6.5 alcohols and phenols Flashcards
what are the methods of preparation of alcohols (4)?
- electrophilic addition of alkenes (H2O, H3PO4, high temp and pressure OR cold conc. H2SO4, followed by H2O, heat)
- mild oxidation of alkenes (cold KMnO4 in KOH)
- nucleophilic substitution of halogenoalkanes (NaOH/KOH, heat under reflux)
- reduction of carbonyl compounds
what are the reagents and conditions for reduction of each carbonyl compounds to what degree of alcohol?
aldehyde to primary alcohol and
ketone to secondary alcohol: LiAlH4 in dry ether / NaBH4 in methanol / H2 with Ni catalyst, heat under high pressure
carboxylic acid to primary alcohol: LiAlH4 in dry ether
what are the reactions of alcohols (5 mechanisms)?
- elimination to alkenes
- nucleophilic substitution
- acid-metal displacement/redox
- condensation with -COOH and -COCl
- oxidation to carbonyl compounds
compare the condensation of alcohols wiith -COOH to alcohols with -COCl
with -COOH: needs conc H2SO4 catalyst, heat under reflux
reaction is reversible
with -COCl: no conditions needed
reaction is irreversible
this is due to electronegative Cl atom drawing shared electron pair towards itself, increasing partial positive charge on C atom
state the reagents and conditions for oxidation of alcohols to each carbonyl
primary alcohol to aldehyde: K2Cr2O7 in dilute sulfuric acid, heat with immediate distillation
primary alcohol to carboxylic acid: KMnO4 / K2Cr2O7 in dilute sulfuric acid, heat under reflux
secondary alcohol to ketone: KMnO4 / K2Cr2O7 in dilute sulfuric acid, heat under reflux
tertiary alcohols cannot be oxidised
state the 3 chemical tests for alcohols
- PCl5 (s)
- KMnO4 and H2SO4, heat
- iodoform test
describe the chemical test and expected observations of PCl5(s)
add PCl5 to a solution of alcohol
white fumes of HCl will be observed
describe the chemical test and expected observations of KMnO4 and H2SO4. describe also how to further differentiate the results
add KMnO4 and H2SO4 to the alcohol in a test tube and warm using a water bath
primary and secondary alcohols: purple KMnO4 decolourises
tertiary alcohols: purple KMnO4 remains purple
to further distinguish, add Tollen’s reagent.
aldehyde will form silver mirror, ketone no ppt observed
describe the chemical test and expected observations of iodoform test
add aqueous iodine in sodium hydroxide to a solution of alcohol in a test tube and warm with water bath
brown iodine decolourises, pale yellow ppt of CH3I observed, if the -CHCH3OH structure is present
what are aryl alcohols and who are they similar to in terms of reactions?
aromatic alcohols whose -OH group is separated from the ring by at least one carbon atom, reactions similar to alcohols
compare the acidity of ethanol, water and phenol
phenol > water > ethanol
the acidity of a compound depends on the stability of its conjugate base.
- ethanol: presence of electron–donating methyl group will increase the intensity of negative charge on the O atom, make it less stable than OH-. smaller extent of dissociation, weaker acid
- phenol: p orbital of O- overlaps with benzene pi electron cloud, delocalisation of negative charge into benzene ring, reducing intensity of negative charge. larger extent of dissociation, stronger acid than OH-
what are the reactions of phenols (4 mechanisms?
- acid-metal displacement/redox
- neutralisation (with NaOH)
- condensation
- electrophilic substitutions (benzene ring)
state the reagents, conditions and processes of condensation of phenol with -COCl
- NaOH to neutralise phenol to O- (stronger nucleophile)
- -COCl
why does phenol -OH not get substituted?
substitution involves breaking C-O bond. p orbital of O atom overlaps with pi electron cloud of the ring resulting in C-O bond having partial double bond character, requires more energy
describe the difference in products of electrophilic substitution for dilute and concentrated HNO3 / X2 in CCl4 vs X2 aqueous
dilute: monosubstituted
conc: polysubstituted (2, 4, 6)
