Oxidative cleavage of alkenes by KMnO4

Steps:

• Hot concentrated acidified KMnO4 cleaves the C=C bond oxidatively: =CH2 yields CO2, =CHR yields RCOOH, =CR2 yields RCOR.
• For but-1-ene (CH3CH2CH=CH2), the double bond is between C3=C4, where C4 is =CH2 (to CO2) and C3 is =CH-CH2CH3 (to CH3CH2COOH).
• Propanoic acid (CH3CH2COOH) forms from the =CH-CH2CH3 fragment.
• Other alkenes lack this fragment, producing different carboxylic acids or ketones.

Why A is correct:

• But-1-ene's terminal =CH2 and =CH-R (R=CH2CH3) yield CO2 and propanoic acid per oxidative cleavage rules.

Why the others are wrong:

• B: But-2-ene (CH3CH=CHCH3) yields two CH3COOH molecules.
• C: 2-Methylpropene ((CH3)2C=CH2) yields (CH3)2C=O and CO2.
• D: 2-Methylbut-1-ene (CH3CH2C(CH3)=CH2) yields CH3CH2C(O)CH3 and CO2.

Final answer: A