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1818 Alcohols,Alcohols,amines andamines andrelatedrelated
compoundscompoundsPrepared by
Emma Bartle
University of Western Australia
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Alcohols play a central role in the interconversion of organic
functional groups.
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18.1 Alcohols18.1 Alcohols
Alcohols contain the functional group
OH, a hydroxyl group
This hydroxyl group is bonded to an sp3
hybridised carbon atom
The oxygen atom is also sp3 hybridised
Two sp3 hybrid orbitals form bonds to
carbon and hydrogen The remaining two sp3 hybrid orbitals each
contain an unshared pair of electrons
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18.1 Alcohols18.1 Alcohols
Alcohols are named the same way as
alkanes, with the following differences
The parent alkane is the longest chain ofcarbon atoms containing the OH group
Change the suffix of the parent alkane
from e to ol, and use a number to
show the location of the OH group
Name and number substituents and list
them in alphabetical order
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18.1 Alcohols18.1 Alcohols
To derive common names for alcohols,
we name the alkyl group bonded to the
OH group and add the word alcohol
Ethanol(Ethyl alcoh ol)
1-Propanol(Propy l alcoho l)
2-Propanol(Is opropy l alcoho l)
1-Butanol(Butyl alcoho l)
OH
OH
OH
OH
2-Butanol
(s ec-Bu tyl alcoho l)2-Methyl-1-propanol
(Is obutyl alcoh ol)
2-Methyl-2-propanol(t ert-Butyl alcohol )
OH
Cyclohexanol(Cyclohexyl alcoho l)
OH
OH
OH
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Write the IUPAC name for each of the following alcohols.
a) is derived from an unbranched alkane so we need to identify only the
alkane and the position of the OH group. Compound
(b) includes a branch on the second last carbon, and the OH group ison the second carbon. Compound (c) is a little more complex. It is a
cyclic alcohol, so we start numbering from the OH group. There is also
a methyl group on the second carbon which is trans- to the OH group
a)
b)
c)
a) octan-1-olb) 4-methylpentan-2-ol
c) trans-2-methylcyclohexanol or 2-methylcyclohexanolstereochemistry,
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18.1 Alcohols18.1 Alcohols
Compounds containing 2 OH groups
are named as diols
Compounds containing 3 OH groups
are named as triols
CH3
CHCH2
HO OH
CH2 CH2
OHOH
CH2 CHCH2
OHHOHO
1,2-Ethanedio lEthylene glycol
1,2-PropanediolPropylene glycol
1,2,3-Propanetrio lGlycerol, Glycerin)
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Compounds containing two hydroxyl groups on adjacent carbons are often
referred to as glycols. Ethylene glycol and propylene glycol are synthesised
from ethylene and propylene respectively,
We classify alcohols as primary (1),
secondary (2) or tertiary (3),Depending on whether the OH
group is on a primary, secondary
or tertiary carbon atom.
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18.1 Alcohols18.1 Alcohols
Physical properties
Both the C-O and O-H bonds of an
alcohol are polar covalent
Alcohols are polar molecules
This is demonstrated for methanol below
H-
H+
H+O
HH
H
C
H
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18.1 Alcohols18.1 Alcohols
Alcohols have higher boiling points and
water solubility than hydrocarbons
CH3 CH2 CH2OH
CH3 CH2 CH2 CH3
CH3OH
CH3 CH3
CH3 CH2 OH
CH3 CH2 CH3
CH3 CH2 CH2 CH2 CH2OH
HOCH2 CH2 CH2 CH2 OH
CH3 CH2 CH2 CH2 CH2 CH3
Structural Formula Name
Molecular
Wei ht(g/mol)
Boiling
Point(C)
Solubilityin Water
methanol 32 65 infinite
ethane 30 -89 insoluble
ethanol 46 78 infinite
propane 44 -42 insoluble
1-propanol 60 97 infinite
butane 58 0 insoluble
1-pentanol 88 138 2.3 g/100 g
1,4-butanediol 90 230 infinite
hexane 86 69 insoluble
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Alcohols associate in the liquid state by
hydrogen bonding
18.1 Alcohols18.1 Alcohols
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Because ofhydrogen bonding between alcohol molecules in the liquid state,
extra energy is required to separate each alcohol molecule from its neighbours,
high boiling points of alcohols compared with alkanes.
Additional hydroxyl groups in a molecule further increase the extent of
hydrogen bonding:
the boiling points of pentan-1-ol (138 C) and butane-1,4-diol (230C),
which have approximately the same molar mass.
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18.1 Alcohols18.1 Alcohols
Preparation of alcohols
Alcohols can be prepared from many
functional groups
Discussed here are:
Preparation from alkenes
Preparation from haloalkanes
Reduction of carbonyl compounds Addition of Grignard reagents to carbonyl
compounds
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Preparation from alkenes Acid catalysed hydration of alkenes
18.1 Alcohols18.1 Alcohols
Obeys Markovnikov's rule (the hydrogen adds to the carbon of the double
bond with the most hydrogen atoms).
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Preparation from haloalkanesNucleophilic substitution with a hydroxide ion
Tertiary haloalkanes are readily converted to alcohols by water.
Secondary and primary alcohols are better prepared by
reaction with hydroxide ions.
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Reduction of carbonyl compounds Aldehydes are reduced to primary alcohols
Ketones are reduced to secondary alcohols
Carboxylic acids and esters can be reduced to form
primary alcohols
18.1 Alcohols18.1 Alcohols
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18.2 Reactions of alcohols18.2 Reactions of alcohols
Reactions discussed here are:
Acidity of alcohols
Basicity of alcohols Reaction with active metals
Dehydration of alcohols to alkenes
Conversion of alcohols to haloalkanes
Oxidation of alcohols to aldehydes,
ketones and carboxylic acids
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18.2 Reactions of alcohols18.2 Reactions of alcohols
Acidity of alcohols
Alcohols have similar acid ionisation
constants (pKa
) to water (15.7)
Aqueous solutions of alcohols have a pH
close to that of pure water
CH3 O H O H
H
[CH3 O-
][ H3 O+]
[ CH3 OH]
CH3 O H O
H
H+
Ka
=
+ +
= .
Ka= .
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18.2 Reactions of alcohols18.2 Reactions of alcohols
The table below gives the pKa for several
low-molar-mass alcohols
( H3 ) 3 OH
( H3 ) 2 HOH
H3 H2OH
H2O
CH3 OH
CH3 COOH
HCl
Co o Ka
-7
15 5
15 7
15 9
17
18
4 8
y roge c lori e
aceticaci
me t a o l
water
et a o l
2- ro a ol
2-met yl-2- ro a ol
Str ct ral
FormulaStro ger
aci
Weaker aci
*A lso give forcom ariso are Ka
values for water,
aceticaci , a y roge c lori e
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18.2 Reactions of alcohols18.2 Reactions of alcohols
Basicity of alcohols
In the presence of a strong acid, theoxygen atom of an alcohol is a weak base
It reacts with the acid by proton transfer toform an oxonium ion
Alcohols can function as both weak acidsand weak bases
CH3 CH2 -O-H H O
H
H O
H
H
H2 SO4CH3 CH2 -O H
H
CH3 CH2 -O H
H HH
O H
Ethylo xon ium ion(pK
a-2.4)
Hydron ium i on(pK
a-1.7)
Ethanol
++
+
++
++
+
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18.2 Reactions of alcohols18.2 Reactions of alcohols
Reaction with active metals
Alcohols react with Li, Na, K and other
active metals to form metal alkoxides
Na is oxidised to Na+ and H+ is reduced to H2
Alkoxide ions are stronger bases than thehydroxide ion
Alkoxide ions can be used as nucleophiles in
substitution reactions
2CH3CH2OH 2 Na 2CH3CH2O-Na
+ H2+ +
So iu m et oxi e
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Conversion to Haloalkanes
Reaction with Hydrohalic Acids (HCl, HBr and HI)
Water-soluble tertiary alcohols react very rapidly with HCl, HBr and HI.
Low-molar-mass, water-soluble primary and secondary alcohols do not
react under these conditions.
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Water-insoluble tertiary alcohols are converted to tertiary halides by bubbling
gaseous HX through a solution of the alcohol dissolved in diethyl ether
or tetrahydrofuran (THF).
Primary and secondary alcohols are converted to bromoalkanes and
iodoalkanes by treatment with concentrated hydrobromic and hydroiodic
acids. For example, heating butan-1-ol with concentrated HBr
gives 1-bromobutane
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From observations of the relative ease of reaction of alcohols with HX
(3 > 2 > 1), it has been proposed that the conversion of tertiary and
secondary alcohols to haloalkanes by concentrated HX occurs by an SNl
mechanism and involves the formation of a carbocation intermediate.
This occurs through the three steps shown below:
Rapid and reversible proton transfer from the acid to the OH group gives an
oxonium ion. This proton transfer converts the leaving group from OH, a poor
leaving group, to H2O, a better leaving group.
Loss of water from the oxonium ion gives a 3 carbocation intermediate.
Step 1
Step 2
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Reaction of the 3 carbocation intermediate (an electrophile)
with a chloride ion (a nucleophile) gives the haloalkane product.
Step 3
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Primary alcohols react with HX by an SN2 mechanism. In
the rate-determining step, the halide ion displaces H2O from
the carbon atom bearing the oxonium ion. The displacement
of H2O and the formation of the CX bond are simultaneous.
Step 1: Rapid and reversible proton transfer to the OH group converts the
leaving group from OH to H2O, which is a better leaving group
Step2: The nucleophilic displacement of H2O by Br then gives the bromoalkane.
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Why do tertiary alcohols react with HX by formation of carbocation
intermediates, whereas primary alcohols react by direct displacement of
the OH2+ group (the protonated OH group)?
1. Electronic factors: Tertiary carbocations are the most stable and mostreadily formed, whereas primary carbocations are the least stable and
hardest to form
2. Steric factors: To form a new carbonhalogen bond, the halide ion must
approach the carbon atom bearing the leaving group from the side directly
opposite that group and begin to form a new covalent bond.
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18.2 Reactions of alcohols18.2 Reactions of alcohols
140oC
Cyclohexanol Cyclohexene
OH
+ H 2 OH2 SO4
180oCCH3 CH2 OH
H2 SO4CH2 =CH 2 + H 2 O
+ H2 OCH3 COH
CH3
CH350oC
H2 SO4CH3 C=CH2
CH3
2- et yl rope e
(Isobutyle e)
Acid-catalysed Dehydration to Alkenes
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18.2 Reactions of alcohols18.2 Reactions of alcohols
When isomeric alkenes are obtained, the
more stable alkene generally predominates
The more stable alkene has the greater
number of substituents on the double bond The acid-catalysed dehydration of alcohols
follows Zaitsevs rule
CH3 CH2 CHCH3
H8 5 %H3 P 4
CH3 CH= CHCH3 CH3 CH2 CH= CH2
1-Butene(20%)
2-Butene(80%)
2-Butanol+
heat
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18.2 Reactions of alcohols18.2 Reactions of alcohols
Oxidation of 1 and 2 alcohols
Oxidation of a 1 alcohol gives an aldehyde or
carboxylic acid, depending on the experimental
conditions
2 alcohols are oxidised to ketones
3 alcohols are not easily oxidised
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18.2 Reactions of alcohols18.2 Reactions of alcohols
The most common reagent used for the
oxidation of a 1 alcohol to carboxylic acid
and 2 alcohol to a ketone is chromic acid
CrO3 H2 OH2 SO4 H2 CrO4+
Chro ic acidChro i (VI)oxide
CH3 (CH2 )6 CH2 OHCrO3
H2 SO4 , H2 OCH3 (CH2 ) 6 CH
O
CH3 (CH2 )6 COH
O
Octa al( ot isolated)
Octa oic acid1-Octa ol
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18.2 Reactions of alcohols18.2 Reactions of alcohols
The mild oxidising agent PCC is used to
oxidise 1 alcohols to aldehydes
CrO3 HClN N
H
CrO3 Cl
-
Py idinium chlo och omate(PCC)
Py idine
+ ++
OHPCC
CH2 Cl2H
O
Ge aniol Ge anial
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18.4 Ethers18.4 Ethers
Ethers contain an atom of oxygen bonded
to 2 carbon atoms
Oxygen is sp
3
hybridised with bondangles of approximately 109.5
H
H O
H
C H
H
H
C
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18.4 Ethers18.4 Ethers
The OR group bonded to the parent
alkane is named as an alkoxy
Common names are derived by listingthe alkyl groups bonded to oxygen in
alphabetical order and adding ether
CH3
CH3
CH3 CCH3
Et 2
H
Et
E h h n
Di h h
h m h p p nm h t ert-bu h , TBE
t rans-2-E h h n
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18.4 Ethers18.4 Ethers
Cyclic ethers contain the ether oxygen
as one of the atoms in the ring
Cyclic ethers are generally known bytheir common names
E hylene
oxi e
Tetrahydro-
furan,T F
Tetrahydro-
yran
1,4-Dioxane
O O
O
OO
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Physical properties
Ethers are moderately polar compounds
However, only weak forces of attraction exist
between ether molecules
18.4 Ethers18.4 Ethers
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18.4 Ethers18.4 Ethers
Ethers have lower boiling points than
alcohols of similar molar mass
CH3 CH2 OH
CH3 OCH3
CH3 CH2 CH2 CH2 CH2 OH
HOCH2 CH2 CH2 CH2 OH
CH3 CH2 CH2 CH2 OCH3
CH3 CH2 CH2 CH2 OH
CH3 CH2 OCH2 CH3
CH3 OCH2 CH2 OCH3 ethylene g lycoldimethyl ethe
90 84 infinite
8 g/100 g3574diethyl ethe
1-butanol 74 117 7.4 g/100 g
slight7188butyl methyl e the
infinite230901,4-butanediol
2.3 g/100 g138881-pentanol
7.8 g/100 g-2446dimethyl ethe
infinite7846ethanol
olubilityin Wate
Boiling
Point(C)
MoleculaWeightNamet uctu al Fo mu la
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18.4 Ethers18.4 Ethers
The effect of hydrogen bonding is
illustrated by comparing the boiling
points of ethanol and dimethyl ether
The difference is due to the polar O-Hgroup in the alcohol
This increases the attractive force
between molecules of ethanol
CH3 CH2 OH CH3 OCH3
bp-24C
Ethanol
bp C
Dim thyl ther
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18.4 Ethers18.4 Ethers
Reactions of ethers
Ethers resemble hydrocarbons in their
resistance to chemical reactions
They do not react readily with oxidising
agents or reducing agents
They are not affected by most acids or bases
at moderate temperature
Because of their good solvent properties
and general inertness to chemical
reaction, ethers are excellent solvents in
which to carry out organic reactions
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SummarySummary
Alcohols
Contain the functional group OH, the
hydroxyl group, bonded to an sp3
hybridised carbon atom
Classified as 1, 2 or 3
IUPAC names are formed by changing
the suffix of the parent alkane to ol Common names are derived by naming
the alkyl group followed by alcohol
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