JoVE Logo

Anmelden

This lesson delves into the conversion of alcohols to corresponding alkyl halides and the mechanism of action for different reagents. Typically, the hydroxyl group is first protonated to convert it to a stable leaving group. Consequently, based on the starting alcohol, the mechanism undergoes either of the nucleophilic substitution routes, SN1 or SN2. Tertiary alkyl halides are made using the two-step SN1 mechanism that occurs via a carbocation intermediate, which is stabilized by hyperconjugation. However, for primary alcohols, the protonation of the hydroxyl group leads to the concerted SN2 route. Secondary alcohols can proceed via either mechanism based on the reaction conditions.

The popular reagents used for converting alcohols to corresponding alkyl halides include the hydrogen halides like hydrogen bromide and hydrogen chloride. However, while it is straightforward with the former, the latter needs an additional catalyst like zinc chloride. This catalyzes the hydroxyl group into a better leaving group enabling the subsequent SN2 process. Other reagents of choice are thionyl chloride and phosphorus tribromide with a similar mechanism. In the presence of relatively weak bases like pyridine/tertiary amine, they generate an excellent leaving group compared to the original leaving group of water.

The most exciting class of reagents is sulfonyls. They react with the alcohols to form corresponding mesylates, tosylates, or triflates to improve their reactivity in an SN2 reaction. In these species, resonance stabilization is inherent to the sulfonyl group. Additional resonance stabilization is contributed by the benzene ring of the tosyl group, and further stability is provided by the strongly electron-withdrawing trifluoromethyl in the triflate.

Stereochemistry

Most importantly, the choice of reagent influences the stereochemistry of the product formed. The use of thionyl chloride leads to an inversion of configuration, while tosyl chlorides retain the chiral configuration in the native alcohol.

Tags

ConversionAlcoholsAlkyl HalidesMechanismReagentsHydroxyl GroupProtonatedLeaving GroupNucleophilic SubstitutionSN1SN2Tertiary Alkyl HalidesCarbocation IntermediateHyperconjugationPrimary AlcoholsConcerted SN2 RouteSecondary AlcoholsReaction ConditionsHydrogen HalidesHydrogen BromideHydrogen ChlorideZinc Chloride CatalystThionyl ChloridePhosphorus TribromidePyridine tertiary Amine BasesSulfonylsMesylatesTosylatesTriflates

Aus Kapitel 10:

article

Now Playing

10.10 : Conversion of Alcohols to Alkyl Halides

Alkohole und Phenole

7.0K Ansichten

article

10.1 : Struktur und Nomenklatur von Alkoholen und Phenolen

Alkohole und Phenole

15.8K Ansichten

article

10.2 : Physikalische Eigenschaften von Alkoholen und Phenolen

Alkohole und Phenole

13.8K Ansichten

article

10.3 : Säure und Basizität von Alkoholen und Phenolen

Alkohole und Phenole

18.3K Ansichten

article

10.4 : Herstellung von Alkoholen durch Additionsreaktionen

Alkohole und Phenole

6.1K Ansichten

article

10.5 : Säurekatalysierte Dehydratisierung von Alkoholen zu Alkenen

Alkohole und Phenole

18.9K Ansichten

article

10.6 : Alkohole aus Carbonylverbindungen: Reduktion

Alkohole und Phenole

10.1K Ansichten

article

10.7 : Alkohole aus Carbonylverbindungen: Grignard-Reaktion

Alkohole und Phenole

5.1K Ansichten

article

10.8 : Schutz vor Alkoholen

Alkohole und Phenole

7.1K Ansichten

article

10.9 : Aufbereitung von Diolen und Pinacol Rearrangement

Alkohole und Phenole

3.2K Ansichten

article

10.11 : Oxidation von Alkoholen

Alkohole und Phenole

12.6K Ansichten

article

10.12 : Herstellung von Alkoholen durch Substitutionsreaktionen

Alkohole und Phenole

5.6K Ansichten

JoVE Logo

Datenschutz

Nutzungsbedingungen

Richtlinien

Forschung

Lehre

ÜBER JoVE

Copyright © 2025 MyJoVE Corporation. Alle Rechte vorbehalten