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

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

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

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

Acid-Base Behavior of Alcohols 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

R pKa H2O 15.7 t-BuOH 18 MeOH 15.5 HCl -2.2 EtOH 15.9 RCO2H 4.7 i-PrOH 17.1 CH4 50 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%

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

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 H2O OH- acetate ester Ester Hydrolysis Acetic acid