Transcript ALCOHOLS

ALCOHOLS
SANTOSH
CHEMISTRY DEPT
Introduction
• Alcohols constitute a class of compounds
which are regarded as hydroxy derivatives of
hydrocarbons.
• Based on the no. of hydroxyl groups the
alcohols are classified as mono-, di- and
trihydric accordingly, if the molecules contain
1, 2 or 3 –OH groups respectively. The alcohols
which contain four or more no. of –OH groups
are called polyhydric alcohols.
Classification CH3, 1o, 2o, 3o
• Primary: carbon with –OH is bonded to one
other carbon.
• Secondary: carbon with –OH is bonded to
two other carbons.
• Tertiary: carbon with –OH is bonded to three
other carbons.
• Aromatic (phenol): -OH is bonded to a
benzene ring.
IUPAC Nomenclature
• Find the longest carbon chain containing
the carbon with the -OH group.
• Drop the -e from the alkane name, add -ol.
• Number the chain, starting from the end
closest to the -OH group.
• Number and name all substituents.
Examples:
CH3
CH3
CH CH2OH
2-methyl-1-propanol
OH
CH3
CH CH2CH3
2-butanol
CH3
CH3
OH
C OH
CH3
2-methyl-2-propanol
Br
CH3
3-bromo-3-methylcyclohexanol
Unsaturated Alcohols
• Hydroxyl group takes precedence. Assign that
carbon the lowest number.
• Use alkene or alkyne name.
OH
CH2
CHCH2CHCH3
pent-4-ene-2-ol
Hydroxy Substituent
• When -OH is part of a higher priority class of
compound, it is named as hydroxy.
• Example:
OH
CH2CH2CH2COOH
4-hydroxybutanoic acid
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Common Names
• Alcohol can be named as alkyl alcohol.
• Useful only for small alkyl groups.
• Examples:
CH3
CH3
CH CH2OH
isobutyl alcohol
OH
CH3
CH CH2CH3
sec-butyl alcohol
Naming Diols
• Two numbers are needed to locate the two
-OH groups.
• Use -diol as suffix instead of -ol.
OH
HO
1,6-hexanediol
Glycols
• 1, 2 diols (vicinal diols) are called glycols.
• Common names for glycols use the name of the
alkene from which they were made.
CH2CH2
CH2CH2CH3
OH OH
OH OH
1,2-ethanediol
1,2-propanediol
ethylene glycol
propylene glycol
Preparation Reactions
Reduction of carbonyl compounds
Hydration of Alkenes
Grignard reactions
Reduction of Carbonyl
Compounds
• Reduction of Aldehydes/ketones
• Reduction of Carboxylic acids/Esters
Reduction of
Aldehydes/Ketones
Hydrogenation
H2
R C H Pt
O
RCH 2OH Primary ROH
H2
R C R'
Pt
O
H
R C R'
OH
Secondary ROH
Reduction of Aldehydes/Ketones
Hydride Reductions
LiAlH 4
RCH
RCH 2OH
or
O
NaBH 4
R C R'
O
LiAlH 4
or
NaBH
4
H
R C R'
OH
Primary ROH
Secondary ROH
Mechanism
Reduction of Carboxylic Acids
and Esters
Lithium Aluminum Hydride
Reduction
R
C OH
LiAlH 4
RCH 2OH + OH -
O
R
C OR'
O
LiAlH 4
RCH 2OH + R'OH
Hydration of
Alkenes
Acid catalyzed Hydration
Oxymercuration-Demercuration
Hydroboration-Oxidation
Acid-Catalyzed Hydration of
Alkenes
Markovnikov addition
Formation of most stable carbocation
Shifts/rearrangements possible
H
R
C
H
C
H
H
+
R
H 2O
R' H
R
C
C
R''
H+
H 2O
H
H
C
C
H
OH H
R
R'
H
C
C
OH H
R''
Hydration of Alkenes via
Oxymercuration/Demercuration
Y Markovnikov addition
Y Typically no shifts/rearrangements
Y Mercurinium ion involvement
H H
R
C C H
R' H
R
C C R''
Hg(OAc)
2
NaBH
4
H 2O
Hg(OAc)
H 2O
R
H
H
C
C H
OH H
2
NaBH
4
R
R'
H
C
C R''
OH H
Hydroboration-Oxidation of
Alkenes
Anti-Markovnikov addition
No shifts/rearrangements
Syn addition
H H
R
C
C H
R' H
R
C
C R''
(BH 3)2
(BH 3)2
OH H 2O 2
R
-
OH
H 2O 2
R
H
H
C
C H
H
OH
R'
H
C
C R''
H
OH
Grignard Addition Reactions
• Addition to Aldehydes/Ketones
• Addition to Esters
• Addition to Epoxides
Grignard Additions to Aldehydes/Ketones
Formation of primary, secondary, and
tertiary alcohols
H C H
RMgX
RCH 2OH
Primary ROH
O
H
R' C H
RMgX
O
R' C R
Secondary ROH
OH
R"
RMgX
R' C R"
O
R' C R
OH
Tertiary ROH
Grignard Additions to Esters
Formation of secondary and tertiary
alcohols
H
C OR + 2R'MgX
R' 2CHOH + ROH
O
Secondary ROH
R'
R" C OR + 2R'MgX
O
R" C OH + ROH
R'
Teriary ROH
Grignard Addition to Epoxides
O
+ RMgX
RCH 2 CH 2 OH
Primary ROH
O
+ RMgX
R'
R'
R'
R
OH
C
C
H
H
R'
Secondary ROH
R' O R'
+ RMgX
R'
R'
R'
R
OH
C
C
R'
R'
Tertiary ROH
R'
Physical Properties
• Unusually high boiling points due to hydrogen
bonding between molecules.
• Small alcohols are miscible in water, but
solubility decreases as the size of the alkyl
group increases.
Boiling Points
Solubility in Water
Solubility decreases as the size
of the alkyl group increases.
CHEMICAL PROPERTIES
The hydroxy gp present in alcohols is a very
reactive gp and the characterstic rxns of
alcohols are the rxns of –OH gp. In general,
these are divided into 3 categories:I. Rxns involving the cleavage of O-H bond
II. Rxns involving the cleavage of C-OH bond
III. Rxns involving both alkyl and hydroxyl gps of
the acohol molecules.
Typical Alcohol Reactions
Salt formation
Dehydration
Oxidation
Alkyl halide
formation
Ester formation
Ether synthesis
Periodic acid cleavage
of glycols
Haloform reaction of
methyl carbinols
THP acetal formation
Conversion of
Alcohols to Salts
Reaction with Active Metals
ROH
Na
-
+
RO Na + H 2
Dehydration of Alcohols
E-1
H
R C C OH
H+
E-2
H
+
R C C OH 2
H
H
rds
H 2O
+
H
H
+
R C C+
H
H
R C C OPOCl 2
H
a dichlorophosphate
intermediate
H
H
H
R C C+
POCl 3
H
H
R C C OH 2
H
R C C OH
H
R C C OPOCl 2
R C C
H
H
R C C
H
1,2-shifts/rearrangements possible
N
Anti periplanar (coplanar) elimination
No 1,2-shifts/rearrangements possible
Oxidation of Alcohols
PCC
Primary
RCH 2OH
KMnO 4
or
K 2Cr 2O 7
²
Secondary
Tertiary
RCHO
R 2CHOH
R 3COH
PCC or
KMnO 4
or
K 2Cr 2O 7
²
PCC or
KMnO
²
RCOOH
R
C R
O
no reaction
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Alcohol Conversion to Alkyl
Halides
Reaction with Hydrogen halides
Reaction with Thionyl chloride
Reaction with Phosphorus
trihalides or pentahalides
Hydrogen Halide Conversion of
Alcohols to Alkyl Halides
RCH 2OH
HX
RCH 2X
SN 2 predominantly
R 2CHOH
HX
R 2CHX
SN 1 or SN 2
R 3COH
HX
R 3CX
SN 1 predominantly
where HX = HI, HBr, or HCl
Mechanisms
Ester Formation from Alcohols
R
C Cl
R'OH
C OR' + HCl
R
O
R
R'OH
C O C R
O
R
O
C OH
O
R
O
R'OH
H
+
R
C OR' + R
C OH
O
O
C OR' + H 2O
O
Distinction b/w 1o, 2o & 3o alcohols
• LUCAS TEST
Lucas reagent : equimolar mixture of c.HCl and anhyd. ZnCl2
Appearance of cloudiness in the rxn mixture indicates the
conversion of alcohol into alkyl halide.
Observation30 alcohol:- reacts immediately & cloudiness appears
immediately.
20 alcohol:- reacts within about 5 minutes when the cloudiness
appears.
10 alcohol:- does not react appreciably at room temp. &
therefore no cloudiness appears.
• An older method known as Victor Meyer’s test is seldom used
these days.
Dihydric Alcohols
Glycols
Periodic Acid Cleavage of
Glycols
H
H
H
C
C H
HIO
OH OH
R
H
H
C
C R
HIO
OH OH
R
R'
R'
C
C R
OH OH
HIO
2H
4
3
O
2R
4
C H + HIO
3
O
2R
4
C H + HIO
C R' + HIO
O
3