An important organic functional group that gives rise to two related groups of compounds is known as the carbonyl group. This involves a carbon atom double-bonded to an oxygen atom. The two groups of organic compounds that contain this group are called the aldehydes and the ketones.
The Carbonyl Functional Group
When a carbon has a double bond connecting it to an oxygen atom, it is able to make two further covalent bonds. The difference between an aldehyde and a ketone is the nature of these bonds. An aldehyde has at least one hydrogen atom bonded to the carbon, making it the end of an organic molecule. A ketone, on the other hand, has two hydrocarbon residues (chains of one or more carbons, with attached hydrogens), attached to the carbonyl group.
An aldehyde therefore can be represented as RCHO, where R represents a hydrocarbon residue (or hydrogen in the case of methanol). In the same way, a ketone can be represented as RCOR.
Properties and Reactions of Aldehydes
Aldehydes are named by taking the name of the hydrocarbon with the same number of carbon atoms and substituting the suffix –al, for the –e at the end of the name. So ethane becomes ethanol, and methane becomes methanal (more commonly known as formaldehyde).
Aldehydes have some polarity, but cannot form hydrogen bonds as a pure liquid, so they have boiling points which are higher than the comparable alkane, but lower than the comparable alcohol. They have strong odours, the smaller ones being rather unpleasant, but the larger, more complex ones being used in the manufacture of perfumes and fragrances.
Aldehydes are formed from the oxidation of primary alcohols, and further oxidation forms carboxylic acids. They are important in organic synthesis because they undergo a reaction called nucleophilic substitution, which, when undertaken with hydrogen cyanide, is a way of adding extra carbon atoms to an organic molecule.
Properties and Reactions of Ketones
Ketones are named by adding the suffix –one to the stem of the alkane with the same number of carbon atoms. So, the smallest ketone: H3C-CO-CH3 is called propanone, because it has three carbon atoms. When the molecules become large enough to have two possible locations for the carbonyl group, it is specified by adding the number of the carbon which has the oxygen bonded to it. Thus, pentan-2-one has the carbonyl on the second carbon, while pentan-3-one has the carbonyl on the third, central carbon.
Many ketones are found in nature as products or intermediates in plant or animal metabolism. Many have pleasant fragrances; both camphor and the ketone causing the musk aroma are ketones.
Aldehydes and ketones are a useful group of compounds, involved in organic synthesis reactions and present in many natural processes.
Sources:
John B Russell, “General Chemistry”, 1992, McGraw-Hill
Jim Clark, “An Introduction to Aldehydes and Ketones”, Chemguide.co.uk, Accessed 18th April 2010
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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