Q.An unsaturated hydrocarbon 'A' adds two molecules of H2 and on reductive ozonolysis gives butane-1,4-dial, ethanal and propanone. Give the structure of 'A', write its IUPAC name and explain the reactions involved.

Concept: Structure determination from ozonolysis products

The hydrocarbon adds two molecules of H₂, so it contains two C=C double bonds (a diene). Reductive ozonolysis ($\ce{O3}$, then $\ce{Zn/H2O}$) cleaves each C=C and turns each doubly-bonded carbon into a carbonyl. To find A, reverse the process — reconnect the carbonyl carbons of the three products with double bonds.

The products are:

• Butane-1,4-dial, $\ce{OHC-CH2-CH2-CHO}$ — a carbonyl at both ends, so it must be the central fragment, joined by a double bond on each side.
• Ethanal, $\ce{CH3-CHO}$ — from a $\ce{CH3-CH=}$ end.
• Propanone, $\ce{(CH3)2C=O}$ — from a $\ce{(CH3)2C=}$ end.

Reconnecting (one end of butane-1,4-dial to ethanal, the other to propanone):

$\ce{(CH3)2C=CH-CH2-CH2-CH=CH-CH3}$ …

Reading the clues

• A adds two molecules of $\text{H}_2$ so it contains two C=C double bonds (a diene).
• Reductive ozonolysis ($\text{O}_3$, then $\text{Zn/H}_2\text{O}$) cleaves each C=C and turns each doubly-bonded carbon into a carbonyl group. To find A, reverse the process: replace each $\text{C=O}$ by a $\text{C=C}$ and reconnect the fragments.

The three carbonyl products are:

• Butane-1,4-dial, $\text{OHC-CH}_2\text{-CH}_2\text{-CHO}$ (a dialdehyde - carbonyls at both ends)
• Ethanal, $\text{CH}_3\text{-CHO}$
• Propanone, $\text{(CH}_3\text{)}_2\text{C=O}$

Reconstructing A

Because butane-1,4-dial carries a carbonyl at each end, it must be the central fragment, joined by a double bond on either side. Its two ends therefore reconnect to the other two fragments:

• one end $\text{=CH-}$ joins the ethanal carbon $\text{CH}_3\text{-CH=}$
• the other end $\text{=CH-}$ joins the propanone carbon $\text{=C(CH}_3\text{)}_2$

Replacing the paired $\text{C=O}$ groups by $\text{C=C}$ bonds:

Numbering the eight-carbon chain to give the substituent the lowest locant places the methyl at C-2 and the double bonds at C-2 and C-6.

A = 2-methylocta-2,6-diene (molecular formula $\text{C}_9\text{H}_{16}$).

Verification by ozonolysis

• $\text{(CH}_3\text{)}_2\text{C=}$ gives propanone, $\text{(CH}_3\text{)}_2\text{C=O}$
• $\text{=CH-CH}_2\text{-CH}_2\text{-CH=}$ gives butane-1,4-dial, $\text{OHC-CH}_2\text{-CH}_2\text{-CHO}$
• $\text{=CH-CH}_3$ gives ethanal, $\text{CH}_3\text{-CHO}$

All three match, confirming the structure. …