Which two organic chemists are known for observing the peroxide effect in adding reagents to unsaturated compounds in 1933?

Correct — B. The peroxide effect was reported in 1933 by Morris S Kharasch and Frank R Mayo, working on the addition of hydrogen bromide to allyl bromide.

The question's phrasing is lifted from their paper's own title, "The Peroxide Effect in the Addition of Reagents to Unsaturated Compounds", which is why the wording sounds unlike a textbook.

They showed that the direction of the addition flips when peroxides are present. The effect is still called the Kharasch effect after the senior author.

Why the others are wrong

• (a)
  Justus von Liebig and Friedrich Wöhler — Justus von Liebig and Friedrich Wohler are nineteenth-century chemists — Wohler's urea synthesis dates from 1828 — working a century before this result and on entirely different problems.
• (c)
  B Steven Bachrach and Roald Hoffmann — Roald Hoffmann shared the 1981 Nobel Prize in Chemistry for orbital-symmetry rules developed in the 1960s. Neither he nor Bachrach has any connection with the 1933 observation.
• (d)
  C John Cadogan and Luis M Campos — Neither John Cadogan nor Luis M Campos worked on the peroxide effect. The option does what SSC often does here, pairing real chemists with a discovery that is not theirs.

Markovnikov's rule says that in adding HX to an unsymmetrical alkene, the hydrogen goes to the carbon already carrying more hydrogens, because the reaction runs through the more stable carbocation.

Peroxides change the mechanism, not the rule. They generate free radicals, the reaction becomes a radical chain of initiation, propagation and termination, and the bromine adds to the other carbon. The product is the anti-Markovnikov one.

The effect works only with HBr. With HCl the hydrogen-chlorine bond is too strong to break in the chain step, and with HI the iodine step is unfavourable, so both keep to Markovnikov addition.

Key facts

• Kharasch and Mayo reported the peroxide effect in 1933.
• The peroxide effect gives the anti-Markovnikov product when HBr adds to an unsymmetrical alkene.
• It operates with HBr but not with HCl or HI.
• The reaction proceeds by a free-radical chain mechanism initiated by peroxides.
• The effect is also known as the Kharasch effect.

Common traps

• Extending the peroxide effect to HCl or HI.
• Calling anti-Markovnikov addition an exception to Markovnikov's rule rather than a different mechanism.

No directly related past PYQ was found.