1

3.9 Reactions of alcohols

1.Combustion2. Dehydration

3. Formation of ethers4.Oxidation

2

Combustion

CH3OH + 2O2 CO2 + 2H2O + Heat

The same as hydrocarbons undergo combustion in the presence of oxygen, alcohols burn with oxygen too.

3

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)

4

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)

5

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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Formation of Ethers

CH3—OH + HO—CH3 CH3—O—CH3 + H2OH+

Low Heat

ANOTHER KIND OF DEHYDRATION

9

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

•Primay•Seconday•Tertiary

ALCOHOLS

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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.

19

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.

20

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.

21

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

Question

Answer: “b”.