Download Topic 10.4 Organic Chemistry Alcohols

Topic 10.4 Alcohols
Assessment Statements
 10.4.1 Describe, using equations, the complete
combustions of alcohols.
 10.4.2 Describe, using equations, the oxidation
reactions of alcohols.
 10.4.3 Determine the products formed by the
oxidation of primary and secondary alcohols.
Alcohols
An alcohol consists of a carbon chain with a hydroxy
group (-OH) attached
Methanol
Ethanol
Propanol
Phenol
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Alcohol Classification
Alcohols may be classified as
- Primary
- Secondary
- Tertiary
Depending on whether the carbon atom
that is attached to the –OH group is
surrounded by one, two or three other
carbon atoms
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Primary Alcohols
A primary alcohol has only one carbon atom attached
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Secondary Alcohols
A secondary alcohol has 2 carbon chains attached
to the group on which the –OH resides.
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Tertiary Alcohols
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Polyhydroxy alcohols
Alcohols that have more than one OH group
are known as polyhydroxyl alcohols. Two
examples are shown below
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Name and classify the following
alcohols.
 2-butanol
(Secondary)
 2-propene-1-ol
(Primary)
 2-methyl-1-butanol
(Primary)
 2-methyl-2-butanol
(Tertiary)
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Properties of Alcohols
The physical properties of alcohols are similar
to those of both water and hydrocarbons
The shorter chain alcohols such as methanol and
ethanol are similar to water, in general they
-- have higher boiling points than hydrocarbons
but lower than water
-- dissolve in water to some degree
-- are more polar than hydrocarbons but less
polar than water
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Physical Properties of ALcohols
State at r.t.p.*
Name
Molecular formula
Solubility In Water
Methanol
CH3OH
Very Soluble
Liquid
Ethanol
C2H5OH
Soluble
Liquid
Propanol
C3H7OH
Soluble
Liquid
Butanol
C4H9OH
Slightly soluble
Liquid
(solid/liquid/gas)
Reactions of Alcohols
Alcohols undergo several types of
reactions including:
Oxidation
Dehydration
Reactions with active metals
Esterification
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Assessment Statements
 10.4.1 Describe, using equations, the complete
combustions of alcohols.
Combustion
An alcohol burns in air to produce
carbon dioxide and water vapour.
ethanol + oxygen → carbon dioxide + water vapour
C2H5OH + 3O2 → 2CO2 + 3H2O
C2H5OH + 3O2 → 2CO2 + 3H2O
Combustion
Flame produced is clean
Combustion of alcohols are useful because:
1. Methanol is less likely than conventional fuel to explode
in an accident.
It is a clean fuel.
2. When alcohol beverage is burnt with food, it gives a
distinct flavour.
Assessment Statements
 10.4.2 Describe, using equations, the oxidation
reactions of alcohols.
 10.4.3 Determine the products formed by the
oxidation of primary and secondary alcohols.
Oxidation
Alcohols are oxidized to alkanals (aldehydes) or
alkanones (ketones)
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Oxidation
 We can oxidise an alcohol (for example, ethanol) by
warming it with acidified potassium dichromate(VI).
 Experimental setup:
(ethanol)
C2H5OH
+ acidified K2Cr2O7
cold
water
heat
CH3COOH
(ethanoic acid)
Oxidation
 The colour of potassium dichromate(VI) changes from
orange to green.
 Oxidation of alcohol is used in:
Breathalyzer to test the amount of alcohol consumed by
drivers.
1º, 2º, 3º Carbons
Primary Alcohols are Oxidized to Alkanals
The most common oxidizing agents are KMnO4 in
basic solution, K2Cr2O7 in acidic solution, or oxygen
from the air. The mechanisms of such reactions are
usually complex
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Oxidation
Chemical equation:
+ 2 [O]
Ethanol
+
Ethanoic
acid
H2 O
Oxidation of 1° Alcohols
 1° alcohol to aldehyde to carboxylic acid
 Difficult to stop at aldehyde
 Use pyridinium chlorochromate (PCC) to limit
the oxidation.
 PCC can also be used to oxidize 2° alcohols to
ketones.
OH
CH3CH2CH2CH2
N H CrO3Cl
O
CH3CH2CH2CH
=>
Chapter 11
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Stopping at Aldehydes
 For IB,
distillation of
aldehyde as it is
produced
increases its yield
Improving Yield of Carboxylic
Acid
 For IB, Heating under reflux
Secondary Alcohols are oxidized to
Alkanones
Example
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Oxidation of 2° Alcohols
 2° alcohol becomes a ketone
 Reagent is Na2Cr2O7/H2SO4
 Active reagent probably H2CrO4
 Color change: orange to greenish-blue
OH
CH3CHCH2CH3
Na2Cr2O7 / H2SO4
O
CH3CCH2CH3
=>
Chapter 11
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Tertiary Alcohols are not easily
oxidized
There is no hydrogen attached to the tertiary
carbon. It is not possible for a carbonyl group
on the tertiary carbon.
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Tertiary Alcohols are not easily
Oxidised
Furthermore the large R groups on the tertiary
carbon, prevent reacting groups. Hence tertiary
alcohols are rather unreactive.
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3° Alcohols Don’t Oxidize
 Cannot lose 2 H’s
 Basis for chromic acid test
=>
Chapter 11
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Question
 Starting with prop- 1- ene, show a chemical pathway to
produce propanoic acid.
Dehydration
Many alcohols undergo intramolecular
dehydration, removing water to form alkenes.
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Reactions with Metals
• Alcohols are far less acidic than water but they
react with highly reactive metals such as
potassium and sodium.
• The metal replaces the hydrogen atom in the
alcohol group forming a salt and hydrogen gas
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Esterification
An alcohol reacts with an alkanoic acid to form
an ester and water.
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Thiols
If the oxygen in the –OH group is replaced with
a sulfur atom, a group with somewhat similar
properties results. The –SH functional group is
known as a thiol or mercaptan.
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Some Thiols
Thiols are well known for their intense, foul odors
Responsible for
skunk odor
Added to natural gas to
give it a rotten egg smell
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Name and classify the following
alcohols.
 2-butanol
(Secondary)
 2-propene-1-ol
(Primary)
 2-methyl-1-butanol
(Primary)
 2-methyl-2-butanol
(Tertiary)
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Alcohols and Natural Products
Urushiol is
the active
ingredient in
poison ivy
and poison
oak.