Aldehydes (R-CO-H) & Ketones (R-CO-R′)
Molecules in both the aldehyde and ketone families
contain the carbonyl group (C=O). In aldehydes, the carbonyl group is always found at the end of the carbon chain. In ketones, the carbonyl group is found at any position other than at the end of the chain.
In nature, ketones are used by animals to send signals: ants use heptan-2-one to warn of danger and pheromones are used as sexual attractants by many animals.
One of the best known aldehydes is formaldehyde, used as an antiseptic and disinfectant. Acetaldehyde is used as a preservative and in the production of resins and dyes.
In nature, ketones are used by animals to send signals: ants use heptan-2-one to warn of danger and pheromones are used as sexual attractants by many animals.
One of the best known aldehydes is formaldehyde, used as an antiseptic and disinfectant. Acetaldehyde is used as a preservative and in the production of resins and dyes.
Naming
Aldehydes
Aldehydes are named by dropping the ending of the name of the longest chain and adding ‘-al. The numbering always places "1" on the carbon in the carbonyl group and the rest of the chain is numbered from there. Since the carbon in the carbonyl group is always designated as "1" there is no need to indicate the location of the carbonyl group in the name.
Aldehydes are named by dropping the ending of the name of the longest chain and adding ‘-al. The numbering always places "1" on the carbon in the carbonyl group and the rest of the chain is numbered from there. Since the carbon in the carbonyl group is always designated as "1" there is no need to indicate the location of the carbonyl group in the name.
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| First compound: The main chain has one carbon (meth), which is singly bonded (an), and it has a carbonyl group at the end of the chain (al). Second compound: The main chain has two carbons (eth), which are singly bonded (an), and it has a carbonyl group at the end of the chain (al). Third compound: Number the chain, starting with "1" on the carbonyl carbon. At the fourth carbon, there is a two carbon branch (4-ethyl) and at the third carbon, there is a one carbon branch (3-methyl). The substituents sit on a six carbon chain (hex), which is singly bonded (an) and has a carbonyl group at the end of the chain (al) |
Naming
Ketones
Ketones are named by dropping the ending of the name of the longest chain and adding ‘-one.’ Since the carbonyl group is found within the main chain, we typically need a number to locate its position. When numbering, consider the carbonyl group to be like a substituent and number as usual (so that substituents have the lowest possible numbers).
Ketones are named by dropping the ending of the name of the longest chain and adding ‘-one.’ Since the carbonyl group is found within the main chain, we typically need a number to locate its position. When numbering, consider the carbonyl group to be like a substituent and number as usual (so that substituents have the lowest possible numbers).
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| First compound: This is the smallest ketone possible. There is no need to locate the carbonyl group with a number - there is no place else it could be found on a chain this short. So, we have a three carbon chain (prop), singly bonded together (an), with a carbonyl group in the middle (one). Second compound: Start the numbering on the lowest carbon on the left side and continue until you have counted nine carbons in succession. This places a chlorine at the fourth carbon (4-chloro) and a methyl at the third carbon (3-methyl). The main chain has nine carbons (non), singly bonded together (an), with a carbonyl group at the sixth carbon (6-one). |
Properties
of Aldehydes & Ketones
The properties of aldehydes and ketones are the result of the carbonyl group:
The properties of aldehydes and ketones are the result of the carbonyl group:
O
║
─ C ─
─ C ─
- polar, but cannot H-bond
- higher mp, bp and water-solubility than analogous hydrocarbons
- lower mp, bp and water-solubility than analogous alcohols
Preparing Aldehydes and Ketones from Alcohols: Oxidation Reactions
An oxidation is any chemical process that involves the loss of electrons (we will discuss this further in the Electrochemistry unit). When alcohols are burned in oxygen, complete oxidation occurs and only CO2 and H2O are produced. If the conditions are manipulated, aldehydes and ketones may be formed. Under the special conditions, oxidizing agents are used to provide the O atoms: substances like KMnO4, H2O2 and K2Cr2O7.
In the following equations (O) will be used to represent the active oxygen atom.
In the structures below, I have drawn in hydrogens that we would not normally include in the structure. However, it makes the reaction easier to understand.
- primary alcohol ↦ produces an aldehyde
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| The active oxygen (O) attacks the primary alcohol and removes two hydrogen atoms - one from the hydroxyl group and the other from the adjacent carbon - these three atoms combine into a unit of water (H-O-H). After their hydrogens are removed, the oxygen and the carbon create another bond, resulting in a carbonyl group (C=O). Notice that the carbonyl group is at the end of the chain, so an aldehyde is produced. |
- secondary alcohol ↦ produces a ketone
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| The active oxygen (O) attacks the secondary alcohol and removes two hydrogen atoms - one from the hydroxyl group and the other from the adjacent carbon - these three atoms combine into a unit of water (H-O-H). After their hydrogens are removed, the oxygen and the carbon create another bond, resulting in a carbonyl group (C=O). Notice that the carbonyl group is in the middle of the chain, so a ketone is produced. |
- tertiary alcohol ↦ not readily oxidized
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| When the active oxygen (O) attacks the tertiary alcohol, there is only the hydrogen in the hydroxyl group. There is no hydrogen on the adjacent carbon, so the reaction cannot proceed. |
From
Aldehydes & Ketones to Alcohols:
Hydrogenation Reactions
Recall that we have seen a hydrogenation reaction in the past. This involved the addition of hydrogen across a C=C double bond. The C=O double bond can also undergo an addition reaction with hydrogen. This is the reverse of the above oxidation reaction.
Recall that we have seen a hydrogenation reaction in the past. This involved the addition of hydrogen across a C=C double bond. The C=O double bond can also undergo an addition reaction with hydrogen. This is the reverse of the above oxidation reaction.
- aldehydes ↦ produce primary alcohols
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| The C=O double bond opens up, leaving a single bond between the C and the O. The bond between the two hydrogens breaks. One H bonds to the O, the other H bonds to the adjacent carbon. This produces a primary alcohol. |
- ketones ↦ produce secondary alcohols
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| The C=O double bond opens up, leaving a single bond between the C and the O. The bond between the two hydrogens breaks. One H bonds to the O, the other H bonds to the adjacent carbon. This produces a secondary alcohol. |
