Question

Q and R in the reaction sequences are respectively:

• A. Hg(OAc)₂, NaBH₄/OH⁻; B₂H₆, H₂O₂/OH⁻
• B. B₂H₆, H₂O₂/OH⁻; H⁺/H₂O
• C. Hg(OAc)₂, NaBH₄/OH⁻; H⁺/H₂O
• D. B₂H₆, H₂O₂/OH⁻; Hg(OAc)₂, NaBH₄/OH⁻

Hint:

Hydroboration-oxidation follows anti-Markovnikov addition, while oxymercuration-demercuration follows Markovnikov addition.

Answer 1

The Correct Option is D

The prompt requires identifying reagents Q and R that transform an alkene into an alcohol. Two main hydration methods exist: oxymercuration-demercuration and hydroboration-oxidation. We will examine these reaction sequences.

Reaction Sequence 1:

The first sequence, converting the alkene to an alcohol, utilizes **oxymercuration-demercuration**, a two-step process:

1. **Oxymercuration:** The alkene reacts with mercury(II) acetate \(Hg(OAc)_2\) in water. \(Hg(OAc)^+\) adds electrophilically to the double bond, followed by water addition to the more substituted carbon.

2. **Demercuration:** The mercurinium ion is reduced with sodium borohydride \(NaBH_4\) in a base (\(OH^-\)), replacing mercury with hydrogen.

Oxymercuration-demercuration follows Markovnikov's rule, placing the alcohol group (-OH) on the more substituted alkene carbon.

Therefore, reagent Q is \(Hg(OAc)_2, NaBH_4/OH^-\)

Reaction Sequence 2:

The second sequence converts the alkene to an alcohol via **hydroboration-oxidation**, also a two-step process:

1. **Hydroboration:** The alkene reacts with diborane \(B_2H_6\) (or a BH3 THF complex). Boron adds to the double bond in a *syn* fashion, preferentially adding to the less substituted carbon due to steric hindrance.

2. **Oxidation:** The resulting trialkylborane is oxidized with hydrogen peroxide \(H_2O_2\) in a base (\(OH^-\)), replacing boron with a hydroxyl group (-OH).

Hydroboration-oxidation results in an anti-Markovnikov addition of water, with the alcohol group (-OH) on the less substituted carbon, and is a *syn* addition.

Therefore, reagent R is \(B_2H_6, H_2O_2/OH^-\)

Conclusion:

Q and R are: \(Hg(OAc)_2, NaBH_4/OH^-\); \(B_2H_6, H_2O_2/OH^-\)