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Chapter 8 Properties of Alcohols

Chapter 8 Properties of Alcohols. Nomenclature Functional Groups RCH 2 —OH = Hydroxy functional group Alcohol = organic molecule having a hydroxy group Replace one H of water with R: H—O—H  R—O—H Ether = replace both H’s of water with R: R—O—R Naming Alcohols

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Chapter 8 Properties of Alcohols

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  1. Chapter 8 Properties of Alcohols • Nomenclature • Functional Groups • RCH2—OH = Hydroxy functional group • Alcohol = organic molecule having a hydroxy group • Replace one H of water with R: H—O—H  R—O—H • Ether = replace both H’s of water with R: R—O—R • Naming Alcohols • Modify the alkane name by dropping –e and adding –ol • CH3OH = methanol • CH3CH2CH2CH2OH = butanol • R-OH = alkanol • Branched Alcohols • Name based on longest chain containing -OH

  2. Number each carbon starting from the closest to –OH • Cyclic Alcohols = cycloalkanols • When referring to –OH as a substituent it is called hydroxy • Simple alcohols often have common names CH3OH = methyl alcohol CH3CH2OH = ethyl alcohol (CH3)2CHOH = isopropyl alcohol (CH3)3COH = tert-butyl alcohol

  3. Structural and Physical Properties of Alcohols • Structure • Alcohol structures are similar to water: sp3 O atom with 2 lone pairs • Steric effects of the R groups slightly alter bond angles • Electronegativity of O shortens and strenthens the O—H bond compared to C—H • DHo(OH) = 104 kcal/mol • DHo(CH) = 98 kcal/mol • Electronegativity of O causes large molecular dipole moment for alcohols

  4. Hydrogen Bonding • Hydrogen bond = weak, partial bond between a heteroatom bound H and another heteroatom (N, O, X, S, etc…) • DHo(OH…..H) = 5 kcal/mol • The sum of the multiple H-bonds in water or alcohols increases the boiling points of these liquids relative to alkanes or haloalkanes Methane = -162o Chloromethane = -24.2o Methanol = 65o Polar No H-Bonds Polar H-Bonds Nonpolar No H-Bonds

  5. Solubility • Alcohols have high water solubility (Like dissolves Like) • Polarities of water and alcohols are similar • Hydroxy group is Hydrophilic (water loving) • Alkyl group is Hydrophobic (water fearing) • Alkanes have no solubility in water (hydrophobic) • Organic molecules with polar functional groups (-OH, -NH2, -CO2H) have much higher water solubilities • The longer the alkyl group, the less water soluble the alcohol (more soluble in hydrocarbons—like dissolves like) • MeOH and EtOH are very similar to water as solvents: many salts will dissolve in them, because they are polar enough to separate the charges • Acid-Base Behavior of Alcohols • Alcohols are Amphoteric = can be acids or bases • Acidity of Alcohols • R—OH + H2O R—O- + H3O+ Ka alkoxide

  6. Alcohols are relatively weak acids • Electronegativity of R—O- allows it to stabilize (-) charge, but not very well • Requires a strong base to fully deprotonate an alcohol EtOH(pKa = 15.9) + OH-  EtO- + H2O (pKa = 15.7) MeOH(pKa = 15.5) + NH2-  MeO- + NH3 (pKa = 35) • Acidity: MeOH > 2-Propanol > t-Butyl Alcohol (1o > 2o > 3o) Steric disruption of solvation of deprotonated product 50% 50% 99.999% ~ 0%

  7. Inductive Effects = transmission of charge through s-bonds • Electronegative substituents increase an alcohol’s acidity • Electronegative groups remove electron density from R—O-, which stabilizes the anion. It is easier to deprotonate = more acidic. • Basicity of Alcohols • Lone pairs of electrons will accept H+ from strong acids ROH + HA  ROH2+ + A- • Conditions for Acid/Base behavior of Alcohols ROH2+ ROH RO- weak base Strong Conjugate Acid pKa = -2 Mild base Strong Base Strong acid Mild acid

  8. H2O • Synthesis of Alcohols by Nucleophilic Substitution • For Industrial Alcohol Synthesis, see your book • Lab Scale Reactions • SN1 Reaction of Tertiary RX ROH • SN2 Reaction of Primary, Secondary RX ROH • Often harder to make the RX than the ROH (get RX from ROH) • E1/E2 products complicate these reactions • Use acetate (weak base) to prevent Elimination reactions OH- acetate ester Ester Hydrolysis Acetic acid

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