Aldehydes and Ketones: Both aldehyde and ketones containing a carbon-oxygen double bond (>C=O). These are also called carbonyl group.
Aldehyde and ketones are collectively called carbonyl compounds. In aldehydes, the carbonyl carbon is bonded to one hydrogen and one alkyl group. Formaldehyde, (HCHO), in which the carbonyl carbon is bonded to two hydrogen atoms is an exception. In ketone, the carbonyl carbon is always bonded to two alkyl groups. These alkyl groups may be same or different.
Electronic structure of carbonyl group: The carbonyl group, like the carbon-carbon double bond of alkenes, is composed of one σ bond and one π bond.
Both the carbon and oxygen are sp2 hybridized. The σ bond is formed by the overlap of an sp2 orbital of carbon and an sp2 orbital of oxygen. The π bond is formed by the overlap of unhybridised p-orbitals of two atoms. The two unshared electron pairs of oxygen occupy the sp2 hybrid orbitals of oxygen. Because the carbonyl carbon is sp2 hybridized, the three atoms attached to it lie in the same plane. The bond angles between the attached atoms are approximately 120º.
Fig: The carbon-oxygen double bond in aldehydes and ketones is composed of an σ bond and a π-bond.
The electrons in the π bond of the carbonyl group are not equally shared. In fact, they are pulled more towards the more electronegative oxygen atom. As a result, the bond is polarized, with the oxygen atom being slightly negative and the carbon atom being slightly positive. This is often indicated as:
Alternatively, the polar nature of the carbonyl group can also be indicated by the following resonance structure:
General methods of preparation of aldehydes and ketones:
Some of the important methods of preparation of aldehydes and ketones are as follows:
1) From alcohols: Aldehydes can be prepared by the oxidation of primary alcohols with mild oxidizing agent such as potassium dichromate and potassium permanganate and sulphuric acid.
On the other hand, ketones can be prepared by the oxidation of secondary alcohols with similar oxidizing agent.
2) Catalytic dehydrogenation of alcohols: Aldehydes may be prepared by passing the vapour of primary alcohols over a copper catalyst heated to about 300ºC.
Similarly, ketones are prepared from secondary alcohols.
3) From carboxylic acids: Aldehydes can be prepared by catalytic decomposition of carboxylic acid. This can be done by heating a mixture of methanoic acid or other acids to 300ºc in the presence of MnO2 which acts as catalyst.
Ketones can be prepared by passing the vapours of fatty acids (other than methanoic acids) over MnO2 at 300ºc.
Aldehydes and ketones can also be prepared by distilling the calcium salts of the acids. e.g.
Calcium acetate Calcium formate (acetaldehyde)
4) From acid chlorides: Aldehyde can be prepared by catalytic hydrogenation in the presence of palladium (Pd) catalyst supported over barium sulphate. The catalytic mixture is poisoned by the addition of a small quantity of suphur or quinoline. This reaction is called Rosenmund reduction.
5) FromAlkynes: Aldehydes and ketones can be prepared by hydration of alkynes in presence of dil. H2SO4 and HgSO4 as catalyst. Water adds to alkynes to form an unstable enol intermediates which rearrange to form aldehydes or ketones.
6) From Alkenes: Aldehydes and ketones can be prepared by the ozonolysis of alkenes. This involves the treatment of the alkenes with ozone to give ozonides. The ozonides are not isolated because they are often explosive in dry state. They are decomposed with Zn+H2O to form aldehydes and ketones.
Physical properties of aldehydes and ketones: The important properties of aldehydes and ketones are as follows-
Most of the aldehydes (except formaldehyde which is a gas) are liquids at room temperature. The lower ketones are colourless liquids and have pleasant smell. The higher members are colourless solids. Aldehydes and ketones upto four carbon atoms are miscible with water. This is due to the presence of hydrogen bonding between the polar carbonyl group and water molecules as –
>C=O———-H─O─H.
With the increase in the size of alkyl group, the solubility decreases. Aldehydes and ketones have relatively high boiling points as compared to that of hydrocarbons of comparable molecular masses. It is due to the presence of polar carbonyl group and therefore, they have stronger intermolecular dipole-dipole interactions between the opposite ends of C=O dipoles.
Chemical reactions of Aldehydes and ketones:
Aldehydes and ketones are highly reactive compounds. Since both these classes have same functional group and shows a large number of common reactions. Both aldehydes and ketones undergo nucleophilic addition reactions. The reactivity of aldehydes and ketones is because of the presence of a polar carbonyl group. As oxygen atom is more electronegative, therefore, it pulls the electron cloud around itself acquiring a partial negative charge (δ-) whereas a partial positive charge (δ+) is developed on the carbon atom. The positively charged carbon atom of carbonyl group is then readily attacked by the nucleophilic species for initiation of the reaction. This leads to the formation of an intermediate anion which further undergoes the attack of H+ ion or other positively charged species to form the final product. It may be represented as:
Relative reactivity of aldehydes and ketones: In general, ketones are less reactive than aldehydes. This is due to the following reasons:
1) Electron releasing effect: In ketones, the carbonyl carbon is attached to two alkyl groups which are electron releasing in nature. These alkyl groups push electrons towards carbonyl carbon and thus, decrease the magnitude of positive charge on it and make it less susceptible to nucleophilic attack.
2) Stearic effect: The size of the alkyl group is more than that of hydrogen. In aldehydes, there is one alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and reactivity decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde. Thus, the reactivity follows the order:
Aromatic aldehydes and ketones:
In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood from the resonating structures of benzaldehyde as shown below:
It is clear from the resonating structures that due to electron releasing (+I effect) of the benzene ring, the magnitude of the positive charge on the carbonyl group decreases and consequently it becomes less susceptible to the nucleophilic attack. Thus, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehydes and ketones. The order of reactivity of aromatic aldehydes and ketones is:
Acidity of α-hydrogen: A carbon atom next to the carbonyl group is called an α-carbon. A hydrogen attached to an α-carbon is referred to as an α-hydrogen. The α-hydrogen of aldehydes and ketones are acidic in nature. The acidity is due to the fact that the anion, which results from the removal of an α-hydrogen by a base B:–, is stabilized by resonance. The resonance-stabilized anion is called Enolate Ion.
The α-carbon of the enolate ion is negatively charged. It can act as a nucleophile. The formation of the enolate ion followed by its addition to a carbonyl group is the process involved in all the condensation reactions of aldehydes and ketones.
Reactions involving alkyl groups
1) Aldol condensation: Aldehydes containing α-hydrogens undergo self addition reaction in the presence of a base to form products called Aldols. The reaction is called Aldol condensation. For example, when two molecules of acetaldehyde combine with each other in the presence of dilute solution of NaOH to form 3-hydroxybutanal (aldol).
2) Cross aldol condensation: The reaction of two different carbonyl compounds (one of which must have an α-hydrogen) in the presence of a base is known as cross or mixed aldol condensation. For examples, acetaldehyde reacts with benzaldehyde which lack of α-hydrogen in presence of a base to form cinnamaldehyde.
Nucleophilic addition reaction: Aldehydes and ketones undergo nucleophilic addition ractions. Examples
1) Addition of bisulphate: Aldehydes and ketones react with a saturated solution of sodium bisulphate (NaHSO3) to form an addition compound.
2) Addition of hydrogen cyanide: Aldehydes and ketones react with HCN in the presence of a acid catalyst to form cyanohydrin.
3) Addition of Grignard reagents: Aldehydes and ketones react with Grignard reagents to give an addition product which on acid hydrolyzed to give an alcohol.
The reaction provides a convenient way of preparing alcohols that contain a larger carbon chain than the starting materials. Formaldehyde reacts with Grignard reagents to produce primary alcohols. Other aldehydes give secondary alcohols. Ketones give tertiary alcohols.
4) Addition of ammonia: Aldehydes (except formaldehyde) reacts with ammonia to form solid aldehyde ammonias.
5) Reaction with ammonia derivatives: Ammonia derivatives (NH2-Z) react with aldehydes and ketones to form compounds containing carbon-nitrogen double bond, together with the elimination of a water molecule.
Ammonia derivatives which react in this way are hydroxylamine, hydrazine, phenyl hydrazine, 2,4-dinitrophenylhydrazine (2,4-DNP) and semicarbazide.
6) Cannizzaro’s Reaction: Aldehydes which lack an α-hydrogen, when heated with conc. NaOH, undergo a disproportionate reaction. One half of the aldehyde molecules are oxidized to a carboxylic acid and one half is reduced to an alcohol. This reaction is known as cannizzar’s reaction. e.g.
