Nomenclature of Aldehydes and Physical Properties of Aldehydes and Ketones

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.

Please choose one of the following topics:

  1. IUPAC Nomenclature of Aldehydes
  2. Practice Problems on Aldehydes
  3. Physical Properties of Aldehydes and Ketones

IUPAC Nomenclature of Aldehydes

Aldehydes are named by using the following rules:

  1. Identify the longest continuous chain of carbons with the acyl or carbonyl carbon as part of the chain.
  2. Number the carbon chain so that the carbonyl (acyl) carbon is always #1.
  3. Locate and identify alphebetically the branched groups by prefixing the carbon number it is attached to. If more than one of the same type of branched group is involved use the Greek prefixes di for 2, tri for three, etc.
  4. After identifying the name, number and location of each branched group, use the alkane name corresponding to the number of carbons in the continuous chain
  5. Drop the "e" and add the characteristic IUPAC ending for all aldehydes, "al"
  6. Alkenals involving Pi bonding will require that the Pi bond is located but the ending will still be "al"

Here are three examples shown in Fig 1 below.

aldehyde structures for practice

Let us consider the structure in Fig 1(a).

  1. We find the longest continuous chain oif carbons with the acyl carbon involved is five.
  2. Numbering the carbons beginning with the acyl carbon on the extreme right as carbon #1
  3. Identifying the branched groups, there is a methyl group on carbon #3 and a Bromine on carbon #2 so we would name and locate them: 2-Bromo-3-methyl
  4. use the alkane name corresponding to the number of carbons in the chain (5) which would be pentane
  5. drop the "e" and add "al" so the name is: 2-Bromo-3-methylpentanal

Let's consider the structure in Fig 1(b) above.

  1. Notice that there is a benzene ring with the characteristic functional group attached to the ring. This would be the parent aromatic aldehyde benzaldehyde
  2. Since we have three substitutions on the Benzene ring we must use numbers and number the ring carbons beginning with the carbon with the aldehyde functional group attached to it as carbon #1. We then proceed to number clockwise.
  3. We notice a Chlorine attached to carbon #3 and a methyl group attached to carbon #4.
  4. Locate and identify these branches: 3-Chloro-4-methyl
  5. Add the parent name benzaldehyde and we have:

    3-Chloro-4-methylbenzaldehyde.

Lets consider the last example in Fig 1(c) above

  1. Indentify the longest contineous chain of carbons with the acyl carbon as one of the carbons and the double bond must be between two of the carbons in the contineous chain which would be six carbons.
  2. Number the carbons in the chain so that the acyl carbon is carbon #1
  3. Locate and identify all branches which is only a methyl attached to carbon # 2.

    2-methyl

  4. Locate the Pi bond in the chain. It is between the caron #4 and 5
  5. Use the alkene name corresponding to the number of carbons (6)

    hexene

  6. Locate the Pi bond by prfixing the lowest carbon # in which the Pi bond is between. It is between carbon #4 and 5 so it would be

    4-pentene

  7. Add to the name and it becomes:

    2-methyl-4-pentene

  8. Drop the "e" and add the characteristic "al" and it becomes:

    2-methyl-4-pentenal

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Practice Examples on Aldehydes

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.

Physical properties of Aldehydes and Ketones

Boiling Points

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.

Water Solubility of Aldehydes and Ketones

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.


Acknowledgements:


Acknowledgements

Send Comments to R.H. Logan:

rhl7460@dcccd.edu


All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
All Rights reserved

Revised: 8/25/97

Answers To The Fig 2 Structures

  1. Fig 2(a) should be 3-Chloro-2,4,4-trimethylhexanal
  2. Fig 2(b) should be 5-Bromo-4-ethyl-2-methylbenzaldehyde
  3. Fig 2(c) should be 2-methyl-4-pentenal

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URL:http://edie.cprost.sfu.ca/~rhlogan/nom_ald.html