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Aldehydes and Ketones

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Aldehydes and ketones are the compounds containing carbonyl (=C=O) functional group. When carbonyl group is attached to two alkyl groups the compound is known as ketone and when attached to an alkyl group and a hydrogen atom, it is called an aldehyde. All aldehydes have a carboyl group bonded on one side to a carbon and on the other side to a hydrogen. In ketones the carbonyl group is situated between two carbon atoms.


Naming 

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When naming aldehydes and ketones according to the IUPAC rules, the carbonyl group (C=O) must be part of the parent chain. Since the carbonyl carbon atom of an aldehyde is always in position number 1, its position is not specified in the name. For ketones, however the position of the carbonyl carbon is given, unless the molecule is small enough that there is no question as to carbonyl placement. Parent chains are named by dropping the final "e" from the name of the corresponding hydrocarbon and adding "al" for aldehydes or "one" for ketones.

Aldehydes and Ketones

The common names of ketones are formed by placing "ketone" after the names of the alkyl groups attached to the carbonyl carbon atom. For aldehydes and for some ketones, other common names are assigned. Three important ones to know are formaldehyde, acetaldehyde and acetone.

Reactions of Aldehydes and Ketones

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Aldehydes and ketones generally undergo the same type of reactions; they both take part in reactions that involve addition of a nucleophile and they both undergo imine formation. However, they do so at different rates. In general, aldehydes are much more reactive than ketones. Partly this is because there is a steric difference between the two types of compound.

For both aldehydes aldehydes and ketones one of the groups bonded to the carbonyl carbon atom is an alkyl or aryl group. However, in aldehydes, the other group bonded to the carbonyl carbon atom is a hydrogen atom, whereas ketones have another alkyl or aryl group bonded to the carbonyl carbon atom. A hydrogen atom is much smaller than an alkyl group, so there is less steric hindrance to nucleophilic attack at an aldehyde carbonyl group.

Aldehydes are more reactive than ketones. For simple compounds containing only an aldehyde or a ketone functional group, this difference in reactivity is unimportant. However, when a compound contains both types of functional group, the reactivity is greater for aldehydes.

Physical Properties 

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Physical state

The lower molecular mass aldehydes have sharp rather unpleasant smell but higher molecular mass aldehydes and ketones are pleasant smelling. In fact some ketones are used in perfumery. Some aromatic aldehydes obtained from natural sources have very pleasant fragrance.

Boiling point

The boiling points of aldehydes and ketones are higher than non-polar alkanes and weakly polar ethers of comparable masses. However, the boiling points of aldehydes and ketones are lower than those of alcohols of comparable molecular masses due to the absence of intermolecular hydrogen bonding.

Solubiltiy

The lower aldehydes and ketones such as methanal, ethanal and propanone are misicible with water in all proportions as they can form hydrogen bonds with water. All aldehydes and ketones are soluble in organic solvents like benzene, ether etc.

How to Prepare Aldehydes and Ketones ?

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One of the bast methods of aldehydes synthesis is by oxidation of primary alcohols. The reaction is often carried out using the Dess-Martin periodinance reagent in dichloromethane solvent at room temperature.

Aldehydes

Carboxylic acids derivatives can be partially reduced to yeild aldehydes. The partial reduction of an ester by diisobutylaluminum hydride for instance is an important laboratory scale method of aldehyde synthesis and mechanistically related processes also occur in biological pathways. The reaction is normally carried out at -78oC (dry-ice temperature) in toluene solution.

Preparation of Aldehydes

Ketone synthesis is similar to those for aldehydes. Secondary alcohols are oxidised by a variety of reagents to give ketones. The choice of oxidant depends on such factors ad reaction scale, cost and acid or base sensitivity of the alcohol. Either the Dess-Martin periodinane or a Cr(VI) reagent such as CrO3 is a common choice.

Preparation of Ketones

Reduction 

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Aldehydes and ketones can be reduced to alcohols using a reducing agent containing the hydride ion H+. The suitable reducing agents are
  • Sodium tetrahydridoborate(III)(sodium borohydride), NaBH4 in water
  • Lithium tetrahydridoaluminate(III)(lithium aluminum hydride), LiAlH4 in dry ether.
Aldehydes are reduced to primary alcohols.

Reduction of Aldehyde
Ketones are reduced to secondary alcohols.

Reduction of Ketones

Oxidation 

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Aldehydes are more easily oxidized than ketones, but as with other organic compounds complete combustion in the presence of oxygen produces carbon dioxide and water.

Oxidation of aldehydes or ketones to 1,2-dicarbonyl compounds by heating with SeO2 by heating or with microwaves. Sometimes oxidation to $\alpha$, $\beta$-unsaturated ketones.

Oxidation of Aldehydes and Ketones

Uses 

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  1. Propanone is used in large quantities as an industrial solvent in paints, varishes and nail polish removers.
  2. Methanal is used in preserving and embalming as a germicide and insecticide, for manufacturing plastic coatings such as Baklite, formica and melanine, for manufacturing polymer adhesives such as those used to glue wood together.
  3. Other aldehydes and ketones are mainly used as intermediates in the manufacture of plastics, dyes and pharmaceuticals as solvents or as perfumes and flavoring agents. Examples of aldehydes and ketones as flavors are benzaldehyde as the flavor of fresh almonds and heptan-2-one as smell of blue cheese.