Introduction to Ketones

James Richard Fromm


Ketones are organic compounds which incorporate a carbonyl functional group, C=O.  The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl substituents.  If at least one of these substituents is hydrogen, the compound is an aldehyde.  If neither is hydrogen, the compound is a ketone.   The general formula for ketones is R2C=O where the R- group represents any alkyl.  Examples:

CH3-

Methyl

CH3CH2-

Ethyl

CH3CH2CH2-

Propyl

CH3CH2CH2CH2-

Butyl

The ketones can be named systematically from the corresponding alkanes by changing the ending of each alkane to -yl followed by the word ketone after both alkyl groups are named, as in the form methyl ethyl ketone for CH3COCH2CH3.  

The common name for a ketone may be listed by molecular weight.  Methyl is chosen first because it has a lower molecular weight than ethyl.

   CH3
   |
   C=O
   |
   CH2CH3
Methyl Ethyl Ketone

The common name may also be listed in alphabetical order

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with respect to the two alkyl groups on the C=O double bond.

If both of the alkyl groups are the same the numerical prefix di- is used, as in the case of dimethylketone.  Dimethyl ketone is more commonly known by the trivial name acetone, and finds significant use as a solvent. 

   CH3
   |
   C=O
   |
   CH3
Dimethyl Ketone
   CH3
   |
   C=O
   |
   CH2CH3
Methyl Ethyl Ketone
   CH2CH3
   |
   C=O
   |
   CH2CH3
Diethyl Ketone
   CH3
   |
   C=O
   |
   CH2CH2CH3
Methyl Propyl Ketone
   CH2CH3
   |
   C=O
   |
   CH2CH2CH3
Ethyl Propyl Ketone

For all except the simplest ketones, this form is not used and the ketone group is indicated systematically by use of the IUPAC system of nomenclature which assigns the suffix -one to ketones.  Further, the parent chain includes the carbonyl group and is numbered so that the carbonyl location is the lowest number. The number of the location must be part of the name whenever there would be an uncertainty.

Dimethyl ketone would be named 2-propanone, while methyl ethyl ketone would be named as a substituted butane, 2-butanone.  However, in the case of both 2-propanone and 2-butanone the number is not necessary since these ketones cannota be arranged in any other order - There is no uncertainty.

   CH3
   |
   C=O
   |
   CH3
Propanone
   CH3
   |
   C=O
   |
   CH2CH3
Butanone
   CH3
   |
   C=O
   |
   CH2CH2CH3
2-Pentanone
   CH2CH3
   |
   C=O
   |
   CH2CH3
3-Pentanone
   CH2CH3
   |
   C=O
   |
   CH2CH2CH3
3-Hexanone
   CH3
   |
CH3CH
   |
   C=O
   |
   CH2CH2CH3
2-Methyl-3-Hexanone

Notice: Propanone, Dimethyl Ketone and Acetone are Identical

A ketone carbonyl function may be located anywhere within a chain or ring, and its position is given by a locator number. Chain numbering normally starts from the end nearest the carbonyl group. In cyclic ketones the carbonyl group is assigned position #1, and this number is not cited in the name, unless more than one carbonyl group is present.

When the suffix form cannot be used, perhaps because another suffix such as that of a carboxylic acid is required, the alternative designation -keto can be used for the ketone group. An example is 2-ketopropanoic acid.

   CH3
   |
   C=O
   |
   C=O
   |
   OH

2-Ketopropanoic Acid

A variety of oxidizing agents can be used to transform a secondary alcohol to a ketone. A common reagent for this reaction is some form of chromium (VI)--chromium in the +6 oxidation state -- in acidic solution. This reagent can be prepared by adding a salt of the chromate (CrO42-) or dichromate (Cr2O72-) ions to sulfuric acid. Or it can be made by adding chromium trioxide (CrO3) to sulfuric acid. Regardless of how it is prepared, the oxidizing agent in these reactions is chromic acid, H2CrO4.

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Copyright 1997 James R. Fromm