Aldehydes and Ketones
Aldehydes and ketones have the
same functional group. Its name is carbonyl group. This group is formed by
oxygen atom attached with a double bond to carbon atom. Compounds which include
this <C=O functional group are called
as carbonyl compounds. Their general formula is CnH2nO. In aldehydes, the car-
bon of carbonyl is bonded to a hydrogen and an alkyl group, whereas in ketones,
carbon of carbonyl group is bonded to the same or different two alkyl groups.
Nomenclature of Aldehydes and Ketones
Aldehydes and ketones are named
according to IUPAC nomenclature system as in the following steps:
A) Nomenclature of Aldehydes
1. The longest carbon chain is chosen. The carbon atom of
carbon group is numbered with 1.
2. -al suffix is added to the end of the name of the corresponding alkane.
3. Alkyl groups are mentioned with their numbers.
As numbering starts from the side
of carbonyl group, the number of carbonyl group doesn’t need to be mentioned.
For example:
B) Nomenclature of Ketones
We apply the steps in nomenclature
of aldehydes.
1. We choose the longest carbon chain and start numbering
from the closest carbon to the carbonyl group.
2. We add –one suffix to the end of the corresponding alkane.
3. If there is branching, alkyl groups are mentioned with
their numbers.
Preparation of Aldehydes and Ketones
Aldehydes and ketones are
synthesized through many methods in industry and laboratories. Here, we will
explain one of the laboratory methods.
Oxidation of Alcohols
Aldehydes and ketones are prepared
by oxidation of primary (1°) and secondary (2°) alcohols with acidic solution
of potassium dichromate (K2Cr2O7 ) or potas- sium permanganate (KMnO4 ).
1- Primary alcohols form aldehydes
as shown in the following equation.
The aldehyde remaining in the
reaction mixture above is readily oxidized and it forms carboxylic acid.
Therefore, oxidation reaction must
be controlled.
2- Ketones are prepared through
oxidation of secondary (20) alcohols.
3- Tertiary alcohols cannot be
oxidized.
Example 1
Write down oxidation products of
the following compounds.
1. -1propanol
2. -2pentanol
Solution:
Physical Properties of Aldehydes and Ketones
All aldehydes and ketones are in
liquid form at room temperature except mathanal (formaldehyde, in gas form).
Although ketones have pleasant
odors, aldehydes have unplaeasant odors. Densities of ketones are lower than
that of water. Owing to the polar property of carbonyl group, these compounds
are accepted as polarized. They can dis- solve in water and organic solvents
like ether. Boiling points of aldehydes and ketones are higher than those of
alkanes, but lower than alcohols which have the same molecular weight with
them. For example:
Chemical Properties of Aldehydes and Ketones
The carbonyl group in aldehydes
and ketones has high ionization property as shown below:
Carbon in the carbonyl group is
positive charged, as nucleophile approaches it, electrophile approaches to
negative charged oxygen.
Aldehydes and ketones can undergo
two-step nucleophilic reactions. These are:
A) 1st step: Nucleophile approaches carbon of carbonyl group and forms a new bond.
This bond breaks double bond between oxygen and carbon and pushes a pair of
electrons to oxygen atom.
B) 2nd step: approach of an electrophile such as H+ ion:
a) Reduction with hydrogen
Aldehydes are reduced to primary
alcohols and ketones are reduced to second- ary alcohols when they react with
H2 with Ni and Pt as catalysts.
b) Reduction to Alkanes
Aldehydes and ketones are reduced
to alkanes via Clemmensen reduction method. In this method, solution of zinc
and mercury in hydrochloric acid is used as catalyst.
c) Reaction with Hydrazine
Aldehydes and ketones react with
hydrazine (H2N-NH2) and produce hydrazone which is known as Schiff base.
This reaction is used to examine
the presence of carbonyl group in aldehydes and ketones. Hydrazone (orange or
yellow color) shows the presence of the carbonyl group.
d) Oxidation
Presence of hydrogen bonded to
carbonyl group in aldehydes causes some re- actions different from those of
ketones. The most important reactions of aldehydes and ketones are oxidation
reactions.
Aldehydes are oxidized into
carboxylic acids by the help of some oxidizing agents. But in ketones, this
does not occur. We can differentiate aldehydes and ketones using following
reactions:
1-Tollens’ Reagent
In order to differentiate
aldehydes, silver ammonium hydroxide is used. In this reaction, silver ion is reduced
in solution and silver metal covers the tube’s sur- face as mirror showing the
presence of aldehyde. This reaction is shown below:
C) Reaction with Fehling’s
Solution
Fehling’s solution is a solution
of copper sulfate salt. It’s basic and dark blue. It is used in oxidation of
aldehydes. In this reaction, copper (II) ions transforms into red copper (I)
oxide. This red copper (I) oxide precipitate shows presence of aldehyde in
mixture. As shown below, ketones do not react with this reagent.
See also
1. Alcohols: nomenclature – synthesis - Properties
2. Alkyl halides: nomenclature - synthesis - Properties
References
1. K. J. Denniston c J. J.Topping c and R. L.Caretc
“General Organic and Biochemistry”c Mc-Graw- Hillc New York
(2004).
2. K.W. Whittenc R.E. Davis and L. M. Peckc “General Chemistry” 7th ed. Holt
Rinehart and Winstonc New York (2010).
3. Clayden, J.; Greeves, N. and Warren, S. (2012) Organic
Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.
4. Streitwieser, Andrew; Heathcock, Clayton H.; Kosower,
Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4:
Medtech (Scientific International, reprint of revised 4th edition, Macmillan,
1998). ISBN 978-93-85998-89-8.
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