B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Aldehydes and ketones are important classes of organic compounds containing the carbonyl group (>C=O). The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. Because of the polarity and reactivity of the carbonyl group, aldehydes and ketones participate in a wide variety of organic reactions.
In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, whereas in ketones the carbonyl carbon is bonded to two carbon groups.
Aldehydes and ketones occur naturally and are also widely used in industrial chemistry, pharmaceuticals, perfumes, flavors, polymers and laboratory synthesis.
The carbonyl carbon in aldehydes and ketones is generally sp2-hybridized. The three sigma bonds around the carbonyl carbon are arranged approximately in a trigonal planar geometry.
The carbon-oxygen double bond consists of one sigma bond and one pi bond. Oxygen is more electronegative than carbon, so the carbonyl group is strongly polarized.
The partial positive charge on carbon makes the carbonyl carbon susceptible to attack by nucleophiles, while the oxygen atom can interact with electrophiles or protons.
Aldehydes in which the –CHO group is attached to an aliphatic carbon chain are called aliphatic aldehydes.
Examples: Methanal, ethanal, propanal.
Aldehydes in which the –CHO group is directly attached to an aromatic ring are called aromatic aldehydes.
Example: Benzaldehyde (C6H5CHO).
When the two groups attached to the carbonyl carbon are identical, the ketone is symmetrical.
Example: Acetone, CH3COCH3.
When the two groups attached to the carbonyl carbon are different, the ketone is unsymmetrical.
Example: CH3COC2H5.
Ketones may also be classified according to whether the carbonyl group is present in an open-chain or cyclic structure.
The longest carbon chain containing the aldehyde group is selected. The suffix -al is used for aldehydes.
| Common name | IUPAC name | Formula |
|---|---|---|
| Formaldehyde | Methanal | HCHO |
| Acetaldehyde | Ethanal | CH3CHO |
| Propionaldehyde | Propanal | CH3CH2CHO |
| Butyraldehyde | Butanal | CH3CH2CH2CHO |
The longest chain containing the carbonyl group is selected and the suffix -one is used. The position of the carbonyl carbon is indicated by a number.
| Common name | IUPAC name | Formula |
|---|---|---|
| Acetone | Propanone | CH3COCH3 |
| Methyl ethyl ketone | Butan-2-one | CH3COCH2CH3 |
Primary alcohols can be oxidized carefully to aldehydes. Controlled oxidation is necessary because aldehydes can undergo further oxidation to carboxylic acids.
Passing primary alcohol vapors over heated copper can produce aldehydes by dehydrogenation.
Acid chlorides can be selectively reduced to aldehydes using hydrogen in the presence of a poisoned palladium catalyst.
The catalyst is commonly palladium supported on calcium carbonate and modified to prevent further reduction of the aldehyde.
Nitriles can be reduced under suitable conditions to imine salts, which on hydrolysis give aldehydes.
Alkenes can undergo ozonolysis followed by reductive work-up to produce aldehydes and/or ketones depending on the structure of the alkene.
Secondary alcohols are oxidized to ketones.
Secondary alcohols give ketones on catalytic dehydrogenation.
Alkynes can undergo hydration followed by tautomerization to produce carbonyl compounds. In suitable cases, the product is a ketone.
Ozonolysis of appropriately substituted alkenes can produce ketones.
Aromatic compounds react with acyl chlorides in the presence of a Lewis acid such as anhydrous AlCl3 to form aromatic ketones.
Aldehydes and ketones are polar compounds because of the carbonyl group. The oxygen atom can participate in hydrogen bonding with water and other hydrogen-bond donors.
The carbonyl carbon is electrophilic because the carbon-oxygen bond is polarized.
Most characteristic reactions of aldehydes and ketones involve nucleophilic addition at the carbonyl carbon.
Protonation or other subsequent steps can convert the initial addition product into the final product.
Aldehydes generally undergo nucleophilic addition more readily than ketones because they have less steric hindrance and possess only one alkyl group attached to the carbonyl carbon.
Ketones have two carbon groups attached to the carbonyl carbon, making nucleophilic attack more sterically hindered and reducing the electrophilic character of the carbonyl carbon through electron donation.
Nucleophilic addition is one of the most important reactions of aldehydes and ketones.
Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins.
Cyanohydrin formation involves nucleophilic attack by cyanide ion on the carbonyl carbon followed by protonation of the oxygen.
Cyanohydrins are useful intermediates in organic synthesis because the nitrile group can be transformed into other functional groups.
Many aldehydes and some ketones react with sodium bisulfite to form crystalline bisulfite addition compounds.
This reaction has practical importance in the separation and purification of certain carbonyl compounds.
Aldehydes and ketones react with hydroxylamine to form oximes.
Oxime formation is a condensation reaction involving replacement of the carbonyl oxygen by the =NOH group.
Aldehydes and ketones react with hydrazine to form hydrazones.
Hydrazone formation is another important condensation reaction of carbonyl compounds.
Aldehydes and ketones react with 2,4-dinitrophenylhydrazine (2,4-DNP) to form characteristic 2,4-dinitrophenylhydrazones.
The resulting derivatives are often crystalline and have characteristic melting points, making the reaction useful in classical identification of carbonyl compounds.
Aldehydes are reduced to primary alcohols, while ketones are reduced to secondary alcohols.
Common reducing agents include sodium borohydride and lithium aluminium hydride.
Aldehydes and ketones can be reduced to hydrocarbons using zinc amalgam and hydrochloric acid.
Carbonyl compounds can also be reduced to hydrocarbons through hydrazone formation followed by strong-base treatment and heating.
Aldehydes are readily oxidized to carboxylic acids.
This easy oxidation distinguishes aldehydes from ketones in many qualitative tests.
Ketones are generally resistant to mild oxidation. Strong oxidation conditions can cause cleavage of carbon-carbon bonds adjacent to the carbonyl group, producing smaller carboxylic acids or other products.
Tollens' reagent is an ammoniacal solution containing a silver(I) complex. Aldehydes reduce silver(I) ions to metallic silver while the aldehyde is oxidized to a carboxylate.
Formation of a silver mirror on the inner surface of a clean test tube is the classical positive observation.
Fehling's reagent contains copper(II) ions in an alkaline complexing medium. Many aliphatic aldehydes reduce Cu(II) to Cu(I), producing red or brick-red copper(I) oxide.
Ketones generally do not give the same positive result under ordinary Fehling test conditions.
Benedict's reagent is also based on the reduction of copper(II) ions under alkaline conditions. Reducing aldehydes can produce colored precipitates of copper(I) oxide.
The test is widely used for detecting reducing substances, particularly in carbohydrate chemistry.
Methyl ketones containing the group CH3CO– give the iodoform reaction with iodine and alkali.
Yellow precipitate of iodoform, CHI3, is produced.
Ethanal can also give the iodoform reaction because it contains the required structural arrangement.
Aldehydes and ketones containing at least one alpha-hydrogen can undergo aldol condensation in the presence of a suitable base or acid catalyst.
The reaction initially forms a beta-hydroxy carbonyl compound, called an aldol. On heating or under suitable conditions, dehydration may occur to produce an alpha,beta-unsaturated carbonyl compound.
The product formed is 3-hydroxybutanal, commonly called aldol.
On heating, it can lose water:
The final product is an alpha,beta-unsaturated aldehyde.
Aldehydes that do not contain an alpha-hydrogen can undergo self-oxidation-reduction, known as the Cannizzaro reaction, in the presence of concentrated alkali.
One molecule of aldehyde is oxidized to a carboxylate while another molecule is reduced to an alcohol.
Formaldehyde and benzaldehyde are common examples of aldehydes capable of undergoing this reaction because they do not possess an alpha-hydrogen.
The Perkin reaction is an important method for the preparation of alpha,beta-unsaturated aromatic acids. Aromatic aldehydes react with acid anhydrides in the presence of a suitable base.
Benzaldehyde and acetic anhydride can be used to produce cinnamic acid under suitable conditions.
Grignard reagents are powerful carbon nucleophiles that react with aldehydes and ketones. After hydrolysis, alcohols are obtained.
Reaction of formaldehyde with a Grignard reagent gives a primary alcohol after hydrolysis.
Other aldehydes generally give secondary alcohols after reaction with Grignard reagents and hydrolysis.
Ketones generally give tertiary alcohols after reaction with Grignard reagents followed by hydrolysis.
| Property | Aldehydes | Ketones |
|---|---|---|
| General structure | R–CHO | R–CO–R′ |
| Hydrogen attached to carbonyl carbon | Present | Absent |
| Oxidation | Readily oxidized to carboxylic acids | Resistant to mild oxidation |
| Tollens' test | Generally positive | Generally negative |
| Fehling's test | Many aliphatic aldehydes give positive result | Generally negative |
| Reactivity toward nucleophiles | Generally higher | Generally lower |
| Example | Ethanal | Propanone |
Formaldehyde, HCHO, is the simplest aldehyde. Its aqueous solution is commonly known as formalin.
Formaldehyde is important in the manufacture of resins, plastics and other industrial chemicals.
Acetaldehyde, CH3CHO, is an important intermediate in organic synthesis and can be oxidized to acetic acid.
Benzaldehyde, C6H5CHO, is the simplest aromatic aldehyde and has a characteristic almond-like odor.
It is used in flavoring, fragrance and organic synthesis.
Acetone, CH3COCH3, is the simplest and most important aliphatic ketone.
It is a volatile, flammable liquid and is widely used as an organic solvent.
Acetophenone, C6H5COCH3, is an aromatic ketone used in fragrance chemistry and organic synthesis.
| Test | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNP test | Positive | Positive |
| Tollens' test | Generally positive | Generally negative |
| Fehling's test | Many aliphatic aldehydes positive | Generally negative |
| Schiff's test | Generally positive | Generally negative |
| Iodoform test | Ethanal gives positive result | Methyl ketones give positive result |
Aldehydes and ketones are extremely important in industrial, pharmaceutical, biological and laboratory chemistry.
Aldehydes and ketones are carbonyl compounds containing the functional group >C=O.
Aldehydes have the general structure R–CHO, while ketones have the structure R–CO–R′.
The carbonyl carbon is electrophilic and therefore undergoes nucleophilic addition reactions.
Aldehydes can be prepared by controlled oxidation or dehydrogenation of primary alcohols, Rosenmund reduction and other methods.
Ketones can be prepared by oxidation or dehydrogenation of secondary alcohols, hydration of alkynes and Friedel-Crafts acylation.
Important reactions include addition of HCN, bisulfite addition, oxime and hydrazone formation, reduction, oxidation, aldol condensation and Cannizzaro reaction.
Tollens' and Fehling's tests are important qualitative tests for aldehydes, while 2,4-DNP is useful for detecting carbonyl compounds.
Aldehydes and ketones have major applications in organic synthesis, pharmaceuticals, solvents, fragrances, flavors and industrial chemistry.