Acidity of Alcohols and Phenols Just like water,the hydroxyl groups in alcohols are weakly acidic-strong bases can generate alkoxide ions. R-O-H+B→R-O+B-H The acidities of alcohols vary greatly,depending on the substituents Alcohol pKa (Lower pKa is stronger acid) Methanol CH,-OH 15.5 Ethanol C,H;-OH 15.9 2-chloroethanol CI-C2H-OH 14.3 2,2,2-trifluoroethanol CFCH2-OH 12.4 t-butylalcohol (CH)C-OH 19.0 cyclohexanol C HI-OH 18.0 phenol C Hs-OH 10.0 water H-OH 15.7 acetic acid CHCO-OH 4.8 hydrochloric acid H-CI -2.2 Notice that electron withdrawing groups on an alcohol increase the acidity by stabilizing the alkoxide formed. CH3CH2-O-H H2O ±CHgCH2-0+H30*K2=1.3x1016 CF3CH2-O-H H2O -CF3CHh-O+H0*K2=4.0x1013 Chl0 Alcohols:Struct +synth (landscape) Page 6
Ch10 Alcohols; Struct + synth (landscape) Page 6 Acidity of Alcohols and Phenols Just like water, the hydroxyl groups in alcohols are weakly acidic – strong bases can generate alkoxide ions. R-O-H + B- R-O - + B-H The acidities of alcohols vary greatly, depending on the substituents. Alcohol pKa (Lower pKa is stronger acid) Methanol CH3-OH 15.5 Ethanol C2H5-OH 15.9 2-chloroethanol Cl-C2H4-OH 14.3 2,2,2-trifluoroethanol CF3CH2-OH 12.4 t-butylalcohol (CH3)3C-OH 19.0 cyclohexanol C6H11-OH 18.0 phenol C6H5-OH 10.0 water H-OH 15.7 acetic acid CH3CO-OH 4.8 hydrochloric acid H-Cl -2.2 Notice that electron withdrawing groups on an alcohol increase the acidity by stabilizing the alkoxide formed. CH3CH2 O-H + H2O CH3CH2 O - + H3O + Ka = 1.3x 10 -16 CF3CH2 O-H + H2O CF3CH2 O - + H3O + Ka = 4.0x 10 -13
Acidity of Phenols Cyclohexanol and phenol are similar in structure,yet their acidities are very different.(Phenol is 10 more acidic). O-H +H20= +H3O* K2=1018 O-H +H20 +H30K2=1010 The phenoxide ion has resonance stabilization since the negative charge can be delocalised over 4 atoms(3 carbons and I oxygen),making it more stable. Phenols are more acidic than related alcohols. Chl0 Alcohols:Swruct +synth (landscape) Page 7
Ch10 Alcohols; Struct + synth (landscape) Page 7 Acidity of Phenols Cyclohexanol and phenol are similar in structure, yet their acidities are very different. (Phenol is 108 more acidic). The phenoxide ion has resonance stabilization since the negative charge can be delocalised over 4 atoms (3 carbons and 1 oxygen), making it more stable. Phenols are more acidic than related alcohols. O-H + H2O O - + H3O + Ka = 10 -18 O-H + H2O O - + H3O + Ka = 10 -10
Different Deprotonations of alcohols (to alkoxides) O-Na" Na*-:OH ←→ +H,O R-Q-H NaH THE R-O:Na++H21 R-O-H Na R-ONa*+2H2↑ Chl0 Alcohols:Struct synth (landscape) Page 8
Ch10 Alcohols; Struct + synth (landscape) Page 8 Different Deprotonations of alcohols (to alkoxides)
Syntheses of Alcohols Previously seen: Nucleophilic substitution of alkyl halide with HO Acid catalyzed hydration of alkenes Oxymercuration-demercuration Hydroboration-oxidation (anti-Markovnikov) Hydroxylation gives diols(syn-OsO:anti-epoxidation-hydrolysis) Acetylides and carbonyl compounds Organometallic Reagents for Alcohol Synthesis When a compound has a covalent bond between a carbon and a metal,it is called an organometallic compound. The two most common types of organometallic are Grignard reagents and organolithium reagents(although there are a variety of others,Cu,Cd,Zn,Hg,etc). C-M bond Electronegativities C世Li Li 1.0 C2.5 88 Na 0.9 Mg 1.3 A11.6 C±Mg K 0.8 86 2013 Pearson Education,Ino A carbon metal bond is polarized with significant negative charge on the carbon because metals are so electropositive These compounds will have nucleophilic carbon atoms. Chl0 Alcohols:Struct +synth (landscape) Page 9
Ch10 Alcohols; Struct + synth (landscape) Page 9 Syntheses of Alcohols Previously seen: Nucleophilic substitution of alkyl halide with HOAcid catalyzed hydration of alkenes Oxymercuration-demercuration Hydroboration-oxidation (anti-Markovnikov) Hydroxylation gives diols (syn – OsO4 : anti – epoxidation-hydrolysis) Acetylides and carbonyl compounds Organometallic Reagents for Alcohol Synthesis When a compound has a covalent bond between a carbon and a metal, it is called an organometallic compound. The two most common types of organometallic are Grignard reagents and organolithium reagents (although there are a variety of others, Cu, Cd, Zn, Hg, etc). A carbon metal bond is polarized with significant negative charge on the carbon because metals are so electropositive. These compounds will have nucleophilic carbon atoms