Ketones
Ketones
Ketones have the general formula CnH2nO
and it contain the oxo (carbonyl) group. In Ketones both available valencies
are attached to carbon atoms and so the keto group occurs within a chain (cf.
ketens).
Nomenclature
The lower members are commonly named according to the alkyl
groups attached to the keto group, e.g.,
CH3COCH3 dimethyl ketone
CH3CH2COCH(CH3)2 ethyl isopropyl ketone
The positions of side-chains or substituents are indicated
by Greek letters, the α-carbon atom being the one adjacent to the keto group,
e.g.,
CH3CHClCOCH2CH2Cl α,β’-dichlorodiethyl ketone
If the two alkyl groups in a ketone are the same, the ketone
is said to be simple or symmetrical; if unlike, mixed or unsymmetrical (cf.
ethers).
According to I.U.P.A.C. system of nomenclature, ketones are
designated by the suffix –one, which is added to thename of the hydrocarbons
from which they are derived. The longest carbon chain containing the keto group
is chosen as the parent hydrocarbon; the positions of side-chain or
substituents are indicated by numbers, and the keto group is given the lowest number
possible. e.g.,
CH3COCH2CH2CH3 pentan-2-one
(CH3)2CHCOCH(CH3)CH2CH3 2,4-dimethylhexan-3-one
Since aldehydes and ketones both contain the carbonyl group,
it might be expected that they would resemble one another. It is therefore instructive
to compare their general methods of preparation and their general properties.
General methods of preparation of ketones
1. By the oxidation or dehydrogenation of a secondary
alcohol.
2. By heating the calcium salt of any monocarboxylic acid
other than formic acid.
(RCO2)2Ca ⟶ R2CO + CaCO3
3. By passing the vapour of any monocarboxylic acid other
than formic acid over manganous oxide at 300oC:
2RCO2H (MnO)⟶ R2CO + CO2 + H2O
A mixture of
monocarboxylic acids gives mixed ketones:
R1CO2H + R2CO2H ⟶ R1COR2
+ CO2 + H2O
R12CO and R22CO
are obtained as by-products.
4. By the ozonolysis or oxidation with the Lemieux reagent of
alkenes of the type R12C=CR22.
General properties of ketones
Dipole moment measurements
of ketones has shown that the values are larger than can be accounted for by
the inductive effect of the oxygen atom, but can be accounted for if carbonyl
compounds are resonance hybrids:
Thus the carbon atom has a positive charge and consequently
can be attached by nucleophilic reagents. The carbonyl group also exhibits
basic properties; it is readily protonated by strong acids to form oxonium
salts, since oxygen is more electronegative than carbon, the second resonating
structure will make a larger contribution than the first.
Hence, protonation increases the electrophilic character of
the carbonyl group and so it can be expected that nucleophilic additions will
be catalysed by acids. It should also be noted that, because of the positive
charge on the carbon atom, the CO group has a strong –I effect. Many addition
reactions of carbonyl compounds may be represented by the general equation.
Reactions given by Ketones
1. Ketones do not readily form ketals when treated with
alcohols in the presence of hydrogen chloride (cf. acetals). Ketals may,
however, be prepared by treating the ketone with ethyl orthoformate :
R2CO
+ HC(OC2H5)3 ⟶ R2C(OC2H5)2 +
HCO2C2H5
2. Ketones condense with chloroform in the presence of
potassium hydroxide to form chlorohydroxy-compounds:
3. Ketones form sodi-derivatives when treated with
sodium or sodamide in ethereal solution e.g.,
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