Oxygen Containing Compounds - Alcohols

Description

  • nomenclature
    • Prefix: hydroxyl, hydroxy.
    • Suffix: -ol, alcohol.
  • physical properties
    • Hydrogen bonding.
    • Higher boiling point than the same compound without the alcohol group.
    • Water soluble as long as molecule does not contain a long hydrophobic region.
  • infrared absorption of OH group: 3300 cm-1 and broad due to hydrogen bonding.

Important reactions

  • substitution reactions: SN1 or SN2, depending on alcohol and derived alkyl halide
    • R-OH + HX <--> R-X + H2O
    • sn1
    • sn2
    • Factors that favor sn1: stable carbocation, tertiary carbon center, protic solvent.
    • Factors that favor sn2: unstable carbocation, primary carbon center, aprotic (but polar) solvent.
    • All substitution reactions need a good leaving group.
    • SN1 = unimolecular reaction, intermediate carbocation formed.
    • SN2 = bimolecular reaction, passes through transition state.
  • oxidation
    • KMnO4 and CrO3 will oxidize primary alcohols to carboxylic acids, but PCC (Pyridine Chlorochromate) and other weak oxidizing agents will only oxidize a primary alcohol to the aldehyde.
    • Secondary alcohols always oxidize to the ketone.
    • Tertiary alcohols do not oxidize.
  • oxidation and reduction
  • pinacol rearrangement in polyhydroxyalcohols; synthetic uses
    pinacol rearrangement
  • Mechanism of pinacol rearrangement:
    pinacol rearrangement mechanism
  • protection of alcohols: the best protecting group for alcohol is the trimethylsilyl group.
    • To protect, add Cl-SiMe3 to R-OH.
    • The alcohol gets "capped" into R-O-SiMe3.
    • To deprotect, add F-.
  • reactions with SOCl2 and PBr3
    • R-OH + SOCl2 --> R-Cl (by products: SO2 + HCl)
    • R-OH + PBr3 --> R-Br (by products: H3PO3, R3PO3, HBr)
  • preparation of mesylates and tosylates
    • mesylate, triflate, tosylate
    • Sulfonates R-SO3- are good leaving groups.
    • The R can be:
      • Methane, which makes methanesulfonate.
      • Toluene, which makes tosylate.
      • Trifluoromethane, which makes triflate.
    • Mesylates can be prepared by reacting an alcohol (R-OH) with mesyl chloride (MsCl).
    • Tosylates can be prepared by reacting an alcohol (R-OH) with tosyl chloride (TsCl).
  • esterification: acid + alcohol = ester
  • esterification
  • inorganic esters: replace the carbon of esters with a different atoms.
  • inorganic esters
  • Reactions involving the formation of inorganic esters:
    PBr3 mechanism
    SOCl2 mechanism
  • Formation of mesylates and tosylates are also reactions that involve inorganic esters.
  • In biochemistry DNA/RNA polymerization, the 3'-OH alcohol group attacks the 5'-phosphate to form an inorganic ester linkage (phosphodiester linkage of DNA/RNA backbone).

General principles

  • hydrogen bonding: hydrogen bonding in alcohols give them a higher boiling point than their corresponding alkanes.
  • acidity of alcohols compared to other classes of oxygen-containing compounds: lower pKa = more acidic.
  • Compound pKa
    COOH (carboxylic acids) 5
    ArOH (phenols) 10
    H2O (water) 16
    ROH (alcohols) 15
    -CH2(CO)-R (alpha hydrogen in aldehydes and ketones) 20
    -CH2(CO)-OR (alpha hydrogen in esters) 25
  • effect of chain branching on physical properties: going from straight chain to branched alkane (with same # carbons) = higher freezing/melting point, lower boiling point.