Download 1.1 10 Oxidation of alcohols and aldehydes

1.1
10
Oxidation of alcohols and aldehydes
By the end of this spread, you should be able to …
1 Describe the oxidation of primary alcohols to form aldehydes and carboxylic acids.
1 Describe the oxidation of secondary alcohols to form ketones.
1 Describe the oxidation of aldehydes to form carboxylic acids.
Key definition
A redox reaction is one in which both
reduction and oxidation take place.
Oxidation of alcohols
You will recall from your AS chemistry studies that primary and secondary alcohols can
be oxidised using an oxidising agent.
s !SUITABLEOXIDISINGAGENTISASOLUTIONCONTAININGACIDIlEDDICHROMATEIONS
H+/Cr2O72−.
s 4HEOXIDISINGMIXTURECANBEMADEFROMPOTASSIUMDICHROMATE+2Cr2O7, and sulfuric
acid, H2SO.
During the reaction, the acidified potassium dichromate changes from orange to green.
Remember that tertiary alcohols are not oxidised by acidified dichromate ions.
Primary alcohols
Primary alcohols can be oxidised to aldehydes and to carboxylic acids (see AS Chemistry
spread 2.2.3).
H
H
H
C
C
H
H
H
Oxidation
OH
H
C
Oxidation
C
H
Primary alcohol
H
O
H
C
H
O
C
H
Aldehyde
OH
Carboxylic acid
Figure 1 Ethanol oxidised to ethanal, and finally to ethanoic acid
The equation for the oxidation of ethanol to the aldehyde ethanal is shown below. Note
that the oxidising agent is shown as [O] – this simplifies the equation.
CH3CH2OH + [O] }m CH3CHO + H2O
When preparing aldehydes in the laboratory, you will need to distil the aldehyde from the
reaction mixture as it is formed. This prevents the aldehyde from being oxidised further to
a carboxylic acid.
Key definition
Reflux is the continual boiling and
condensing of a reaction mixture to
ensure that the reaction takes place
without the contents of the flask boiling
dry.
When making the carboxylic acid, the reaction mixture is usually heated under reflux
before distilling the product off.
CH3CH2OH + 2[O] }m CH3COOH + H2O
Secondary alcohols
Secondary alcohols can be oxidised to ketones (see AS Chemistry spread 2.2.3). Unlike
aldehydes, ketones are not oxidised further.
H
H
OH
H
C
C
C
H
Oxidation
H
H
H
H
Secondary alcohol
H
O
H
C
C
C
H
H
H
Ketone
Figure 2 Propan-2-ol can be oxidised to propanone
The equation for the oxidation of propan-2-ol to the ketone propanone is:
CH3CHOHCH3 + [O] }m CH3COCH3 + H2O
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Module 1
Rings, acids and amines
Oxidation of alcohols and aldehydes
Oxidising aldehydes
You learnt in AS chemistry that aldehydes can be oxidised further to carboxylic acids.
Acidified potassium dichromate(VI) is a suitable oxidising agent, and the reaction is
normally carried out under reflux. The orange potassium dichromate(VI) changes colour,
becoming green.
H
H
CH3
C
H
O
C
C
H
Aldehyde
+ [O]
Oxidised
H
Signifies use of an
oxidising agent
H
H
CH3
C
C
O
C
H
H
Carboxylic acid
OH
Figure 3 Oxidation of an aldehyde to a carboxylic acid
STRETCH and CHALLENGE
Acidified potassium dichromate(VI) is an oxidising agent that oxidises primary alcohols,
secondary alcohols and aldehydes. During oxidation, dichromate(VI) ions are reduced
and the colour changes from orange to green.
You can write a fully balanced equation for the reaction by combining the oxidation
half-equation for the organic compound with the reduction half equation for the
dichromate(IV). The example below shows oxidation of the aldehyde ethanal.
CH3CHO(l) + H2O(l) }m CH3COOH(aq) + 2H+(aq) + 2e–
Cr2O72–(aq) + 14H+(aq) + 6e– }m 2Cr3+(aq) + 7H2O(l)
oxidation
reduction
The two equations can be combined to give an overall equation. To do this, the first
equation is multiplied by 3 to make the number of electrons in each equation the same,
and then the two equations are added together. The overall equation becomes
2–
+
3+
3CH3CHO(l) + Cr2O7 (aq) + 8H (aq) }m 3CH3COOH(aq) + 2Cr (aq) + 4H2O(l)
Figure 4 Potassium dichromate(VI) is
reduced in its reactions with aldehydes
and changes colour from orange to green
You will learn more about writing equations for redox reactions in spread 2.2.9.
‘Waiter, my wine is sour’
When a bottle of wine is opened, oxygen enters the bottle and an oxidation process
begins. Over time, the taste and colour of the wine change, and often a brownish
tint can be seen when the glass containing the wine is held against a white background.
This occurs because compounds in the wine called
tannins are oxidised when exposed to air. This oxidation
produces small quantities of hydrogen peroxide, which
then oxidises ethanol to ethanal. Ethanal produces the
oxidised odour and masks the
natural fragrance of a wine.
After a bottle of wine has
been opened, ethanal can
form within as little as two
hours. White wines and older
reds are particularly
susceptible to oxidation.
Figure 5 Wine tasting
Figure 6 Wine tasters smell
the wine before tasting in
order to sample the bouquet
Questions
1 (a) Draw the displayed formulae
of a primary alcohol, a
secondary alcohol and a
tertiary alcohol that would be
isomeric with pentan-1-ol.
(b) What products, if any, would
be formed if the three
alcohols in part (a) were
oxidised? Name each
organic product.
(c) Name a suitable oxidising
agent and the reaction
conditions for the oxidation.
2 What precautions would you
need to take when oxidising a
solution of propan-1-ol if you
were hoping to make propanal?
3 In an oxidation equation, what
does the symbol [O] represent?
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