Peroxide Effect In Addition Reactions

The peroxide or Kharasch effect reverses the normal orientation of $HBr$ addition to alkenes, and it operates only with $HBr$, not with $HCl$ or $HI$. Peroxides initiate a free radical chain: they decompose to give radicals that abstract hydrogen from $HBr$, generating bromine radicals. The bromine radical adds to the terminal carbon of but-1-ene because that produces the more stable secondary carbon radical $C{H}_3-C{H}_2-C{H}^{\bullet }-C{H}_{2}Br$ rather than a primary one. This radical then abstracts hydrogen from another $HBr$ molecule, placing hydrogen on the internal carbon and giving 1-bromobutane, the anti-Markovnikov product. The Markovnikov option is wrong because peroxides switch the mechanism from ionic to radical, changing the regiochemistry. The third option pairs the correct product with the wrong mechanism, since anti-Markovnikov orientation here arises from radicals, not carbocations. The claim of no reaction is false because peroxides actually accelerate $HBr$ addition. The reason the effect is limited to $HBr$ is energetic: for $HCl$ the hydrogen-abstraction step is endothermic and for $HI$ the halogen-addition step is endothermic, so neither sustains an efficient chain, whereas both propagation steps for $HBr$ are exothermic. This follows the NCERT account of the peroxide effect and connects radical stability with reaction orientation. A consistency check: only $HBr$ shows this effect because its bond energetics make both radical steps exothermic.