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Part 5.
The chemistry of
ALDEHYDES and KETONES
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 methods of synthesising aldehydes or
ketones reagents apparatus procedures
Part 5.3
Methods of preparing aldehydes and ketones
Sub-index for this page
5.3.1
The structure and classification of alcohols
(reminders)
5.3.2
Introduction to the oxidation of alcohols
5.3.3
Oxidation of aliphatic primary alcohols to
aliphatic aldehydes
5.3.4
The oxidation of secondary alcohols to
ketones
5.3.5 The
preparation of aromatic ketones
INDEX of ALDEHYDES
and KETONES revision notes
All Advanced A Level Organic
Chemistry Notes
[SEARCH
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5.3.1
The structure and classification of alcohols (reminders)
Aliphatic alcohols
You need to know the structures of the sub-classes of alcohols - primary,
secondary and tertiary,
Structure reminder
and the relative structures of aldehydes and ketones.
Note: An aryl group means a benzene
ring C6H5- or a substituted benzene ring e.g. CH3C6H5-.
TOP OF PAGE
and sub-index
5.3.2
Introduction to the oxidation of alcohols
Primary aliphatic alcohols R-OH, R is H or alkyl:
When oxidised they form aldehydes e.g.
-
 ==>
, ethanol
==> ethanal -
 ==>
, 2-methylpropan-1-ol
==> 2-methylpropanal -
 ==>
,butan-1-ol ==> butanal -
 ==>
, pentan-1-ol ==> pentanal
Secondary aliphatic alcohols
R-CH(OH)-R', R or R' are both alkyl (can be aryl):
When oxidised they form relatively stable ketones e.g.
-
 ==>
 ,
propan-2-ol ==> propanone
-
 ==>
 ,
butan-2-ol ==> butanone
(butan-2-one)
-
 ==>
 ,
pentan-3-ol
==> pentan-3-one
TOP OF PAGE
and sub-index
5.3.3 Oxidation of aliphatic primary alcohols to
aliphatic aldehydes
It is
possible using the same reagent of aqueous sodium/potassium
dichromate(VI)–sulphuric acid to oxidise a primary alcohol to an
aldehyde.
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 (just heating up to ~60oC with an electric
heater is ok.
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.
After the initial heating, the
exothermic reaction can maintain the required temperature.
Note the oxidation state colour
change of the orange dichromate(VI) ion to the green chromium(III)
ion.
Ideally the temperature should be
higher than the boiling point of the aldehyde and lower than the boiling
point of the alcohol - so the product, not the reactant, will
preferentially distil over into the condenser and collection flask AND
before it can be further oxidised to the corresponding carboxylic acid.
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, is not oxidised further to a
carboxylic acid, especially as aldehydes are more easily oxidised than
the original primary alcohol.
RCH2OH ===>
RCHO ===> RCOOH
For ethanal, the boiling point is so
low, you need to cool the collection flask with ice or very cold water.
primary alcohol
==> aldehyde (e.g. preparation of ethanal and propanal)
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 ==> propanal:
CH3CH2CH2OH + [O]
===> CH3CH2CHO + H2O
butan–1–ol ==> butanal:
CH3CH2CH2CH2OH + [O]
===> CH3CH2CH2CHO + H2O
TOP OF PAGE
and sub-index
5.3.4 The 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 can reflux first the alcohol/K2Cr2O7/H2SO4(aq) for a
short time (see diagrams at the end), but not too long, in
case the ketone is oxidised to a 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.
(Its the same intermolecular forces as
for the aldehyde)
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).
primary alcohol
==> aldehyde (e.g. preparation of propanone and butanone)
reduction half reaction: Cr2O72–(aq) + 14H+(aq)
+ 6e– ===> 2Cr3+(aq) + 7H2O(l)
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)
Examples using
simplified symbol equations e.g.
propan-2-ol ==> propanone:
CH3CH(OH)CH3 + [O]
===> CH3COCH3 + H2O
butan–2–ol ==> butanone:
CH3CH(OH)CH2CH3
+ [O] ===> CH3COCH2CH3 + H2O
A refluxing system (left diagram)
can be used if the reaction is slow.
After completion, fit the still head and
thermometer, rearrange the condenser and fractionally distil off the ketone
(right
diagram).
The aldehyde can also be fractionally
distilled to get a more purer sample.
For ethanal, the boiling point is so low,
you need to cool the collection flask with ice or very cold water.
INDEX of ALDEHYDE
and KETONE revision notes
All Advanced Organic
Chemistry Notes
TOP OF PAGE
and sub-index
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+
===>
+ HCl
===>
+ HCl
