Chapter 8 Properties of Alcohols I. Nomenclature A. Functional Groups 1) RCH2—OH = Hydroxy functional group 2) Alcohol = organic molecule having a hydroxy group 3) Replace one H of water with R: H—O—H R—O—H 4) Ether = replace both H’s of water with R: R—O—R B. Naming Alcohols 1) Modify the alkane name by dropping –e and adding –ol a) CH3OH = methanol b) CH3CH2CH2CH2OH = butanol c) R-OH = alkanol 2) OH Branched Alcohols a) Name based on longest chain containing -OH 3-methylheptanol OH 4-methly-3-propyl-2-octanol b) Number each carbon starting from the closest to –OH OH 1-propanol 3) HO H (S)-3-hexanol OH 2,2,5-trimethyl-3-heptanol Cyclic Alcohols = cycloalkanols OH Cl 1 1 cyclohexanol 4) 5) OH 1-methylcyclobutanol 1 OH cis-3-chlorocyclopentanol When referring to –OH as a substituent it is called hydroxy 3-hydroxypropylamine Simple alcohols often have common names CH3OH = methyl alcohol H2N OH CH3CH2OH = ethyl alcohol (CH3)2CHOH = isopropyl alcohol (CH3)3COH = tert-butyl alcohol II. Structural and Physical Properties of Alcohols A. Structure 1) Alcohol structures are similar to water: sp3 O atom with 2 lone pairs 2) 3) Steric effects of the R groups slightly alter bond angles Electronegativity of O shortens and strenthens the O—H bond compared to C—H a) DHo(OH) = 104 kcal/mol b) DHo(CH) = 98 kcal/mol 4) Electronegativity of O causes large molecular dipole moment for alcohols R H O + - B. Hydrogen Bonding 1) Hydrogen bond = weak, partial bond between a heteroatom bound H and another heteroatom (N, O, X, S, etc…) 2) DHo(OH…..H) = 5 kcal/mol 3) 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 Polar Nonpolar No H-Bonds H-Bonds No H-Bonds C. Solubility 1) Alcohols have high water solubility (Like dissolves Like) a) Polarities of water and alcohols are similar OH b) Hydroxy group is Hydrophilic (water loving) c) Alkyl group is Hydrophobic (water fearing) d) Alkanes have no solubility in water (hydrophobic) e) Organic molecules with polar functional groups (-OH, -NH2, -CO2H) have much higher water solubilities 2) The longer the alkyl group, the less water soluble the alcohol (more soluble in hydrocarbons—like dissolves like) 3) 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 III. Acid-Base Behavior of Alcohols A. B. Alcohols are Amphoteric = can be acids or bases Acidity of Alcohols Ka 1) R—OH + H2O R—O- + H3O+ alkoxide [ H 3O ][ RO ] Ka [ ROH ] pKa log Ka pH at which acid is half dissociate d 2) 3) 4) R pKa 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) 50% 50% MeOH(pKa = 15.5) + NH2- MeO- + NH3 (pKa = 35) ~ 0% 5) 99.999% Acidity: MeOH > 2-Propanol > t-Butyl Alcohol (1o > 2o > 3o) Steric disruption of solvation of deprotonated product 6) Inductive Effects = transmission of charge through s-bonds a) Electronegative substituents increase an alcohol’s acidity CCl3CH2OH b) C. > CHCl2CH2OH > CH2ClCH2OH > CH3CH2OH Electronegative groups remove electron density from R—O-, which stabilizes the anion. It is easier to deprotonate = more acidic. Basicity of Alcohols 1) Lone pairs of electrons will accept H+ from strong acids ROH + HA ROH2+ + Aweak Strong base Conjugate Acid pKa = -2 2) Conditions for Acid/Base behavior of Alcohols ROH2+ Mild base Strong acid ROH Strong Base Mild acid RO- IV. Synthesis of Alcohols by Nucleophilic Substitution A. B. For Industrial Alcohol Synthesis, see your book Lab Scale Reactions H2O 1) SN1 Reaction of Tertiary RX ROH OH2) SN2 Reaction of Primary, Secondary RX ROH 3) Often harder to make the RX than the ROH (get RX from ROH) 4) E1/E2 products complicate these reactions 5) Use acetate (weak base) to prevent Elimination reactions O CH3CH2Br + - + CH3C O Na acetate O CH3CH2O CCH3 OH SN 2 O CH3CH2O CCH3 ester O - CH3CH2OH + HO C CH3 Ester Hydrolysis Acetic acid