Ketones
| Ketones |
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| R 1 and R 2 are organic radicals (alkyl, aryl or the like). The carbonyl group is marked in blue . |
Ketones are chemical compounds that contain a non-terminal carbonyl group (> C = O) as a functional group . A ketone group [C – C (O) –C] contains three carbon atoms. All ketones contain at least three carbon atoms. In contrast to the aldehydes , here the carbonyl carbon is linked to carbon atoms in both directions. Ketones can be viewed as oxidation products of secondary alcohols . The ketones that can be derived from the alkanes are also called alkanones . Correspondingly, the ketones attributable to alkenes and alkynes are referred to as alkenones (see enones ) or alkynones . The simplest ketone is acetone . A simple aromatic ketone is benzophenone (diphenyl ketone ). A mixed ketone is acetophenone (methyl phenyl ketone ). Physiologically, certain ketones are produced as metabolic products in the liver.
nomenclature
According to the IUPAC nomenclature, ketones are given the suffix -one, provided that the carbonyl group is the group with the highest priority in the molecule . Accordingly, the ketone derived from propane is called propanone (CH 3 –CO – CH 3 , common name: acetone ). In the case of ketones with more than three carbon atoms, the carbonyl group is located in the main chain and is given the smallest possible number. This can either be in front of the name or, in the case of more complex molecules, directly in front of the ending -on. For example, states: CH 3 -CH 2 -CH 2 -CO-CH 3 , accordingly, 2-pentanone or pentane-2-one. If the carbonyl group does not have the highest priority, the prefix Oxo- is used. For example, CH 3 -CO-CH 2 -CHO 3 is called oxobutanal.
However, the ketones very often have names that consist of the designation of the alkyl radicals followed by the ending -ketone ( radical-functional nomenclature ). Thus butanone also called methyl ethyl ketone (MEK). If one of the substituents is a phenyl group , the ending -phenon is common, as is the case with acetophenone .
properties
Low molecular ketones are colorless, easily mobile liquids and, due to the polarity of the carbonyl group, are soluble in water. Low molecular weight ketones are characterized by a mostly pleasant, fruity odor . Higher molecular ketones are solid substances.
Due to the + I effect of the alkyl substituents on the carbonyl carbon atom, ketones have a lower reactivity than aldehydes and do not tend to polymerize . In contrast to the aldehydes, there is no reaction with ketones in the Fehling test , since the ketone group cannot be further oxidized while retaining the carbon structure.
Manufacturing
- Ketones can be made by oxidizing secondary alcohols .
- Another production method is the ozonolysis of alkenes.
- The most important manufacturing method for aromatic ketones is Friedel-Crafts acylation .
- According to the ketone synthesis by Gilman and van Ess , the analogous ketone is obtained from a carboxylic acid (R 1 +1) and a lithium alkyl compound (R 2 ) by hydrolysis
- Another manufacturing method is the reaction of Weinreb amides with alkyl lithium compounds and subsequent aqueous-acidic processing.
- The so-called lime salt distillation is a now historical synthesis , in which calcium salts of carboxylic acids are heated dry and the ketone released in gaseous form is recondensed (for example to produce acetone from calcium acetate ).
- Acetone is a by- product in the industrial phenol synthesis using the cumene hydroperoxide process (Hock process).
- Water addition to alkynes.
Reactions
Like aldehydes , ketones enter into addition and condensation reactions .
Ketones (R 1 , R 2 , R 3 = organyl radical) are in equilibrium with their tautomeric form , the enols ( keto-enol tautomerism ), if a hydrogen atom is bonded to the α-carbon atom:
To protect against undesirable reactions, ketones are converted with diols to form full acetals (see protection group ):
When the full acetal is reacted with water, the ketone is formed again under acid catalysis, i.e. H. the protective group is split off.
In contrast to aldehydes, ketones can only be attacked oxidatively under drastic reaction conditions. A bond between carbonyl carbon and a neighboring carbon is split. The resulting molecular fragments are carboxylic acids .
Important representatives of the ketones
use
- Acetone : solvents, detergents
- Cyclohexanone : Manufacture of Perlon
- Raspberry Ketone : smell of raspberries
Spectroscopy of ketones
In IR spectra of ketones and aldehydes , the characteristic band of the C = O stretching vibration is found in the range from 1690 to 1750 cm −1 .
See also
literature
- K. Peter C. Vollhardt, Neil E. Schore: Organic chemistry. 4th edition. Wiley-VCH, Weinheim 2005, ISBN 3-527-31380-X .
Web links
Individual evidence
- ↑ Joachim Buddrus: Fundamentals of Organic Chemistry , Walter de Gruyter Verlag, Berlin, 4th edition, 2011, p. 479, ISBN 978-3-11-024894-4 .