Hydroxy Compounds. (Chapter 34). Phenol. OH. 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). . O -. H +. R +. Three tendencies of reactions. 3. Attack other substrates.
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Hydroxy Compounds (Chapter 34)
Phenol OH Hydroxy compounds Aliphatic Monohydric Alcohols 1o Primary RCH2OH (one –R) 2o Secondary R2CHOH (two –R) 3o Tertiary R3COH (three –R)
O- H+ R+ Three tendencies of reactions 3. Attack other substrates Nu: :B • Nucleophiles attack • alkyl group 2. Bases that attack the hydrogen atom
Nucleophilic Substitution • In acidic medium, -OH is protonated to facilitate C-O bond cleavage (-OH2+ is a better leaving group) • RCH2OH + H+ RCH2-OH2+ • SN1 mainly (down-grading of Nu: in acidic medium)
Bubbling HX(g) HBr Br + H2O OH Halide Formation HX is produced ‘in situ’ NaBr + H2SO4 NaHSO4 + HBr HBr + C4H9OH C4H9Br + H2O
Halide Formation PX3 ( P + X2) or SOCl2 PCl3 + 3 C2H5OH 3 C2H5Cl + P(OH)3 SOCl2 + 2 C2H5OH 2 C2H5Cl + SO2 + H2O
Mechanism R-O+H-Zn-Cl2 R+ + Cl- RCl R-OH + ZnCl2 Lucas reaction • Use to distinguish 1o, 2o,3o alkanols • Reagent: ZnCl2(s) in conc.HCl • SN1 mainly, R-Cl is formed • Observation: • 3o Two distinct layers formed immediately • 2o Two distinct layers appear in 10 min. • 1o A cloudy appearance after a few hour
Mechanism(E1): CH3CHCH3 + H+ CH3CHCH3 CH3C+HCH3 + H2O CH2=CHCH3 + H+ OH OH2+ Elimination • Dehydration, -H2O • Tend to be first order, 2 steps, leaving group led. • 3o alkanols eliminate most readily • Unlike haloalkanes, SN and E do not occur in competition. Each set of reagents do just one job. (PI3 for SN, c.H2SO4/Al2O3 as water grabbers)
excess c.H2SO4,170oC CH3CH2CHCH3 CH3CH2CH=CH2 OH or Al2O3,350oC + CH3CH=CHCH3 (major) Intramolecular Dehydration Saytzeff’s 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. H2SO4 • 2CH3CH2OH CH3CH2OCH2CH3 • 140oC • For 1o alkanol (2o,3o Alkenes form) • Not suitable for unsymmetrical ether • SN2 mechanism
Mechanism (SN2) c. H2SO4 CH3CH2OH CH3CH2OH CH3CH2O+H2 CH3CH2O+HCH2CH3 + H2O CH3CH2OCH2CH3 + H+ 140oC Intermolecular Dehydration
Strength increase ? As Acids Ka CH3-O-H + H2O CH3O:- + H3O+ pKa values: HCl -7 CH3COOH 14.8 CH3OH 15.5 H2O 15.7 CH3CH2OH 15.9 (CH3)2CHOH 17 (CH3)3COH 18
Reaction with sodium e.g. 2CH3OH + 2Na 2CH3O- Na+ + H2 CH3O- Methoxide ion A stronger base than OH-. Why?
As Nucleophiles Esterification: c.H2SO4 Alkanol + Acid Ester + water reflux • Excess acid or alkanol is used to drive the eqm. to • the formation of ester. • c.H2SO4 is used to • Catalyse the reaction • Shift the equilibrium position to the product side • by removing H2O
R’ :O R’ R’ H O O+H H+ C OH HO C C O+ R H OH OH R’ R’ -H2O -H+ H+ shift R’COOR C O+H2 H-O C H-O+= O O R R Mechansium of esterification R
Oxidation Oxidizing Agent: K2Cr2O7/H+ 1o alkanol [O] [O] RCH2OH RCHO RCOOH aldehyde alkanoic acid 2o alkanol [O] R2COH R2C=O ketone 3o alkanol Cannot be oxidized
O R H OH HO Cr + C R OH O R H :O R C + H2O + H2CrO3 C O R O OH Cr R O Mechanism of Oxidation 2o alkanol
R H R [O] C C O H OH H O R .. H OH HO Cr :O C H :O- O OH O Cr C O R R H+ O C O HO Mechanism of Oxidation 1o alkanol
e.g. OH I2/NaOH CH3CHC2H5 C2H5COO-Na+ + CHI3 (a yellow ppt.) Triiodomethane Formation Substrate: Alkanol with CH3C(OH)- Reagent: I2 in NaOH(aq) , a mold O.A. Serve as a qualitative test to identify compound with the above structure.
.. OH Phenol Acid strength C6H5OH(aq) C6H5O-(aq) + H+(aq) Ka = 1x10-10, much stronger than aliphatic alkanols. • Reason: • Non-bonded e- of oxygen takes part • in the delocalized e- system. • weakened O-H bond
is stabilized by delocalization of the negative charge into the benzene ring. ..- O:- O O .. -.. O- Phenol
Reaction of phenols • Reaction with sodium • C6H5OH + Na C6H5O-Na+ + ½ H2 • (more vigorous than aliphatic alkanol) 2. Reaction with NaOH C6H5OH + NaOH C6H5O-Na+ +H2O
O O O O OH O-Na+ O-C-R O-C-R R-C-O-C-R NaOH O R-C-O-Cl Reaction of phenols -OH takes part in e- system, NOT a good Nu: