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Why aren't the aldehydes methanal/formaldehyde and ethanal/acetaldehyde also ketones? Ketones are defined by replacing any H - C - H in an alkane with C=O, which is true for both of these: methanal replaces two hydrogen atoms in methane with C=O, while ethanal does it with ethane.

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    $\begingroup$ They are not ketones because the carbonyl group is at a terminal position. In a ketone, both neighbouring atoms of the carbon in $\ce{C=O}$ need to be carbon atoms. In the aldehydes, one of the neighbours is a hydrogen atom. $\endgroup$ – Klaus-Dieter Warzecha Nov 4 '16 at 11:31
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    $\begingroup$ That definition of ketones is wrong! $\endgroup$ – orthocresol Nov 4 '16 at 12:09
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Aldehydes deserve a different name because they have different properties, for example being easily oxidized to acids, as in the Benedict test which distinguishes "reducing" sugars like glucose (an aldehyde) from non-reducing ones like fructose (a ketone). The relation between aldehydes and ketones is like the relation between alcohols and ethers, both of which can be described as R-O-R, but the properties are significantly different when one of the R's is a hydrogen.

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According to the Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995), ketones are defined as follows:

ketones:
Compounds in which a carbonyl group is bonded to two carbon atoms: $\ce{R2C=O}$ (neither $\ce{R}$ may be $\ce{H}$). NOC Rule C-311.1. (…)

The mentioned nomenclature of organic chemistry (NOC) Rule C-311.1 refers to the obsolete 1979 IUPAC recommendations:

311.1 – The generic name “ketone” is given to compounds containing an oxygen atom doubly bound to a single carbon atom with the carbonyl group $\ce{>C=O}$ joined to two carbon atoms. (…)

The corresponding rule in the also obsolete 1993 IUPAC recommendations reads:

R-5.6.2.1 Ketones. The generic term “ketone” refers to compounds containing a carbonyl group, $\ce{>C=O}$, joined to two carbon atoms. (…)

The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) refers to the definition of the 1995 glossary of class names:

P-64.1.1 Ketones are defined classically as compounds in which a carbonyl group is bonded to two carbon atoms: $\ce{R2CO}$ (neither $\ce{R}$ may be $\ce{H}$). (…)

All definitions include the criterion that the carbonyl group is bonded to two carbon atoms. Since in methanal (formaldehyde) as well as in ethanal (acetaldehyde) the carbonyl group is not bonded to two carbon atoms, these compounds are not ketones.

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Functional groups are identified based on their characteristic reactions. Aldehydes and ketones are structurally similar, and reactivity is certainly closely related for both groups. However, there are reactions where the outcome is very different for aldehydes than for ketones. Here is one of them:

Oxidation

Aldehydes react with a variety of oxidizing agents to form carboxylic acids (formaldehyde is often over-oxidized to carbon dioxide).

$$\ce{RC(O)H ->[\ce{CrO3}][\ce{H2SO4}] RCO2H}$$

Ketones, on the other hand, are hard to oxidize. Acetone (dimethyl ketone, 2-propanone) is sometimes used as a cosolvent in the chromic acid oxidation (above).

$$ \ce{RC(O)R ->[\ce{CrO3}][\ce{H2SO4}]} \text{no reaction} $$

That is not to say that ketones cannot be oxidized, but different reagents are needed:

$$\ce{RC(O)R ->[\ce{RCO3H}] RCO2R}$$

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