Overview
This lecture explores the chemistry of alcohols, including their synthesis, reactions to form other compounds (like ethers and epoxides), transformations into better leaving groups, and their oxidation to aldehydes, ketones, and carboxylic acids.
Alcohol Basics and Synthesis
- Alcohols contain a hydroxyl (-OH) group bonded to a carbon.
- Ethanol is the alcohol in alcoholic drinks; methanol is toxic and can cause blindness or death.
- Alcohols are used in products like hand sanitizers for their ability to denature proteins and destroy pathogens.
- Alcohols can be synthesized via acid-catalyzed hydration of alkenes (Markovnikov or anti-Markovnikov addition).
- Substitution of haloalkanes with sodium hydroxide can produce alcohols.
- Diols (compounds with two -OH groups) can be made by oxidizing alkenes with osmium tetroxide.
Alcohols as Reactants
- Alcohols are weak nucleophiles and weakly acidic (pKa ≈ 16).
- Deprotonation forms alkoxides (RO-)—strong nucleophiles used to synthesize ethers.
- Ethers (R-O-R') are made by reacting an alkoxide with an alkyl halide via the SN2 mechanism.
- Ethers are stable but can be broken down with strong acids to yield alcohols and alkyl halides.
Making Alcohols Better Leaving Groups
- Hydroxyl groups are poor leaving groups; conversion to alkyl halides improves reactivity.
- Alcohols can be converted into alkyl halides using hydrogen halides, phosphorus tribromide, or thionyl chloride (last works for primary/secondary alcohols).
- Tosyl chloride or mesyl chloride convert alcohols into tosylates/mesylates (good leaving groups) without changing stereochemistry.
Epoxides and Their Reactions
- Epoxides are three-membered cyclic ethers formed from halohydrins or alkenes.
- Epoxides are reactive due to ring strain and undergo ring-opening reactions.
- Acid-catalyzed opening (SN1-like): nucleophile attacks the more substituted carbon.
- Base-catalyzed opening (SN2): nucleophile attacks the less substituted carbon.
Oxidation of Alcohols
- Oxidation is loss of electrons or loss of C-H/gain of C-O bonds.
- Primary alcohols oxidize to aldehydes and further to carboxylic acids with strong oxidants (e.g., chromic acid).
- Secondary alcohols oxidize to ketones; tertiary alcohols do not oxidize.
- Milder oxidants (e.g., PCC, PDC, DMP) can stop at the aldehyde stage.
- Chromic acid gives an orange-to-green color change during oxidation.
Key Terms & Definitions
- Alcohol — Organic molecule with a hydroxyl (-OH) group bonded to carbon.
- Alkoxide — Negatively charged conjugate base of an alcohol (RO-).
- Ether — Compound featuring an oxygen atom bonded to two carbons.
- Epoxide — Three-membered cyclic ether with significant ring strain.
- Tosylate/Mesylate — Modified alcohols with improved leaving group ability.
- Oxidation — In organics, loss of H or gain of O to carbon.
Action Items / Next Steps
- Prepare for a lab experiment applying these alcohol reactions in the next session.
- Review oxidation reagents and their effects on different types of alcohols.