Transcript Document

3.9 Reactions of alcohols
1.Combustion
2. Dehydration
3. Formation of ethers
4.Oxidation
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The same as hydrocarbons undergo combustion
in the presence of oxygen, alcohols burn with
oxygen too.
Combustion
CH3OH + 2O2
CO2 + 2H2O + Heat
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Dehydration of Alcohols to form Alkenes
Alkenes can add water to yield alcohols.
In a reverse reaction, alcohols can lose water
molecule when they are heated with an acid catalyst
such as H2SO4.
H OH
H+, heat
H-C-C-H
H H
alcohol (ethanol)
H-C=C-H + H2O
H H
alkene (ethene)
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Dehydration of Alcohols to form Alkenes
During the dehydration of an alcohol, a H– and –
OH are removed from adjacent carbon atoms of the
same alcohol to produce a water molecule. A
double bond forms between the same two carbon
atoms to produce an alkene.
H OH
H+, heat
H-C-C-H
H H
alcohol (ethanol)
H-C=C-H + H2O
H H
alkene (ethene)
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Saytzeff’s Rule
The dehydration of a secondary alcohol can result in
the formation of either of two products.
Saytzeff’s rule state that the major product is the one
that results when the hydrogen is removed from the
carbon atom with the smallest number of hydrogen
atoms.
“THE POOR GET POORER” rule
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Saytzeff’s Rule
The major product is the one that results when the
hydrogen is removed from the carbon atom with the
smallest number of hydrogen atoms. “The poor get
poorer”
2H
3H
H
+H2O
H+
Heat
2-Butene (major product: 90%)
+H2O
OH
2-Butanol
1-Butene (minor product: 10%)
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THE MOST SUBSTITUTED
C=C
IS THE MAJOR PRODUCT
Formation of Ethers
ANOTHER KIND OF DEHYDRATION
Ethers form when the dehydration of alcohols occurs at
lower temperatures in the presence of an acid catalyst.
R—OH + HO—R’
H+
Low Heat
R—O—R’ + H2O
The components of water are removed from two molecules:
an H– from one alcohol and the –OH from another.
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Formation of Ethers
ANOTHER KIND OF DEHYDRATION
CH3—OH + HO—CH3
H+
Low Heat
CH3—O—CH3 + H2O
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Oxidation of Alcohols
The reactions of alcohols have a central role in organic chemistry because
alcohols can be converted to many of the other functional groups.
Reduction is
• a gain of electrons
• a less positive oxidation number
• a gain of hydrogen atoms
• the loss of oxygen atoms
• the loss of bonds to oxygen.
Oxidation is
• a loss of electrons
• a more positive oxidation number
• a loss of hydrogen atoms
• the addition of oxygen atoms.
• more bonds to oxygen
2 Bonds to O
1 Bond to O
Alkane
Alcohol (1°)
OH
[O]
CH3
CH3
CH3
CH2
3 Bonds to O
Aldehyde
[O]
O
CH3
C
Carboxylic acid
O
[O]
H
CH3
C
OH
[O]
CO2 + H 2O
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Oxidation numbers
• Each C-H is a -1
• Each C-O is a +1
• Sum total C-H and C-O to determine
Oxidation Number
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Oxidation numbers
• Each C-H is a -1
• Each C-O is a +1
What are the oxidation numbers for
CH3OH
CH2O
HCO2H
Hint: Write the expanded formula to
determine the number of bonds to C
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OXIDATION REACTIONS
•Primay
•Seconday
•Tertiary
ALCOHOLS
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OXIDATION REACTIONS
Oxidation of Primary Alcohol
Primary alcohol [O] aldehyde [O] carboxylic acid
Example: oxidation of methanol
CH3OH  CH2O  HCO2H
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Oxidation of Secondary Alcohol
Secondary alcohol
[O]
Ketone
Double bonds forms
Two hydrogen atoms removed
OH
R1 CH R2
[O]
O
R1 C R2
2° Alcohol
H2O
Ketone
R1, R2: alkyl groups
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Metabolic example
C H 3
O H
O
C H
C O H
C H 3
O
O
C
C O H
Lactic acid
dehyrogenase
Lactic acid
Pyruvic acid
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OH
CH3
O
[O]
CH2
CH3
C H
OH
CH3
CH2
ethanol
+
6+
Cr
orange
[O]
[O]
Oxidation of Alcohol in the body
CO2 + H2O
O
CH3
CO H
Acetic acid
+
3+
Cr
green
Breathalyzer test for
suspected drunk drivers
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Oxidation of Tertiary Alcohol
Tertiary alcohol
[O]
No reaction
Only one hydrogen atom, not the two required to be removed
OH
R1-C-R2
[O]
NR
R3
3° Alcohol
R1, R2, R3: alkyl groups
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Question
What is the dehydration product
when 3-methyl-2-butanol reacts with
acid in the presence of heat?
a.
trans-2-butene
b.
2-methyl-2-butene
c.
trans-2-methyl-2-butene
d.
3-methyl-1-butene
Answer: “b”. The alkene forms so that the carboncarbon double bond is more substituted. This is
Saytzeff's rule.
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Question
What is the product of this
reaction?
a.
b.
c.
d.
a carboxylic acid
a ketone
an aldehyde
no product forms
Answer: “d”. Tertiary alcohols cannot oxidize.
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Question
The hydration of 1-methylcyclopentene
gives
a.
b.
c.
d.
1-methylcyclopentanol.
2-methylcyclopentanol.
3-methylcyclopentanol.
None of these form.
Answer”a”. This reaction is the addition of water to a
double bond. Do you remember Markovnikov's rule? The
hydrogen from water adds to the end of the double bond
with the most hydrogens.
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Question
What product forms and what type of
reaction is this?
a. The reaction is an oxidation and the product is
b. The reaction forms ether and the product is
c. The reaction is an oxidation and the product is
d. The reaction forms ether and the product is
Answer: “b”.
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