Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene?

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Accepted Answer

> [!TLDR]
> The alkene must have a carbon‑carbon double bond that, when cleaved by ozone, gives exactly propanal ($\mathrm{CH_3CH_2CHO}$) and pentan‑3‑one ($\mathrm{CH_3CH_2COCH_2CH_3}$). Reconnecting the two carbonyl fragments across the original double bond yields **3‑ethylhex‑3‑ene**.

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### Why this approach works

Ozonolysis is a clean chemical “scissors” that cuts a $\mathrm{C=C}$ bond and replaces each carbon of the double bond with a carbonyl group ($\mathrm{C=O}$).  
- If the carbon had two hydrogens attached, you get an aldehyde ($\mathrm{-CHO}$).  
- If it had one hydrogen and one alkyl group, you still get an aldehyde.  
- If it had two alkyl groups (no hydrogen), you get a ketone ($\mathrm{C=O}$ with two $\mathrm{R}$ groups).

So the two products tell us exactly what was attached to each end of the original double bond. To find the alkene, we simply “glue” the two carbonyl fragments back together — removing the oxygen atoms and joining the two carbons with a double bond.

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### Step‑by‑step reconstruction

1. **Identify the carbonyl fragments**  
   - Propanal: $\mathrm{CH_3CH_2CHO}$  
     The aldehyde carbon (the $\mathrm{C=O}$) came from a carbon that had **one hydrogen** and one alkyl group ($\mathrm{CH_3CH_2-}$) attached to the original double bond.  
   - Pentan‑3‑one: $\mathrm{CH_3CH_2COCH_2CH_3}$  
     The ketone carbon (the $\mathrm{C=O}$) came from a carbon that had **two alkyl groups** ($\mathrm{CH_3CH_2-}$ and $\mathrm{CH_3CH_2-}$) attached to the original double bond — no hydrogen.

2. **Reconnect the two carbons**  
   Remove the oxygen from each carbonyl and join the two carbons with a double bond.  
   - Left fragment (from propanal): $\mathrm{CH_3CH_2CH=}$  
   - Right fragment (from pentan‑3‑one): $\mathrm{=C(CH_2CH_3)_2}$

Putting them together:  
   $$\mathrm{CH_3CH_2CH=C(CH_2CH_3)_2}$$

3. **Name the alkene**  
   The longest chain containing the double bond has 6 carbons:  
   $\mathrm{CH_3CH_2CH=C(CH_2CH_3)_2}$  
   Nu

Isomer Name	Common Name	Structure (Condensed)	Boiling Point (°C)
Pentane	n-Pentane	$CH_3(CH_2)_3CH_3$	36.1
2-Methylbutane	Isopentane	$(CH_3)_2CHCH_2CH_3$	27.7
2,2-Dimethylpropane	Neopentane	$C(CH_3)_4$	9.5

Q.Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene?

Concept understanding — Chain Isomerism

Chain Isomerism: The Skeleton of a Molecule

The Intuition

The Precise Statement

A Concrete Example: Pentane ( $C_5H_{12}$ )

When Does Chain Isomerism Occur?

Why this approach works

Step‑by‑step reconstruction

Q.Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene?

Why this approach works

Step‑by‑step reconstruction

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