Chemical Reactions

Step 1: Markovnikov's Rule

Markovnikov's rule states that in the electrophilic addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens, and the halogen to the carbon with fewer hydrogens, forming the more stable carbocation.

Step 2: Evaluate Options

Option (a): HBr to propene

Propene ( ) reacts with HBr:

H adds to the carbon (more hydrogens), and Br to the carbon, following Markovnikov's rule.

Result: Applicable.

Option (b): HBr to propene with peroxides

With peroxides, HBr addition to propene ( ) is free radical, yielding anti-Markovnikov product:

Br adds to the carbon, opposite to Markovnikov's rule.

Result: Not applicable.

Option (c): Br to ethene

Ethene ( ) is symmetrical, and Br addition forms:

No regioselectivity is needed, so Markovnikov's rule does not apply.

Result: Not applicable.

Option (d): Hydroboration-oxidation

Hydroboration-oxidation of propene ( ) gives:

Boron adds to the less substituted carbon, yielding anti-Markovnikov addition.

Result: Not applicable.

Step 3: Final Answer

Only option (a) follows Markovnikov's rule.