Mechanisms of Alcohol Reactions

Introduction to Alcohol Reactions

  • Alcohols undergo various reactions involving conversion to better leaving groups such as bromides, chlorides, or sulfonates.

Mechanisms of Alcohol Conversion

SN2 Reaction with Phosphorus Tribromide (PBr3)

  • Inapplicability to Tertiary Alcohols

    • Reactions do not work on tertiary alcohols due to steric hindrance; key mechanism involves SN2 which does not favor crowded positions.

    • Absence of carbohydrate intermediates prevents rearrangements and makes stereochemistry predictable.

  • Example Reaction:

    • React r-isomer of 2-butanol with PBr3 in a 3:1 ratio.

    • Results in:

    • 3 equivalents of 2-bromobutane (s-isomer)

    • 1 equivalent of phosphoryl acid (H3PO3)

Mechanism of Reaction with PBr3

  • Step 1: Alcohol reacts with phosphorus tribromide.

    • Phosphorus acts as an electrophile, and alcohol's oxygen attacks phosphorus, breaking a phosphorus-bromine bond and releasing Br⁻.

    • Forming a complex where alcohol's oxygen is positively charged and bonded to phosphorus.

  • Step 2: Formation of good leaving group

    • Hydroxyl group is transformed; behaves like a better leaving group (e.g., water).

    • Resulting leaving group: di-bromyl phosphate, which facilitates subsequent nucleophilic attack.

  • SN2 Step:

    • Br⁻ attacks the secondary carbon, resulting in inversion of stereochemistry during substitution.

    • Remaining bromines on phosphorus allow reaction with more alcohol molecules (3:1 ratio).

Alternative Reaction: Conversion to Alkyl Chloride with Thionyl Chloride (SOCl2)

  • Reaction with SOCl2

    • Involves alcohol and SOCl2 in presence of base (commonly pyridine).

    • Pyridine neutralizes HCl byproduct.

    • Converts alcohol to alkyl chloride with inverted stereochemistry.

    • Byproducts: SO2 and pyridinium hydrochloride.

Formation of Sulfonates from Alcohols

  • Reagent: Sulfonyl chloride (e.g., tosyl chloride, TsCl).

  • Reactions yield sulfonates, a good leaving group, allowing substitution without altering stereochemistry.

  • Example:

    • Alcohol + tosyl chloride + pyridine → alkyl tosylate.

    • This maintains configuration through retention (no inversion).

Summary of Sulfonate Reactions

  • Sulfonate formation is rapid and allows for both SN2 reactions and E2 eliminations depending on the reagent used.

    • With alkyl tosylate and good nucleophiles (like CN⁻), SN2 substitution creates new carbon-carbon bonds.

    • Tosylate can also participate in E2 eliminations using strong bases.

Dehydration Reactions of Alcohols

Acid-Catalyzed Dehydration of Alcohols to Alkenes

  • Overview: Converts alcohols to alkenes via elimination of water, reversible process influenced by reaction conditions.

  • Mechanism:

    • Protonation of hydroxyl group, forming a good leaving group.

    • Elimination forms carbocations; stability depends on the preference of secondary or tertiary sites.

    • Temperature influences reaction rates: higher temperatures favor reactions for primary and secondary alcohols.

  • Example: Isobutylene formation from tert-butanol via acid-catalyzed dehydration.

Determining Major Products in Eliminations

  • Zaitsev vs. Hoffman Products:

    • More substituted (Zaitsev) alkenes tend to be the major product; less substituted (Hoffman) form less favorable pathways.

Pinacol Rearrangements

  • Reactions of Glycols (1,2-diols):

    • Dehydration of pinacol leads to a rearrangement to pinacolone involving hydronium, carbocation stabilization, and methyl shifts.

Oxidation of Alcohols

Categories of Alcohol Oxidation

  • Primary Alcohols:

    • Can oxidize to aldehydes (via PCC) or carboxylic acids (via chromic acid).

  • Secondary Alcohols:

    • Convert to ketones via chromic acid or PCC in organic solvents.

  • Tertiary Alcohols:

    • Generally, not oxidizable due to lack of hydrogen atoms on the carbon bonded to the hydroxyl group.

Reagents for Oxidation

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  • Chromium-based Oxidants:

    • Chromic acid (H2CrO4), sodium/potassium dichromate in sulfuric acid, or Jones reagent.

  • PCC (Pyridinium chlorochromate): Utilized when water is absent for selective oxidation of primary alcohols to aldehydes (avoiding carboxylic acid formation).

Periodic Acid Oxidation of Glycols

  • Oxidation Mechanism: Cleavage of C-C bonds in 1,2-diols by periodic acid (HIO4) results in carbonyl formation.

  • Forms cyclic perioate intermediate facilitating the cleavage, producing two equivalents of aldehyde/ketone depending on structure.

  • Importance: Only cis glycols can be oxidized by periodic acid due to required cyclic intermediate formation.

Conclusion

  • Mastery of the listed reactions is crucial for succeeding in organic chemistry, particularly in understanding stereochemistry, reaction mechanisms, and predicting product outcomes across different alcohol transformations.