Introduction to Aldehydes

James Richard Fromm


The common names of aldehydes are derived from the names of the corresponding carboxylic acids.

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The systematic names for aldehydes are obtained by adding -al to the name of the parent alkane.

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The presence of substituents is indicated by numbering the parent alkane chain from the end of the molecule that carries the em.gif (834 bytes)CHO functional group. For example,

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Another method of arriving at the same systematic way of naming aldehydes is from the corresponding alkanes, by changing the ending -ane to -al or to the longer form -yl aldehyde. Methane thus gives rise to methanal, CH2O, which is more widely known by its trivial name formaldehyde, while ethane gives rise to ethanal, CH3CH2O, which is more widely known by its trivial name acetaldehyde. The remaining aldehydes are generally named systematically although some older forms still prevail; propanal is sometimes still referred to as propionaldehyde. Aldehydes can be reduced to alcohols or oxidized to carboxylic acids.  The parent chain is the longest chain that includes the aldehyde group.  It's name is made by replacing the ending -e of parent alkane name with -al. The numbering of the chain always starts with the carbon of the aldehyde group being 1. For example:

   H
    |
   C=O
    |
   H
Methanal
  CH3
   |
  C=O
   |
  H
Ethanal
CH3-CH2
          |
         C=O
          |
         H
Propanal
            CH3
             |
   CH3-CH
             |
            C=O
             |
            H
2-Methylpropanal

In all cases the aldehyde function has a higher status than either an alcohol, alkene or ketone and provides the nomenclature suffix. The other functional groups are treated as substituents.  In other words, the -al ending takes precedence over -ol, -ene, or -one.  In those events when the -CHO functional group is not part of the parent chain it is generally referred to as an aldo functional group.

        CHO
          |
    H-C-CHO
            |
       CHO

2-Ketopropanedial

Oxidation of Aldehydes

Aldehydes are the most easily oxidized of all organic compounds, though ketones resist oxidation strongly. They can also be slowly oxidized by oxygen in the air. In this example, potassium dichromate, together with hydrogen sulfate (sulfuric acid), easily oxidizes ethanal to ethanoic acid, chromium sulfate, potassium sulfate and water.

3CH3CHO + K2Cr2O7 + 4H2SO4 arrow2.gif (113 bytes) 3CH3COOH + Cr2(SO4)3 + K2SO4 + 4H2O

The addition of hydrogen across a C=O double bond raises several important points. First, and perhaps foremost, it shows the connection between the chemistry of primary alcohols and aldehydes. But it also helps us understand the origin of the term aldehyde. If a reduction reaction in which H2 is added across a double bond is an example of a hydrogenation reaction, then an oxidation reaction in which an H2 molecule is removed to form a double bond might be called dehydrogenation. Thus, using the symbol [O] to represent an oxidizing agent, we see that the product of the oxidation of a primary alcohol is literally an "al-dehyd" or aldehyde. It is an alcohol that has been dehydrogenated.

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The choice of oxidizing agents to convert a primary alcohol to an aldehyde is much more limited. Most reagents that can oxidize the alcohol to an aldehyde carry the reaction one step further em.gif (834 bytes) they oxidize the aldehyde to the corresponding carboxylic acid.

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A weaker oxidizing agent, which is just strong enough to prepare the aldehyde from the primary alcohol, can be obtained by dissolving the complex that forms between CrO3 and pyridine, C6H5N, in a solvent such as dichloromethane that doesn't contain any water.

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Copyright 1997 James R. Fromm