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Alcohols

Alcohols. −. Contain a hydroxyl (-OH) group. +. Intermolecular forces: dipole-dipole, H-bonding H-bonds between alcohol molecules: high boiling points H-bonds with water: up to 4-carbon alcohols soluble in water -OH group can act as a weak base or a weak acid. + Strong acid.

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Alcohols

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  1. Alcohols − • Contain a hydroxyl (-OH) group + • Intermolecular forces: dipole-dipole, H-bonding • H-bonds between alcohol molecules: high boiling points • H-bonds with water: up to 4-carbon alcohols soluble in water • -OH group can act as a weak base or a weak acid + Strong acid + Strong base alkoxide alcohol oxonium ion

  2. Alcohol Nomenclature • Parent chain = longest chain containing C with -OH • Root name: replace –e with –ol • ethane  ethanol, butene  butenol, etc. • Give –OH the smallest possible number • –OH has priority over double bonds, alkyl groups • Two –OH groups  -diol; three –OH groups  -triol • Add to end of root name (propane  propanediol) 1,2-ethanediol (ethylene glycol) antifreeze 5-methyl-3-hexanol

  3. Alcohol Naming Practice 2-propanol (isopropyl alcohol) 2,4-dimethyl-3-pentanol 3,5-dimethyl-2,4-heptanediol 4-penten-2-ol

  4. Classes of Alcohols Primary (1°) alcohol 1-butanol OH C attached to 1 other C Secondary (2°) alcohol OH C attached to 2 other C’s 4-phenyl-2-hexanol Tertiary (3°) alcohol OH C attached to 3 other C’s 1-methylcyclohexanol

  5. Reactions of Alcohols Strong base alkoxide • Reaction with strong bases • alcohol as proton donor (weak acid) • Reaction with strong acids • alcohol as proton acceptor (weak base) • Dehydration • reverse of hydration of alkenes • requires H+ catalyst • Oxidation • increase # of C-O bonds Strong acid oxonium ion H+ - H2O alkene oxidizing agent or aldehyde ketone

  6. Dehydration Mechanism Step 1: electrophilic H+ catalyst attacks nucleophilic O atom Step 2: H2O dissociates, leaving behind a carbocation H2O + Step 3: Electrons from neighboring C-H bond form  bond, regenerating H+ catalyst +

  7. Hydration and Dehydration H+ H2O + Hydration and dehydration are in equilibrium Can change [H2O] to favor one reaction or the other Change Favors Increase [H2O] Formation of Alcohol (hydration) Decrease [H2O] Formation of Alkene (dehydration)

  8. Possible Dehydration Products H+ ? The most-substituted alkene product is favored (most stable) Major product Least H’s on double bond

  9. Oxidation of Alcohols • Oxidation: increases oxidation number • More C-O bonds (add O) or increases bond order • Fewer C-H bonds (remove H) • Needs an oxidizing agent • CrO3, Cr2O72-, MnO4- ,or PCC (pyridinium chlorochromate) PCC stops at aldehyde 1° alcohol +1 -2 -2 -2 -1 oxidizing agent oxidizing agent 0 +1 +3 +1 -2 0 0 +1 +1 +1 aldehyde carboxylic acid CrO3 (Cr6+) Cr3+

  10. Breathalyzer Tests oxidized oxidized ethanol ethanal (acetaldehyde) ethanoic acid (acetic acid) + Cr6+ + Cr3+

  11. Breathalyzer Tests oxidized oxidized ethanol ethanal (acetaldehyde) ethanoic acid (acetic acid) + Cr6+ + Cr3+ oxidized oxidized methanol methanal (formaldehyde) methanoic acid (formic acid)

  12. Oxidation of Alcohols 0 -2 +1 0 oxidizing agent 2° alcohol 0 +2 -2 0 0 +1 ketone propanone (acetone) 2-propanol oxidizing agent No reaction 3° alcohol

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