Key area:
Oxidation of foods
Overview
In this section, learn how
oxidation reactions in foods
convert alcohols to aldehydes
and ketones, and study the role
of
antioxidants
in
the
preservation of foods.
.
a) Oxidation of alcohols
Learning intention
In this section, learn about the
products
of
oxidation
of
primary, secondary and tertiary
alcohols.
Find
out
about
important mild oxidising agents
and learn how to spot an
oxidation reaction in a carbon
compound.
After completing this lesson you should be able to :
Branched-chain alcohols, with no more than eight carbon atoms in their longest chain:
can be named from structural formulae
structural formulae can be drawn from names
molecular formulae can be written from names
Draw the structural formula for, and name isomeric alcohols, including primary secondary and
tertiary alcohols where appropriate.
Hydrogen bonding can be used to explain the properties of alcohols including, boiling points, melting points,
viscosity and solubility/miscibility in water. Structures of diols and triols and the effect of hydrogen bonding on
properties, including, boiling points, melting points, viscosity and solubility/miscibility in water, of these
molecules.
In the laboratory, hot copper(II) oxide or acidified dichromate(VI) solutions can be used to
oxidise primary and secondary alcohols:
Primary alcohols are oxidised, first to aldehydes and then to carboxylic acids.
Secondary alcohols are oxidised to ketones
Tertiary alcohols cannot be oxidised
When applied to carbon compounds, oxidation results in an increase in the oxygen to hydrogen
ratio and reduction results in a decrease in the oxygen to hydrogen ratio.
Introducing alcohol
This page explains what alcohols are, and what the
difference is between primary, secondary and tertiary
alcohols.
Naming alcohols, aldehydes and ketones
Information from BBC Bitesize on the rules for naming
organic compounds.
Alkanals
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Summary of Aldehyde
Nomenclature rules
1.Aldehydes take their name
from their parent alkane chains.
The -e is removed from the end
and is replaced with -al.
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Akanones
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Summary of Ketone Nomenclature rules
1) Ketones take their name from their
parent alkane chains. The ending -e is
removed and replaced with -one.
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Classification of alcohols
H
H
H
C
C
H
OH
CH3
H
H
C
CH3
O
H
H3C C
CH3
H
H
H
H
C
C
C
H
OH
H
propan-2-ol
O
H
CH3
Secondary alcohol,
TWO C’s joined to the
C bonded to
the OH group
Primary alcohol,
ONE C joined to the
C bonded to
the OH group
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Tertiary alcohol,
THREE C’s joined to
the C bonded to
the OH group
CH3
H
H3C
C
CH3
OH
2-methylpropan-2-ol
COPY
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Oxidation reactions
The most important oxidation reactions are oxidation of alcohols (alkanols)
and aldehydes (alkanals), using a variety of oxidising agents.
Oxidation just means joining with oxygen.
Complete combustion is an extreme oxidation reaction.
Example: complete combustion of methanol
2 CH3OH + 3 O2 → 2 CO2 + 4 H2O
Alcohols burn in oxygen to produce carbon dioxide and water.
In organic chemistry, oxidation can mean either adding oxygen or removing
hydrogen.
The oxidations to remember are:
1.primary alcohol →aldehyde →carboxylic acid;
2.secondary alcohol→ketone →no further oxidation;
3.tertiary alcohol →not oxidised.
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Oxidation of Alcohols
Primary alcohols can be oxidised by a number of oxidising agents, in two
stages, 1st Stage - Hydrogen is lost; 2nd Stage - oxygen is gained.
When applied to carbon compounds, oxidation results in an increase in
the oxygen to hydrogen ratio.
H
1st
R
C
O
oxidation
H
+
O
R
O
R
C
H
+
H2O
aldehyde
O H
2nd
C
H
aldehyde
+
O
oxidation
O
R
C
O
H
Carboxylic acid
Secondary alcohols can be oxidised to form ketones,
Tertiary alcohols do not undergo oxidation.
O
R
C
ketone
R1
Experiment
The oxidising agents
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It is important to remember the colour changes which occur in the reactions,
from starting to final colours.
Oxidising agents must always be reduced.
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Reaction sequence involving primary and secondary alcohols
H
hydration
H
C
C
C
H
oxiation
O
H
- 2H
H
H
propan-1-ol
H
H
H
H
+ H2O
H
H
C
C
H
H
C
C
H
H
H
H
O
alkanal
hydration
H
H
C
H
H
C
O
oxidation
H
C
H
H
H
H
propan-2-ol
secondary alcohol
- 2H
H
C
H
H
H
C
C
H
H
alkanoic acid
H
+ H2O
+O
H
propanoic acid
H
propene
oxiation
C
propanal
primary alcohol
C
H
H
C
C
O
propanone
alkanone
H
O
C
O
H
Higher Chemistry
Unit 2 – Nature’s Chemistry
Alcohols
Multiple Choice Questions
1. Which of the following structural formulae
represents a tertiary alcohol?
Answer
B.
2. Ethanol is prepared from glucose by the process of
A. respiration
B. fermentation
C. catalytic hydration
D. photosynthesis
Answer
B.
3. Which alkanol can, on dehydration, produce a pair
of isomeric alkenes?
A. propan-2-ol
B. pentan-3-ol
C. hexan-3-ol
D. heptan-4-ol
Answer
C.
4. Which product(s) would be expected upon dehydration
of the following alcohol?
A. 2-methylbut-2-ene only
B. 2-methylbut-1-ene and 2 methylbut-2-ene
C. 2-methylbut-1-ene only
D. 2-methylbut-1-ene and 3-methylbut-1-ene
Answer
B.
5. What type of reaction takes place when
propene is formed from propanol?
A. condensation
B. hydrolysis
C. dehydration
D. hydration
Answer
C.
6. When an alcohol is completely oxidised the
products are
A. carbon dioxide and hydrogen
B. carbon dioxide and water
C. carbon monoxide and hydrogen
D. carbon dioxide and water
Answer
B.
7. When acidified potassium dichromate is
used to oxidise an alcohol the colour
changes from
A. blue to red/orange
B. black to brown
C. green to orange
D. orange to green
Answer
D.
8. The type(s) of alcohol which can be oxidised
are
A. primary only
B. secondary only
C. tertiary only
D. none of the above answers
Answer
D.
9. 2-methyl hexan-1-ol and
3-methyl hexan-1-ol are
A. isomers which are both primary alcohols
B. isomers which are both primary alcohols
C. not isomers but both primary alcohols
D. not isomers but both secondary alcohols
Answer
A.
10.2-methyl pentan-2-ol
A. is a primary alcohol and can be oxidised
B. is a secondary alcohol and can be oxidised
C. is a tertiary alcohol and can be oxidised
D. is a tertiary alcohol and cannot be oxidised
Answer
D.