Transcript Reactions of Alcohol

Aspects
4/12/2016
Dr Seemal Jelani
Chem-240
1
Structure of Alcohols
The hydroxyl groups of alcohols are good hydrogen
bonding donors and acceptors
4/12/2016
Dr Seemal Jelani
Chem-240
2
 180.9 0
109.3 0
 The functional group
of an alcohol is an
OH (hydroxyl) group
bonded to an sp3hybridized carbon
4/12/2016
Dr Seemal Jelani
Chem-240
3
 The oxygen atom of an alcohol is also
sp3 hybridized
 Two sp3 hybrid orbitals of oxygen form
sigma bonds to atoms of carbon and
hydrogen, and the remaining two sp3
hybrid orbitals each contain an
unshared pair of electrons
4/12/2016
Dr Seemal Jelani
Chem-240
4
The measured C-O-H bond angle
in methanol is 108.9°, very close to
the perfectly tetrahedral angle of
109.5°.
4/12/2016
Dr Seemal Jelani
Chem-240
5
Physical properties
4/12/2016
Dr Seemal Jelani
Chem-240
6
Detail Physical properties
Dipole-dipole interaction
 The attraction between the positive end of
one dipole and the negative end of another
Hydrogen bonding
 The attractive interaction between a
hydrogen atom bonded to an atom of high
electronegativity (most O or N)
4/12/2016
Dr Seemal Jelani
Chem-240
7
and a lone pair of electrons on another atom of high
electronegativity (again, most commonly O or N).
δ–
H δ+
O
δ+
C
H
H
H
4/12/2016
Dr Seemal Jelani
Chem-240
8
Why alcohols are considered as
Polar compounds
Due of the presence of the polar-OH
group, alcohols are polar
compounds, with partial positive
charges on carbon and hydrogen
and a partial negative charge on
oxygen
4/12/2016
Dr Seemal Jelani
Chem-240
9
The attraction between the
positive end of one dipole and
the negative end of another is
called dipole-dipole
interaction.
4/12/2016
Dr Seemal Jelani
Chem-240
10
When the positive end of one of the
dipoles is a hydrogen atom bonded to
O or N (atoms of high electronegativity)
and the negative end of the other dipole
is an O or N atom, the attractive
interaction between dipoles is
particularly strong and is given the
special name of hydrogen bonding.
4/12/2016
Dr Seemal Jelani
Chem-240
11
Polarity of the
C-O-H bond in
alcohol
4/12/2016
Dr Seemal Jelani
Chem-240
12
δ−
δ−
4/12/2016
Dr Seemal Jelani
δ+
δ+
Chem-240
13
•There is extensive hydrogen bonding
between alcohol molecules in
the pure liquid
•It shows the association of ethanol
molecules by hydrogen bonding
between the partially negative oxygen
atom of one ethanol molecule and the
partially positive hydrogen atom of
another ethanol molecule.
4/12/2016
Dr Seemal Jelani
Chem-240
14
4/12/2016
Dr Seemal Jelani
Chem-240
15
Effect of Hydrogen bonding
 The presence of additional hydroxyl
groups in a molecule further increases
the extent of hydrogen bonding, as can
be seen by comparing the boiling
points of hexane (bp 69°C), 1-pentanol
(bp 138°C), and 1,4-butanediol (bp
230°C)
4/12/2016
Dr Seemal Jelani
Chem-240
16
Boiling Points and Solubilities in Water of Five Groups
of Alcohols and Hydrocarbons of Similar Molecular
Weight
Mol wt BP
S/water
 CH3OH
Methanol
 CH3CH3 Ethane
 CH3CH2OH Ethanol
 CH3CH2CH3 Propane
 CH3CH2CH2OH
1-Propanol
 CH3CH2CH2CH3
Butane
4/12/2016
Dr Seemal Jelani
32
30
46
44
60
65
289
78
242
97
Infinite
Insoluble
Infinite
Insoluble
Infinite
58
0
Insoluble
Chem-240
17
 CH3CH2CH2CH2OH
74
1-Butanol
 CH3CH2CH2CH2CH3
Pentane
72
 HOCH2CH2CH2CH2OH 90
1,4-Butanediol
 CH3CH2CH2CH2CH2OH 88
1-Pentanol
 CH3CH2CH2CH2CH2CH386
 Hexane
4/12/2016
Dr Seemal Jelani
Chem-240
117
8 g/100 g
36
230
Insoluble
Infinite
138
2.3 g/100 g
69
Insoluble
18
Acidity and Basicity of Alcohols
 Alcohols can function as both weak
acids (proton donors) and weak bases
(proton acceptors)
 In dilute aqueous solution, only
methanol (pKa 15.5) is more acidic than
water.
4/12/2016
Dr Seemal Jelani
Chem-240
19
..
..
 CH3-O-H + H-O-H
..
..
4/12/2016
Dr Seemal Jelani
.. .. +
CH3-O : + H-O-H
..
H
Chem-240
20
Acidity is associated not only with
the tendency of compound to yield
hydrogen in H2O but also to accept
an electron pair to form a covalent
bond.
4/12/2016
Dr Seemal Jelani
Chem-240
22
The strengths of weak acids are measured on the pKa
scale. The smaller the number on this scale, the
stronger the acid is.
The less the charge is
delocalized, the less
stable the ion, and the
weaker the acid.
General
Rule
1. Acidity increases as the electronegativity increases
Example:
H – CH3 < H – NH2 < H – OH < H – F < H – SH < H – Cl
2. Within the Family, acidity increases as the size
increases
Example:
H – F < H – Cl < H – Br < H – I
 Ethanol has about the same acidity as water
 Higher-molecular-weight, water soluble
alcohols are slightly weaker acids than water
 Thus, although alcohols have some acidity,
they are not strong enough acids to react
with weak bases such as sodium bicarbonate
or sodium carbonate
4/12/2016
Dr Seemal Jelani
Chem-240
26
In a polar solution, acidity of alcohols is lower, than acidity of water
due to the less efficient solvation of the alkoxides (steric reason).
Stronger base
CH3CH2OH + OH-
Stronger acid
CH3CH2O- + H2O
pKa = 15.9
pKa = 15.7
An alkoxide
Primary alcohols
Secondary alcohols
Tertiary alcohols
Acidity
CH3CH2OH,
pKa = 15.9
Primary alcoxides
CH3CH(OH)CH3,
pKa = 17.1
(CH3)3COH,
pKa = 19.2
Secondary alcoxides
Tertiary alcoxides
Basicity
For simple alcohols, such as
methanol and ethanol, acidity
depends primarily on the degree
of solvation and stabilization of
the alkoxide ion by water
molecules
4/12/2016
Dr Seemal Jelani
Chem-240
28
The negatively charged oxygen
atoms of the methoxide and
ethoxide ions are almost as
accessible for solvation as the
hydroxide ion is; therefore, these
alcohols are about as acidic as
water.
4/12/2016
Dr Seemal Jelani
Chem-240
29
pKa Values for Selected Alcohols in
Dilute Aqueous Solution* Stronger Acid
 Compound Structural
 Hydrogen chloride
 Acetic acid
 Methanol
 Water
4/12/2016
Dr Seemal Jelani
Formula
HCl
CH3COOH
CH3OH
H2O
Chem-240
pKa
27
4.8
15.5
15.7
30
 Ethanol
 2-Propanol
 2-Methyl-2-
CH3CH2OH
(CH3)2CHOH
(CH3)3COH
15.9
17
18
propanol
Weaker acid
4/12/2016
Dr Seemal Jelani
Chem-240
31
Conclusion
 To summarize, when trying to predict the
mechanisms of reactions involving an
hydroxy group, you need to keep in mind
that it is both a weak acid and a weak
base, so consider adding a proton or
taking a proton away in the initial steps of
mechanisms when there is a strong acid or
base present, respectively.
4/12/2016
Dr Seemal Jelani
Chem-240
32
 In addition, an important mechanistic
theme in many of the reactions of
alcohols is that the -OH group, a poor
leaving group, reacts with protons or a
variety of strong electrophiles to
create –OH2+ or analogous group, a
much better leaving group, enabling
subsequent substitution or elimination
reactions to take place.
4/12/2016
Dr Seemal Jelani
Chem-240
33
Reactions of
Alcohol
4/12/2016
Dr Seemal Jelani
Chem-240
34
Oxidation and Reduction
 Ways of recognizing oxidation/reduction reactions:
 Oxidation and reductions always occur together
Oxidation
Reduction
An atom loses
electrons
An atom gains
electrons
An atom gains a
bond to oxygen
An atom loses a
bond to oxygen
An atom loses a An atom gains a
bond to hydrogen bond to hydrogen
35
Reactions of Alcohols
 This same definition can also be applied to the
oxidation of alcohols by potassium dichromate
(K2Cr2O7).
The oxidation
requires that there
are hydrogens to be
removed on the
carbon to which the
hydroxyl is bound
36
Reaction of Alcohols with Active
Metals
 Alcohols react with Li, Na, K, and other active
metals to liberate hydrogen and form metal
alkoxides
 In this oxidation/reduction reaction, Na is
oxidized to Na+ and H+ is reduced to H2
2CH3 OH + 2 Na
2CH3 O_Na +
+ H2
sodium methoxide
4/12/2016
Dr Seemal Jelani
Chem-240
37
 The sulfuric acid removes the alcohol from the air
into a liquid solution.
 The alcohol reacts with potassium dichromate to
produce:
* chromium sulfate
* potassium sulfate
* acetic acid
* water
Silver nitrate is used as a catalyst
4/12/2016
Dr Seemal Jelani
Chem-240
38
4/12/2016
Dr Seemal Jelani
Chem-240
39
 Application: Breathalyzer
4/12/2016
Dr Seemal Jelani
Chem-240
40
Reactions of Alcohols
 The oxidation of primary (1°) alcohols is a way
for preparing aldehydes and carboxylic acids.
 The oxidation of secondary (2°) alcohols is a way
for preparing ketones.
 The oxidation of tertiary (3°) alcohols does not
occur because there are not hydrogens attached
to the carbon to which the hydroxyl is attached
41
Reactions of Alcohols and Thiols
42
Reactions of Alcohols
 In biological reactions the
coenzyme NAD+ is often used
as the oxidizing agent.
 The NAD+ takes the
electrons away from alcohols
to produce aldehydes,
carboxylic acids and ketones.
43
Reactions of Alcohols
 Example
 The oxidation of malate to oxaloacetate that occurs in
the citric Oacid cycle:
O
O
O
C
HO
O
NAD+
C
H
O
C
CH2
CH2
C
C
O
L-malate
44
C
NADH + H+
O
O
oxaloacetate
 Another reaction the dehydration of alcohols to
produce alkenes
OH
C
H
C
alcohol
45
H+
heat
C
C
alkene
+
H2O
Preparations of Alcohols,
 Like the complement hydration reaction, dehydration
can also produce multiple products.
46
Preparations of Alcohols
 Multiple products occur whenever there are
a different number of hydrogen atoms
attached to the two carbons that are on
either side of the carbon to which the
hydroxyl is attached.
 There is a rule that can be used to predict
which of the two products is predicted to
be the major product.
47
 In a dehydration of an alcohol, the hydrogen
will be removed from the neighboring
carbon atom that carries the fewest
hydrogen atoms.
4/12/2016
Dr Seemal Jelani
Chem-240
48
 Examples of
dehydration of
alcohols
49
Reactions of Alcohols with
Aldehydes and Ketones
 Aldehydes and
ketones can react with
alcohols to form
hemiacetals,
hemiketals, acetals
and ketals.
 This is because
carbohydrates are
rich in aldehydes,
ketones and
alcohols
50
O
H
C
CH2
H
C
OH
HO
C
H
H
C
H
C
CH2
glucose
OH
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
OH
CH2
fructose
OH