Download Ch12-alcohols-II

Alcohols
Alcohols as Acids
Alcohols as Acids
Alcohols as Acids
Alcohols as Acids
resonance in phenols
Alcohols
Methanol and ethanol are industrial chemicals:
solvents, antifreeze, fuel, antiseptic, social / recreational
Isopropyl alcohol is prepared by hydration of
propene.
All alcohols with four carbons or fewer are
readily available.
Most alcohols with five or six carbons are
readily available.
Sources of alcohols
Reactions discussed in earlier chapters
Hydration of alkenes
Hydroboration-oxidation of alkenes
Hydrolysis of alkyl halides
Sources of alcohols
New methods:
Reduction of aldehydes and ketones
Reduction of carboxylic acids
Reduction of esters
Reaction of Grignard reagents
Preparation of Alcohols
by
Reduction of Aldehydes and Ketones
Reduction of Aldehydes Gives Primary Alcohols
R
R
C
H
O
H
C
H
OH
Example: Catalytic Hydrogenation
O
CH3O
CH
+
H2
Pt, ethanol
CH3O
CH2OH
(92%)
Reduction of Ketones Gives Secondary Alcohols
R
R
C
R'
O
H
C
R'
OH
Example: Catalytic Hydrogenation
H
O
+
H2
OH
Pt
ethanol
(93-95%)
Retrosynthetic Analysis
R
H
C
R
OH
H:–
R
C
R'
O
C
O
H
H
H
C
R
OH
H:–
R'
Metal Hydride Reducing Agents
H
H
+
Na H
–
B
H
Li
+
Al
H
H
H
Sodium
borohydride
H
–
Lithium
aluminum hydride
act as hydride donors
Examples: Sodium Borohydride
Aldehyde
O2N
O2N
O
NaBH4
CH2OH
CH
methanol
(82%)
Ketone
O
NaBH4
ethanol
H OH
(84%)
Lithium aluminum hydride
more reactive than sodium borohydride
cannot use water, ethanol, methanol etc.
as solvents
diethyl ether is most commonly used solvent
Examples: Lithium Aluminum Hydride
Aldehyde
O
CH3(CH2)5CH
1. LiAlH4
diethyl ether
2. H2O
CH3(CH2)5CH2OH
(86%)
Ketone
O
(C6H5)2CHCCH3
1. LiAlH4
diethyl ether
2. H2O
OH
(C6H5)2CHCHCH3
(84%)
Selectivity
neither NaBH4 or LiAlH4
reduces isolated
double bonds
O
1. LiAlH4
diethyl ether
2. H2O
(90%)
H
OH
Preparation of Alcohols By Reduction
of Carboxylic Acids and Esters
Reduction of Carboxylic Acids
Gives Primary Alcohols
R
R
C
HO
O
H
C
H
lithium aluminum hydride is only
effective reducing agent
OH
Example: Reduction of a Carboxylic Acid
O
COH
1. LiAlH4
diethyl ether
2. H2O
CH2OH
(78%)
Reduction of Esters
Gives Primary Alcohols
Lithium aluminum hydride preferred for
laboratory reductions
Sodium borohydride reduction is too slow
to be useful
Catalytic hydrogenolysis used in industry
but conditions difficult or dangerous to duplicate
in the laboratory (special catalyst, high
temperature, high pressure
Example: Reduction of a Carboxylic Acid
O
COCH2CH3
1. LiAlH4
diethyl ether
2. H2O
CH2OH +
(90%)
CH3CH2OH
Summary
Synthesis of Alcohols
Sodium borohydride / Lithium aluminum hydride
Reductions