Question

Which product is obtained when 1-pentene reacts with HCl in the presence of benzoyl peroxide?

• A. 1-chloro pentane
• B. 2-chloro pentane
• C. 1-chloro, 3-methyl butane
• D. 2,2-dimethyl 1-chloropropane

Hint:

The peroxide (Kharasch) effect only works for one specific hydrogen halide.

Answer 1

The Correct Option is B

Step 1: Peroxides are known to reverse the regiochemistry of addition of HX to unsymmetrical alkenes, an effect called the peroxide effect or Kharasch effect, but this reversal only operates for HBr. Step 2: The peroxide effect requires a free radical chain mechanism. The initiation step would need to generate a chlorine radical from HCl, but the H-Cl bond is very strong, so this step is highly endothermic and does not proceed under normal conditions. Step 3: Because the radical chain cannot be sustained with HCl, benzoyl peroxide has no real effect on this reaction, and the addition proceeds by the normal ionic (electrophilic addition) mechanism instead. Step 4: In the ionic mechanism, Markovnikov's rule applies: the proton adds to the carbon of the double bond that already carries more hydrogens, and chloride adds to the carbon that can form the more stable carbocation. \[ CH_2=CH-CH_2-CH_2-CH_3 + HCl \rightarrow CH_3-CHCl-CH_2-CH_2-CH_3 \] Step 5: Here the proton adds to C1, and chlorine adds to C2, the more substituted secondary carbon, giving 2-chloropentane as the major product. \[\boxed{\text{2-chloropentane}}\]