Hydroxy Compounds (Chapter 34)

Hydroxy compounds Aliphatic Monohydric Alcohols 1 o Primary RCH 2 OH (one –R) 2 o Secondary R 2 CHOH (two –R) 3 o Tertiary R 3 COH (three –R) Phenol OH

Three tendencies of reactions R + O - H + Nu: 1.Nucleophiles attack alkyl group :B 2. Bases that attack the hydrogen atom 3. Attack other substrates

Nucleophilic Substitution In acidic medium, -OH is protonated to facilitate C-O bond cleavage (-OH 2 + is a better leaving group) RCH 2 OH + H + RCH 2 -OH 2 + S N 1 mainly (down-grading of Nu: in acidic medium)

Halide Formation OH HBr Br+ H 2 O Bubbling HX(g) HX is produced in situ NaBr + H 2 SO 4 NaHSO 4 + HBr HBr + C 4 H 9 OH C 4 H 9 Br + H 2 O

Halide Formation PX 3 ( P + X 2 ) or SOCl 2 PCl C 2 H 5 OH 3 C 2 H 5 Cl + P(OH) 3 SOCl C 2 H 5 OH 2 C 2 H 5 Cl + SO 2 + H 2 O

Lucas reaction Use to distinguish 1 o, 2 o,3 o alkanols Reagent: ZnCl 2 (s) in conc.HCl S N 1 mainly, R-Cl is formed Observation: 3 o Two distinct layers formed immediately 2 o Two distinct layers appear in 10 min. 1 o A cloudy appearance after a few hour Mechanism R-OH + ZnCl 2 R-O + H-Zn - Cl 2 R + + Cl - RCl

Elimination Dehydration, -H 2 O Tend to be first order, 2 steps, leaving group led. 3 o alkanols eliminate most readily Unlike haloalkanes, S N and E do not occur in competition. Each set of reagents do just one job. (PI 3 for S N, c.H 2 SO 4 /Al 2 O 3 as water grabbers) Mechanism(E1): CH 3 CHCH 3 + H + CH 3 CHCH 3 CH 3 C + HCH 3 + H 2 O CH 2 =CHCH 3 + H + OHOH 2 +

Intramolecular Dehydration excess c.H 2 SO 4,170 o C CH 3 CH 2 CHCH 3 CH 3 CH 2 CH=CH 2 OH or Al 2 O 3,350 o C + CH 3 CH=CHCH 3 (major) Saytzeffs rule: In the elimination reactions, the major product should be the one with greater number of alkyl groups attached to the C=C bond.(higher substituted alkenes are more stable.)

Intermolecular Dehydration c. H 2 SO 4 2CH 3 CH 2 OH CH 3 CH 2 OCH 2 CH o C For 1 o alkanol (2 o,3 o Alkenes form) Not suitable for unsymmetrical ether S N 2 mechanism

Intermolecular Dehydration CH 3 CH 2 OH CH 3 CH 2 O + H 2 CH 3 CH 2 O + HCH 2 CH 3 + H 2 O CH 3 CH 2 OCH 2 CH 3 + H + c. H 2 SO o C CH 3 CH 2 OH Mechanism (S N 2)

As Acids K a CH 3 -O-H + H 2 O CH 3 O: - + H 3 O + pK a values: HCl-7 CH 3 COOH 14.8 CH 3 OH 15.5 H 2 O 15.7 CH 3 CH 2 OH 15.9 (CH 3 ) 2 CHOH17 (CH 3 ) 3 COH18 Strength increase ?

Reaction with sodium e.g. 2CH 3 OH + 2Na 2CH 3 O - Na + + H 2 CH 3 O - Methoxide ion A stronger base than OH -. Why?

As Nucleophiles Esterification: c.H 2 SO 4 Alkanol + Acid Ester + water reflux Excess acid or alkanol is used to drive the eqm. to the formation of ester. c.H 2 SO 4 is used to 1.Catalyse the reaction 2.Shift the equilibrium position to the product side by removing H 2 O

Mechansium of esterification R C O R OH H+H+ C O+HO+H R :O:O H C R HO OH O+O+ RH C R H-O O+H2O+H2 O R H + shift C R H-O + = O R -H 2 O-H + RCOOR

Oxidation 1 o alkanol [O] [O] RCH 2 OH RCHO RCOOH aldehyde alkanoic acid Oxidizing Agent: K 2 Cr 2 O 7 /H + 2 o alkanol [O] R 2 COH R 2 C=O ketone 3 o alkanol Cannot be oxidized

Mechanism of Oxidation 2 o alkanol R H C ROH + HOCr OH O O R H C RO Cr OH O :O + H 2 O R C R O + H 2 CrO 3

Mechanism of Oxidation 1 o alkanol R H C HOH [O] R C H O HOCr OH O O H C R O.. R H C :O - O Cr OH O :O H+H+ R C HO O

Triiodomethane Formation Substrate: Alkanol with CH 3 C(OH)- Reagent: I 2 in NaOH(aq), a mold O.A. e.g. OH I 2 /NaOH CH 3 CHC 2 H 5 C 2 H 5 COO - Na + + CHI 3 (a yellow ppt.) Serve as a qualitative test to identify compound with the above structure.

Phenol Acid strength C 6 H 5 OH(aq) C 6 H 5 O - (aq) + H + (aq) K a = 1x10 -10, much stronger than aliphatic alkanols. Reason: Non-bonded e - of oxygen takes part in the delocalized e - system. weakened O-H bond OH..

Phenol O-O-.. is stabilized by delocalization of the negative charge into the benzene ring. O: - O.. - O -..

Reaction of phenols 1.Reaction with sodium C 6 H 5 OH + Na C 6 H 5 O - Na + + ½ H 2 (more vigorous than aliphatic alkanol) 2. Reaction with NaOH C 6 H 5 OH + NaOH C 6 H 5 O - Na + +H 2 O

Reaction of phenols OH O - Na + NaOH R-C-O-C-R O O O-C-R O R-C-O-Cl O O-C-R O -OH takes part in e - system, NOT a good Nu: