Aldehydes are organic compounds that have an acyl group, R-C=O with a Hydrogen bonded to the carbonyl or acyl carbon (double bonded carbon). Ketones are a part of another acyl family with the acyl group, R-C=O having another alkyl group, R', connected to the carbonyl carbon. Aldehydes and Ketones have similar chemistry activity because of the fact that they both have a carbonyl carbon. The bond is polar with the Oxygen atom the negative end and the carbon end being positive. This results in nucleophiles being attracted to the carbonyl carbon and bonding with it. This similarity in nucleophilic attack makes the chemistry very close as we will see.
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Aldehydes are named by using the following rules:
Let us consider the structure in Fig 1(a).
Let's consider the structure in Fig 1(b) above.
Lets consider the last example in Fig 1(c) above
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Below in Fig 2 are three structures. Please identify the IUPAC name of each structure.
When you have finished identifying the IUPAC names, you may check for the correct answers.
None of the Hydrogen atoms conected to an aldehyde or ketone are bonded to an Oxygen or Nitrogen so they do not attract other molecules with the strong Hydrogen bonding. For this reason the aldehydes do not have as high a boiling point for the same sized alcohol that does have a Hydrogen bonded to an Oxygen. However aldehydes and ketones do have the carbonyl structure which is polar since the Oxygen is much higher in electronegativity than carbon atom. Therefore aldehydes and ketones will exhibit dipole-dipole interactions as well as the weak London dispersion intermolecular forces which make them have higher boiling points compared to the hydrocarbons and the ethers. For example, propanal and acetone have boiling points of 49oC and 56oC respectively. The difference is due to the slightly higher molecular mass of acetone. This is to be compared with 1-propanol of 97oC and Ethyl Methyl Ether at 8oC and butane at 0oC. The extra strong hydrogen bonding between the 1-Propanol molecules would account for its higher boiling point.
The carbonyl Oxygen with its lone pairs allow for ketones and aldehydes to hydrogen bond with Hydroxylic compounds like water and alcohols. Therefore the low molecular mass (up to four carbons) of aldehydes and ketones allow them to be very soluble in water. This solubility is similar to alcohols and ethers which also have Oxygen atoms with lone pairs of electrons. Aldehydes and ketones of greater than five carbons generally will not be soluble in water as are the alcohols. This is because the increased size of the hydrocarbon portion will prevent water molecules from being attracted to the organic molecules. In other words the solvation process is hampered and the water molecules are not capable of surrounding each organic molecule and separating them. The polar water molecules have little attraction for hydrocarbons.
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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.
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All textual content copyrighted (c) 1997 R.H. Logan, Instructor of Chemistry, DCCCD All Rights reserved