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Aldehydes and Ketones

The carbon atom of a carbonyl group must form two other bonds in addition to the carbon oxygen double bond in order to have four bonds. The nature of these two additional bonds determines the type of carbonyl containing compound it is.

  1. Aldehyde - In an aldehyde one of the two additional bonds that the carbonyl carbon atom forms must be ti a hydrogen atom. The other may be to a hydrogen atom, an alkyl or cycloalkyl group, or an aromatic ring system.
  2. Ketone - In a ketone both of the additional bonds of the carbonyl carbon atom must be to another carbon atom that is part of an alkyl, cycloalkyl or aromatic group.
Aldehyde and Ketone

The carbonyl group of an aldehyde is flanked by a hydrogen atom, while the carbonyl group of a ketone is flanked by two carbon atoms.

 

Naming Aldehydes and Ketones

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The systematic name for an aldehyde is obtained from the parent alkane by removing the final -e and adding -al. For ketones the final -e is replaced by -one, and a number indicates the position of the carbonyl group wherever necessary. The carbon chain in ketones is numbered such that the carbonyl carbon gets the lowest possible number.

In aldehyde the carbonyl group is always at the end of the chain and is always assumed to be carbon number 1. The positions of the other substituents are specified by numbers as usual. The following examples illustrates these principles.
Naming Aldehydes and Ketones
The names in parentheses are common names that are used much more often than the systematic names.

An alternative system for naming ketones specifies the substituents attached to the C=C group. For example, the compound 2-butanone is used in the system as described.

2-Butanone

However this molecule also can be named methyl ethyl ketone and is commonly referred to in industry as MEK (methyl ethyl ketone).
Methyl Ethyl Ketone

Reactions of Aldehydes and Ketones

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1. Oxidation


Aldehyde are oxidized to carboxylic acids by a variety of oxidizing agents, including potassium dichromate. Aldehyde are also oxidized to carboxylic acids by the oxygen in the air.

Oxidation of Aldehyde

Ketones in contrast resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen. The fact that aldehydes are so easy to oxidize and ketone does not. To distinguish between these types of compounds simple chemical tests are carried out.

Suppose that we have a compound we know is either an aldehyde or a ketone. To determine which it is we can treat the compound with a mild oxidizing agent. If it can be oxidized it is a aldehyde otherwise its a ketone. One reagent which has been used for this purpose is Tollens reagent.

Oxidation of Ketones

2. Reduction


Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.
Aldehyde Reduction

The reduction of a C = O double bond under these conditions is slower than the reduction of a C = C double bond. Thus if the same molecule contains both C = O and C = C double bonds, the C = C double bond is reduced first.

Synthesis of Aldehydes and ketones

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1. By ozonolysis of alkenes

Alkenes react with ozone to form ozonides which on subsequent reductive cleavage with zinc dust and water or H2/Pd give aldehydes, Ketones or a mixture of both depending on the substitution pattern of the alkene.

Ozonolysis of Alkenes

Zinc dust removes H2O2 formed, which otherwise can further oxidize the aldehyde formed to acids.

Using a suitable alkene, the desired aldehyde or ketone can be formed.

Example:

Aldehyde from Alkene

2. By hydration of alkynes

Ethynes adds water in the presence of H2SO4 and HgSO4 to give acetaldehyde.

Hydration of other alkynes under similar condition gives ketones.

Alkynes Hydration

3. By oxidation of methylbenzenes

Oxidation of an aromatic compound leaving a methyl group at the benzene ring with CrO3 in the presence of acetic anhydride followed by hydrolysis gives the corresponding benzaldehyde.

Oxidation of Methylbenzenes


Further oxidation of the benzaldehyde to benzoic acid is prevented as the aldehyde is trapped by acetic anhydride as a non-oxidisable benzylidene diacetate derivative. This reaction is called the Etard reaction.

4. From Nitriles

Partial reduction of nitriles with acidified stannous chloride SnCl2/HCl at room temperature gives aldehydes. In the first step imine hydrochloride is obtained which on subsequent hydrolysis with boiled water gives aldehyde. This specific type of reduction of nitriles is called Stephen's reduction.

SnCl2 + 2HCl $\rightarrow$ SnCl4 + 2 [H]

Formation of Acetaldehyde

Similarly benzonitrile gives benzaldehyde.

Benzonitrile Gives Benzaldehyde

Ketones are obtained when Grignard's reagent add on to nitriles. The imine salt intermediate that is formed on hydrolysis gives ketone.

Formation of Ketone from Grignard Reagent

Formation of Acetone
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Propionaldehyde
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