Download - 1 3.9 Reactions of alcohols 1.Combustion 2. Dehydration 3. Formation of ethers 4.Oxidation
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Combustion
CH3OH + 2O2 CO2 + 2H2O + Heat
The same as hydrocarbons undergo combustion in the presence of oxygen, alcohols burn with oxygen too.
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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-C=C-H + H2O
H H H H
alcohol (ethanol) 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-C=C-H + H2O
H H H H
alcohol (ethanol) 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”
H
OH
H+
Heat
+H2O
+H2O
2-Butene (major product: 90%)
1-Butene (minor product: 10%)2-Butanol
2 H 3H
THE MOST SUBSTITUTED C=C IS THE MAJOR PRODUCT
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Formation of Ethers
Ethers form when the dehydration of alcohols occurs at lower temperatureslower temperatures in the presence of an acid catalyst.
R—OH + HO—R’ R—O—R’ + H2OH+
Low Heat
ANOTHER KIND OF DEHYDRATION
The components of water are removed from two molecules: an H– from one alcohol and the –OH from another.
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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.
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
CH3 CH3 CH3 CH2
OH
CH3 C H
O
CH3 C OH
O
CO2 H2O+
Alkane Alcohol (1°) Aldehyde Carboxylic acid
1 Bond to O 2 Bonds to O 3 Bonds to O
[O] [O] [O][O]
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.
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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
CH3OH CH2O HCO2H
Oxidation of Primary Alcohol
Primary alcohol aldehyde carboxylic acid
Example: oxidation of methanol
[O] [O]
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Oxidation of Secondary Alcohol
Secondary alcohol Ketone[O]
R1 CH
OH
R2[O]
R1 C
O
R2 H2O
R1, R2: alkyl groups
2° AlcoholKetone
Double bonds formsTwo hydrogen atoms removed
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Metabolic example
Lactic aciddehyrogenase
Lactic acid Pyruvic acid
C H 3 C H
O H
C O H
O
C H 3 C
O
C O H
O
16ethanol orange Acetic acid green
Breathalyzer test for suspected drunk driversCH3 CH2
OH
CH3 C H
O
Cr6+ [O]+ + Cr3+O
CH3 CH2
OH
CH3 C H
O
CO2 H2O+ Oxidation of Alcohol in the body[O][O]
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Oxidation of Tertiary Alcohol
Tertiary alcohol No reaction[O]
R1, R2, R3: alkyl groups
3° Alcohol
Only one hydrogen atom, not the two required to be removed
R1-C-R2
R3
OH[O]
NR
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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
Question
Answer: “b”. The alkene forms so that the carbon-carbon double bond is more substituted. This is Saytzeff's rule.
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What is the product of this reaction?
a. a carboxylic acidb. a ketonec. an aldehyded. no product forms
Question
Answer: “d”. Tertiary alcohols cannot oxidize.
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The hydration of 1-methylcyclopentene gives
a. 1-methylcyclopentanol.b. 2-methylcyclopentanol.c. 3-methylcyclopentanol.d. None of these form.
Question
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.