Part 4.
The chemistry of ALCOHOLS
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
Part 4.5
The controlled oxidation of alcohols with selected oxidising agents
Sub-index for this page
4.5.1
Reminders of the
structure of the sub-classes of aliphatic alcohols
4.5.2
Possible
oxidation sequences starting with an alcohol
4.5.3
Summary of oxidising agents and products from oxidised alcohols
4.5.4 Oxidation of primary alcohols to
aldehydes
4.5.5 Oxidation of primary alcohols to carboxylic acids
4.5.6
Oxidation of secondary alcohols to
ketones
4.5.7
Oxidation of tertiary alcohols
4.5.8
Oxidations of
alcohols with potassium manganate(VII) and industrial processes
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4.5.1 Reminders of the
structure of the sub-classes of aliphatic alcohols
Aliphatic alcohols
You need to know the structures of the sub-classes of alcohols - primary,
secondary and tertiary.
TOP OF PAGE and
sub-index
4.5.2
Possible oxidation sequences starting with an alcohol
Most alcohols can be readily oxidised
to aldehydes and ketones.
Aldehydes are easily oxidised further to carboxylic
acids.
The reagent is often potassium dichromate(VI) K2Cr2O7
, acidified with diluted sulphuric acid H2SO4(aq)
(colour change is orange to green).
However the oxidation products depend on the original structure of the alcohol.
The alcohol functional group -OH in aliphatic alcohols is classified into primary, secondary and tertiary types.
(When the -OH is attached directly to a benzene ring the molecule is called a phenol
and their oxidation is not included in this section).
Primary aliphatic alcohols R-OH, R is H or alkyl:
When oxidised they form aldehydes and further oxidation gives a relatively stable carboxylic acid e.g.
-
==> ==>
-
==> ==>
-
==>
==>
-
==> ==>
Secondary aliphatic alcohols
R-CH(OH)-R', R or R' are both alkyl (can be aryl):
When oxidised they form relatively stable ketones (see NOTE below) e.g.
-
==>
,
propan-2-ol ==> propanone
-
==>
,
butan-2-ol ==> butanone (butan-2-one)
-
==>
,
pentan-3-ol
==> pentan-3-one
Tertiary aliphatic alcohols RR'R"C-OH,
where R,R' or R" are all alkyl (or aryl):
These are relatively stable to oxidation (see NOTE
1. further down) e.g.
-
2-methylpropan-2-ol ,
or
2-methylbutan-2-o l,
or
3-methylpentan-3-o l,
or
Footnotes:
-
Ketones and
carboxylic acids are relatively stable to further oxidation because a strong C-C
bond must be broken in the process. Prolonged oxidation with H2SO4(aq)/K2Cr2O7
or using a more powerful oxidising agent, results in the formation of carbon
dioxide, water and carboxylic acids of shorter carbon chain length than the
original alcohol or ketone.
-
When a primary alcohol
is oxidised to an aldehyde, the oxidation to the carboxylic acid is rapid. If
the aldehyde formed first is the desired product, it must be immediately
distilled off to prevent further oxidation.
-
Important examination note:
TOP OF PAGE and
sub-index
4.5.3 Summary of some oxidising agents and the products
from oxidised alcohols
homologous
series change on oxidation |
molecular
structure change Apart from CH3OH
R = alkyl or aryl |
(a) heat
with mod conc. H2SO4 and K2Cr2O7(aq) (lab method) |
(b) reflux
with KMnO4/NaOH(aq) (lab method) |
(c) oxygen + catalyst or thermal decomposition (industrial
methods) |
primary
alcohol ==> aldehyde ==> carboxylic acid |
RCH2OH
==> RCHO ==> RCOOH |
YES, can separate intermediate aldehyde, or allow complete oxidation to
carboxylic acid |
YES but only get RCOOH and of little synthetic use |
e.g. CH3CH2OH ==> CH3CHO
(Cu/500oC) |
secondary alcohol ==> ketone |
R2CHOH ==> R2C=O |
YES |
YES but of little synthetic use |
(CH3)2CHOH ==>
CH3COCH3 (Cu/500oC) |
tertiary
alcohol ==> ? |
R3C–OH fairly stable
(if oxidised C–C bonds broken ==> lower RCOOH,
CO2, H2O products) |
not readily oxidised – no synthetic use |
not readily oxidised – no synthetic use |
not readily oxidised – no synthetic use |
I've included other oxidising agents
and industrial processes, but the potassium dichromate(VI)/sulfuric acid
reagent is the most important to know for school chemistry and their
application is described in the experiments in the next few sections.
TOP OF PAGE and
sub-index
4.5.4 Oxidation of primary alcohols to
aldehydes
It is
possible using the same reagent of aqueous sodium/potassium
dichromate(VI)–sulphuric acid to oxidise a primary alcohol to either the
aldehyde, or the carboxylic acid, depending on the reaction conditions.
In order to selectively isolate the
aldehyde this initial oxidation
products must be removed from the reaction mixture as quickly as possible,
otherwise oxidation proceeds to the carboxylic acid (diagram
PD1 below).
The 25% sulphuric acid is placed in
the flask and gently simmered. The alcohol and aqueous sodium/potassium
dichromate(VI) solution is dripped onto the hot acid. Immediately, the
orange dichromate(VI) is reduced by the alcohol to the green
chromium(III) ion and the alcohol is oxidised to the aldehyde or ketone.
The diagram shows a
bunsen burner being used to supply the heat ('my days'), these days its
more likely, and safer, to use an electrical heater that the round
bottomed flask fits in snugly.
A spot of theory to
explain the separation of the aldehyde/ketone from the reaction mixture.
For the same carbon
number, the boiling point of the polar aldehyde/ketone (δ+C=Oδ–, but no H
bonding) is lower than the original more polar alcohol (δ–O–Hδ+,
hydrogen bonding in the alcohol) whose bpt. is higher. Therefore, as long as the bpt. of the
aldehyde/ketone is not too high, in the set–up shown above, the aldehyde rapidly
distils over and condenses in
the collection tube/flask with some water.
This rapid in situ
extraction ensures that most of the aldehyde (or ketone), 9.1(a) is not oxidised further.
If the carboxylic acid of the same carbon number
is required from a primary alcohol, the mixture is refluxed using the set–up
illustrated in diagram PD5.
(i) primary alcohol
==> aldehyde
Cr2O72–(aq)
+ 3RCH2OH(aq) + 8H+(aq) ===>
3RCHO(aq) + 2Cr3+(aq) + 7H2O(l)
reduction half reaction:
Cr2O72–(aq) + 14H+(aq)
+ 6e– ===> 2Cr3+(aq) + 7H2O(l)
oxidation half reaction:
RCH2OH(aq) ===> RCHO(aq) + 2H+(aq) + 2e–(aq) (R = alkyl or aryl)
Examples using
simplified symbol equations e.g.
ethanol ==> ethanal:
CH3CH2OH + [O] ==>
CH3CHO + H2O
propan–1–ol
(1–propanol, n–propyl alcohol, n–propanol) ==> propanal
CH3CH2CH2OH
+ [O] ==> CH3CH2CHO + H2O
TOP OF PAGE and
sub-index
4.5.5 Oxidation of primary alcohols to carboxylic acids
The technique illustrated on the right
(diagram PD5) is called heating under
reflux, a method which enables a reaction to be carried out at a higher
temperature than room temperature to speed up the reaction AND retain the
solvent (reaction medium e.g. water) and any volatile reactant or product
(e.g. an alcohol/aldehyde/ketone). As the mixture boils, the vapours of the
solvent or volatile reactant/product are condensed back into the flask in
the vertical condenser, so any volatile reactant is used up and no volatile
product lost (at least at this stage in a preparation!).
Following on from 4.5.4 where the primary
alcohol is oxidised to the aldehyde, the procedure is repeated under
reflux conditions with excess of the potassium dichromate(VI)/sulfuric acid
mixture.
The further oxidation is ...
(ii) aldehyde ==>
carboxylic acid
Cr2O72–(aq)
+ 3RCHO(aq) + 8H+(aq) ==> 3RCOOH(aq)
+ 2Cr3+(aq) + 4H2O(l) (R =
alkyl or aryl)
oxidation half–reaction:
RCHO(aq) + H2O(l) ==> RCOOH(aq)
+ 2H+(aq) + 2e–(aq)
Examples using
simplified symbol equations:
ethanal
==> ethanoic acid, CH3CHO + [O] ==>
CH3COOH
propanal
(propionaldehyde) ==> propanoic acid (propionic
acid)
CH3CH2CHO
+ [O] ==> CH3CH2COOH
butanal
(butyraldehyde) ==> butanoic acid (butyric acid)
CH3CH2CH2CHO
+ [O] ==> CH3CH2CH2COOH
so overall for reflux
conditions (i) + (ii) gives
(iii) primary alcohol
==> carboxylic acid
2Cr2O72–(aq)
+ 3RCH2OH(aq) + 16H+(aq) ==>
3RCOOH(aq) + 4Cr3+(aq) + 11H2O(l)
oxi'n half–reaction:
RCH2OH(aq) + H2O(l)
==>
RCOOH(aq) + 4H+(aq) + 4e–(aq)
(R = alkyl or aryl)
Examples using
simplified symbol equations:
ethanol ==> ethanoic acid
CH3CH2OH
+ 2[O] ==> CH3COOH + H2O
propan–1–ol
(1–propanol, n–propyl alcohol, n–propanol) ==> propanoic
acid
CH3CH2CH2OH
+ 2[O] ==> CH3CH2COOH + H2O
TOP OF PAGE and
sub-index
4.5.6 Oxidation of secondary alcohols to
ketones
In the case of secondary
alcohols you only get the ketone if you distil the product off immediately
as shown in diagram PD1 above.
You do NOT reflux the alcohol/K2Cr2O7/H2SO4(aq) for a long time, in
case the ketone is oxidised to lower carbon number carboxylic acids,
carbon dioxide and water etc. if the carbon chain is broken
However, ketones are quite stable
to further oxidation due to the strong carbon–carbon (C–C) bonds that have to be broken.
So, as with the aldehyde, you can distil
off the ketone, having a lower boiling point than the parent secondary
alcohol.
To be on the safe side
it is better to make the ketone under the
same restricted reaction conditions used to produce the aldehyde (details
above with diagram PD1).
Cr2O72–(aq)
+ 3R2CHOH(aq) + 8H+(aq) ==>
3R2C=O(aq) + 2Cr3+(aq) + 7H2O(l)
oxidation half–reaction:
R2CHOH(aq) ==> R2C=O(aq)
+ 2H+(aq) + 2e–(aq) (R = alkyl or aryl)
TOP OF PAGE and
sub-index
4.5.7 Oxidation of tertiary alcohols
Tertiary alcohols, R3COH (R = alkyl or aryl), are
not readily oxidised because strong carbon–carbon
bonds have to be broken.
If a tertiary alcohol is refluxed with a
powerful oxidising agent, they will break down to give lower chain carboxylic acids,
carbon dioxide and water and therefore of no synthetic use.
TOP OF PAGE and
sub-index
4.5.8
Oxidations of alcohols with potassium manganate(VII) and industrial
processes
School laboratory preparation
Heating a
primary alcohol with a aqueous sodium hydroxide and potassium manganate(VII)
mixture under reflux (diagram PD2) will
give the sodium salt of the carboxylic acid and it is not possible to
isolate the intermediate aldehyde.
However, the acid/dichromate(VI) method
9.1(a) under reflux is better, and the carboxylic acid is less liable to further
degradative oxidation. The complex reaction can be summarised as:
RCH2OH(aq) +
NaOH(aq) + 2[O] ==>
RCOO–Na+(aq) + 2H2O(l)
(R = alkyl or aryl)
After removing the
excess KMnO4/MnO2 the weak acid is freed from its
sodium salt by adding strong dilute hydrochloric acid.
RCOO–(aq) + H+(aq)
==> RCOOH(aq/s)
Oxidation of secondary alcohols
Ketones
are produced by refluxing secondary alcohols with NaOH/KMnO4(aq),
but further oxidation is likely to take place because this reagent is a
stronger oxidising agent than acidified potassium dichromate(VI).
(CH3)2CHOH + [O]
==>
(CH3)2C=O + H2O
(propan–2–ol ==>
propanone)
Industrial oxidation processes
Many
chemical feedstocks are oxidised directly with molecular
oxygen/transition metal catalyst to produce useful products in industry.
e.g. oxidation of primary alcohols
2CH3OH + O2 ==>
2HCHO + 2H2O
(Ag/500oC, methanol ==> methanal)
or 2CH3CH2OH + O2
==>
2CH3CHO + 2H2O
(Ag/500oC, ethanol ==> ethanal)
and the latter reaction
can also be achieved via a thermal decomposition using a different catalyst,
e.g. CH3CH2OH ==>
CH3CHO + H2 (Cu/500oC),
which is still an
oxidation, right carbon (–1) to (+1) and 2 x hydrogen (+1) to (0).
e.g. oxidation of secondary
alcohols
2(CH3)2CHOH(g) + O2(g)
==>
2(CH3)2C=O(g) + 2H2O(g)
(Ag/500oC)
and this reaction can
also be achieved via a thermal decomposition using a different catalyst.
(CH3)2CHOH(g)
==>
(CH3)2C=O(g) + H2(g) (Cu/500oC)
which is still an
oxidation, right carbon (–1) to (+1) and 2 x hydrogen (+1) to (0).
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INDEX of notes on ALCOHOLS
chemistry
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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