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🥼organic chemistry review

Olah's Reagent

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Olah's reagent, also known as magic acid, is a strong superacid system used in organic chemistry for the activation and conversion of alcohols into alkyl halides. It plays a crucial role in the preparation of alkyl halides from alcohols, a common transformation in organic synthesis.

5 Must Know Facts For Your Next Test

  1. Olah's reagent is a mixture of antimony pentafluoride (SbF5) and fluorosulfuric acid (HSO3F), creating a superacid system with a Hammett acidity function (H0) of around -20.
  2. The strong acidity of Olah's reagent allows it to protonate alcohols, forming a carbocation intermediate that can be substituted by a halide ion to produce an alkyl halide.
  3. Olah's reagent is particularly useful for the preparation of tertiary alkyl halides, which are otherwise difficult to synthesize due to steric hindrance.
  4. The reaction with Olah's reagent typically proceeds via an $\SN1$ mechanism, where the rate-determining step is the formation of the carbocation intermediate.
  5. Olah's reagent is a versatile reagent that can also be used for other organic transformations, such as the activation of alkanes for electrophilic substitution reactions.

Review Questions

  • Explain the role of Olah's reagent in the preparation of alkyl halides from alcohols.
    • Olah's reagent, a superacid system composed of antimony pentafluoride (SbF5) and fluorosulfuric acid (HSO3F), is used to activate alcohols for the substitution of the hydroxyl group with a halide ion. The strong acidity of Olah's reagent protonates the alcohol, forming a carbocation intermediate that can then be attacked by a halide nucleophile, resulting in the formation of an alkyl halide. This transformation is particularly useful for the synthesis of tertiary alkyl halides, which are otherwise difficult to obtain due to steric factors.
  • Describe the mechanism by which Olah's reagent facilitates the conversion of alcohols to alkyl halides.
    • The reaction of alcohols with Olah's reagent typically proceeds via an $\SN1$ mechanism. The first step involves the protonation of the alcohol by the superacid, forming a carbocation intermediate. This carbocation is then attacked by a halide ion, such as fluoride or chloride, to displace the hydroxyl group and produce the corresponding alkyl halide. The strong acidity of Olah's reagent is crucial in stabilizing the carbocation intermediate, which is the rate-determining step of the reaction.
  • Discuss the versatility of Olah's reagent and its potential applications beyond the preparation of alkyl halides from alcohols.
    • In addition to its use in the conversion of alcohols to alkyl halides, Olah's reagent is a versatile reagent that can be employed in various other organic transformations. The superacid system can be used to activate alkanes for electrophilic substitution reactions, allowing for the functionalization of otherwise unreactive hydrocarbon substrates. Furthermore, Olah's reagent has been utilized in the protonation and activation of other organic functional groups, such as ethers and esters, enabling a range of synthetic manipulations. The exceptional acidity and reactivity of Olah's reagent make it a valuable tool in the arsenal of organic chemists, expanding the scope of possible reactions and transformations in organic synthesis.