Q.Choose the correct option.
Identify 'A' in the following reaction:
CH3-C(CH3)=CH2 --A--> CH3-CO-CH3 + CO2 + H2O
a. KMnO4/H+ b. alkaline KMnO4 c. dil. H2SO4/1% HgSO4 d. NaOH/H2O2

Step 1. Read the transformation. CH3-C(CH3)=CH2 (2-methylpropene) is converted to CH3-CO-CH3 (acetone) plus CO2 and H2O.

Step 2. Match to the reaction type. Section 15.2.4 describes hot acidic KMnO4 (or acidic K2Cr2O7) as cleaving a C=C bond outright: a carbon bearing two carbon substituents becomes a ketone, while a carbon bearing hydrogen (here, the terminal =CH2, which has two H's and no carbon substituent) is oxidised all the way to CO2 and water. Here the internal carbon (bearing two CH3 groups) becomes the ketone acetone, and the terminal =CH2 carbon is fully destroyed to CO2 + H2O -- exactly matching the given products.

Step 3. Rule out the other options. Alkaline KMnO4 (option b) is the MILD, cold-dilute hydroxylation reagent that gives a 1,2-diol, not a cleavage product. Dilute H2SO4/1% HgSO4 (option c) is the alkyne-hydration reagent, irrelevant to an alkene substrate. NaOH/H2O2 (option d) is not a reagent used anywhere in this chapter's alkene reactions.

Recognising the full oxidative-cleavage signature (a terminal alkene carbon destroyed to CO2+H2O, an internal carbon surviving as a ketone) as specifically the hot-acidic-KMnO4 reaction

• Confusing hot ACIDIC KMnO4 (cleaves the double bond) with cold DILUTE ALKALINE KMnO4 (only hydroxylates, giving a diol, section 15.2.4) -- the two reagents give very different products from the same alkene.
• Not noticing that CO2 formation specifically signals a terminal =CH2 carbon being fully destroyed, which only the acidic KMnO4/H+ cleavage produces.