What is the major organic product formed when 1-methylcyclopentene...

When 1-methylcyclopentene is treated with hot acidic $\text{KMnO}_4$, the internal $\text{C=C}$ double bond undergoes complete oxidative cleavage, resulting in the opening of the five-membered ring.

The doubly bonded carbon atom bearing the methyl group has no vinylic hydrogen atoms, so it is oxidized to a ketone group.

The other doubly bonded carbon atom has one vinylic hydrogen atom, so it is oxidized to a carboxylic acid group.

The resulting straight-chain molecule contains six carbon atoms in total: the five carbons from the ring and the one from the methyl group.

Numbering from the carboxylic acid carbon as position 1, the ketone carbonyl is at position 5.

Therefore, the structure of the product is $\text{CH}_3\text{-CO-CH}_2\text{-CH}_2\text{-CH}_2\text{-COOH}$, and its IUPAC name is 5-oxohexanoic acid.

Answer: 5-Oxohexanoic acid