted Answer

### The Magic of Ozonolysis: Reverse-Engineering Alkenes

Ozonolysis is a classic and highly testable reaction in organic chemistry. You can think of ozone ($O_3$) as a pair of molecular scissors that specifically targets and snips carbon-carbon double bonds. Once the bond is cut, the reaction caps each of the newly freed carbon ends with an oxygen atom, creating two carbonyl compounds (aldehydes or ketones).

But what happens when you are given the products and asked to find the original reactant? This is where we use a simple, foolproof trick to reverse-engineer the alkene.

### The Setup: Face-to-Face

In this problem, we are told that the ozonolysis of an unknown compound $A$ yields an equimolar mixture of **acetone** and **propionaldehyde**.

First, let's draw the structures of our products:
- Acetone: $H_3C-C(=O)-CH_3$
- Propionaldehyde: $O=CH-CH_2-CH_3$

To find the original alkene, we write both products side-by-side, ensuring that their carbonyl oxygen atoms are directly facing each other.

### The Execution: Stitching the Carbons

Now for the magic step. Since ozonolysis added those oxygen atoms by breaking a double bond, we simply reverse the process. We erase the two oxygen atoms and stitch the two carbonyl carbon atoms back together with a double bond.

$$H_3C-C(=O)-CH_3 + O=CH-CH_2-CH_3 \xrightarrow{	ext{Remove O atoms}} H_3C-C(CH_3)=CH-CH_2-CH_3$$

### Naming the Final Product

We have successfully reconstructed the original alkene. Now, we just need to determine its IUPAC name.

1. **Find the longest chain:** The longest continuous carbon chain containing the double bond has 5 carbons, making the parent name a **pentene**.
2. **Number the chain:** We number from the left to give the double bond the lowest possible locant (position 2).
3. **Identify substituents:** There is a methyl group attached to carbon-2.

Putting it all together, the name of the alkene is **$2$-methyl-$2$-pentene**.

**A Quick Note on Reaction Conditions:** The presence of an aldehyde (propionaldehyde) in the products confirms that this was a **reductive ozonolysis** (typically using $Zn/H_2O$ or $(CH_3)_2S$). If the reaction had been oxidative (using $H_2O_2$), the aldehyde would have been further oxidized into a carboxylic acid (propanoic acid). Always keep an eye on the reagents!

Animated Solution for Chemistry - Organic Chemistry: Ozonolysis of an organic compound $A$ produces acetone and propionaldehyde in equimolar mixture. Identify $A$ from the following compounds.

Select Answer:

Visualized Solution

Ozonolysis Reaction

The Reverse Trick

Aligning the Products

Stitching the Carbons

Final IUPAC Name

Reductive vs Oxidative

The Sigma Insight: Hydrocarbons

The Magic of Ozonolysis

The Setup

The Execution

Naming the Final Product

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