13.1 Aliphatic Carbonyl Compounds
🎯 Learning Outcomes
After the lesson, learners should be able to:
- write some examples of aliphatic aldehydes and ketones
- name the given aldehydes and ketones in IUPAC and common systems
- describe briefly the nomenclature and isomerism of aliphatic aldehydes and ketones
- show the preparation of aldehydes and ketones from
- dehydrogenation, oxidation of alcohol
- ozonolysis of alkenes
- acid chloride
- gem dihaloalkane
- and catalytic hydration of alkynes
- state physical properties of aldehydes and ketones
- describe the structure and nature of the carbonyl group
- explain chemical properties of aliphatic aldehydes and ketones, i.e. addition of H2, HCN and NaHSO3
- describe action of aldehyde and ketone with ammonia derivatives, i.e. NH2OH, NH2-NH2, phenyl hydrazine and semicarbazide
- explain Aldol condensation, Cannizzaro’s reaction, Clemmensen’s reduction, and Wolff–Kishner reduction
- describe action with PCl5 and action with LiAlH4
- explain action of methanal with ammonia and phenol
- distinguish between aliphatic aldehydes and ketones by using 2,4-DNP reagent, Tollens’ reagent and Fehling’s solution
- define formalin and state its uses
1 🧪 Introduction, Nomenclature & Isomerism
🧪 Introduction
These are compounds with the general formula CnH2nO (for saturated, acyclic aldehydes and ketones) and the functional group >C=O. These are also called carbonyl compounds.
If the carbonyl carbon is bonded with at least one hydrogen atom, it is an aldehyde; if it is bonded with both carbon atoms, it is a ketone. e.g.,
In other words, if the carbonyl group is bonded with two alkyl groups, it is a ketone; if it is bonded with one or no alkyl group, it is called an aldehyde.
🏷️ Nomenclature
Common nomenclature
The common names of the aldehydes are derived from the names of acids they give on oxidation. e.g.:
| Formula | Common name | Corresponding acid |
|---|---|---|
| HCHO | Formaldehyde | HCOOH (Formic acid) |
| CH₃CHO | Acetaldehyde | CH₃COOH (Acetic acid) |
| C₂H₅CHO | Propionaldehyde | C₂H₅COOH (Propionic acid) |
| CH₃CH₂CH₂CHO | n-butyraldehyde | CH₃CH₂CH₂COOH (n-butyric acid) |
The common names of the ketones are obtained by writing the names of the alkyl groups attached to the carbonyl group followed by the word ‘ketone’ at the end. e.g.
| Formula | Common name |
|---|---|
| CH₃COCH₃ | Dimethyl ketone (Acetone) |
| CH₃COC₂H₅ | Ethyl methyl ketone |
| C₂H₅COC₂H₅ | Diethyl ketone |
IUPAC nomenclature
In this system, both the aldehydes and ketones are named according to the general rules of IUPAC. The secondary suffixes of aldehydes and ketones are -al and -one, respectively. e.g.:
🔀 Structural Isomerism in Aldehydes and Ketones
(A) Chain Isomerism
Higher aldehydes and ketones show chain isomerism. e.g., Butanal and 2-methylpropanal are chain isomers.
Similarly, Pentan-2-one and 3-methylbutan-2-one are chain isomers.
(B) Position Isomerism
Aldehydes do not show this type of isomerism. It is because the –CHO group is always present at one end of the chain.
Higher ketones exhibit position isomerism. e.g., Pentan-2-one and Pentan-3-one are position isomers of each other. These also illustrate metamerism, because the carbon atoms are distributed differently on the two sides of the carbonyl group.
(C) Functional Isomerism
Aldehydes and ketones can show functional isomerism with one another. e.g.
2 ⚗️ Preparation of Aldehydes & Ketones
1. From alcohols
(a) By oxidation of alcohols
Aldehydes can be prepared by oxidation of primary alcohols by acidified or alkaline KMnO4, K2Cr2O7, etc.
The aldehyde should be removed by distillation as soon as it is formed; otherwise, it further oxidises to carboxylic acid.
Oxidation of secondary alcohol gives ketones.
Catalytic dehydrogenation of alcohols (studied in properties of alcohols).
2. From alkenes (by ozonolysis)
Alkenes on treatment with ozone give unstable ozonides, which on hydrolysis with zinc and water give carbonyl compounds.
- Alkenes of type RCH=CH₂ or RCH=CHR give aldehydes.
- Alkenes of the type R₂C=CH₂, R₂C=CHR or R₂C=CR₂ give at least one ketone.
3. From acid chlorides (Rosenmund’s reduction)
Aldehydes except formaldehyde are prepared by reduction of acid chlorides by Rosenmund’s catalyst, i.e., Pd deposited on BaSO4 poisoned with quinoline or sulphur to prevent further reduction of aldehydes. This is called Rosenmund’s reduction.
Ketones are prepared by treating acid chloride with dialkyl cadmium.
This is a historically valid method, but organocadmium compounds are toxic and not commonly used in modern synthetic chemistry.
4. From alkynes (by hydration)
When alkynes are passed through dilute H₂SO₄ in the presence of HgSO₄, carbonyl compounds are obtained.
The addition follows Markovnikov’s rule.
5. From Gem-dihaloalkanes
Terminal geminal dihalides on hydrolysis with an alkali solution give aldehydes, and non-terminal geminal dihalides give ketones. e.g.,
3 🌡️ Physical Properties & Structure of the Carbonyl Group
🌡️ General physical properties
- Lower aldehydes generally have strong, characteristic odours. Some aldehydes and ketones have pleasant odours and are used in perfumes and flavouring materials.
- Lower aldehydes and ketones are generally volatile liquids. As molecular mass increases, their boiling points increase, and sufficiently higher members may be solids.
- Generally, aldehydes and ketones possess higher boiling points than alkanes and ethers of comparable molecular masses. This is due to the intermolecular dipole–dipole interactions.
The oxygen of the carbonyl group carries a partial negative charge, whereas carbon carries a partial positive charge. The intermolecular force operates as:
Aldehydes and ketones containing four or fewer carbon atoms are soluble in water due to the dipole–dipole interactions. The solubility of the carbonyl compounds decreases as we move up the homologous series, as the hydrophobic hydrocarbon portion reduces solubility.
⚛️ Structure of the carbonyl group
In the carbonyl group, both carbon and oxygen atoms are sp2 hybridised. One of the p-orbitals of each carbon and oxygen atom remains unhybridised. The three sp2 hybrid orbitals of carbon lie in a single plane with an angle of about 120° between them. One sp2 hybrid orbital of C forms one σ bond with an sp2 orbital of O. The other two sp2 hybrid orbitals form σ bonds with either carbon or hydrogen. The unhybridised p-orbital of the carbonyl carbon laterally overlaps with the remaining half-filled p-orbital of the O atom to form a π bond.
Practice drawing the orbital diagram of the carbonyl group.
The carbonyl group is highly polarised because oxygen, being more electronegative than the carbon atom, draws the shared pair of electrons towards itself, developing a partial positive charge (δ⁺) in carbon and a partial negative charge (δ⁻) in itself.
4 🔬 Nucleophilic Addition Reactions
The C=O bond is strongly polar.
Carbonyl carbon → δ⁺ Carbonyl oxygen → δ⁻
Therefore, nucleophiles attack the carbonyl carbon.
1. Addition of hydrogen cyanide (HCN)
Aldehydes and ketones react with HCN (prepared in situ by mixing NaCN and H₂SO₄) to give cyanohydrins.
Cyanohydrins on hydrolysis (acidic or basic) give 2-hydroxy acids (simply, the cyanide/nitrile group on hydrolysis produces carboxylic acids).
2. Addition of sodium bisulphite (NaHSO₃)
Aldehydes and many ketones react with saturated sodium bisulphite solution to form crystalline bisulphite addition products.
Nucleophilic addition followed by elimination (Addition of ammonia derivatives)
Aldehydes and ketones react with ammonia derivatives to form an addition product, which on elimination of water gives the final product. For simplicity and exam point of view, these reactions are shown in single steps.
(a) Reaction with hydrazine (NH₂NH₂)
Aldehydes or ketones react with hydrazine to give hydrazones.
(b) Reaction with hydroxylamine (NH₂OH)
Aldehydes or ketones react with hydroxylamine to give oximes.
(c) Reaction with semicarbazide (NH₂NHCONH₂)
Aldehydes and ketones react with semicarbazide to give semicarbazone.
(d) Reaction with phenylhydrazine (NH₂NH–Ph)
Aldehydes and ketones on treatment with phenylhydrazine give phenylhydrazone.
(e) Reaction with 2,4-dinitrophenylhydrazine (2,4-DNP or 2,4-DNPh)
(the 2,4-DNP test)
When aldehydes and ketones are treated with 2,4-dinitrophenylhydrazine, a yellow, orange or red crystalline precipitate of aldehyde/ketone-2,4-dinitrophenylhydrazone is obtained. It is a characteristic reaction of carbonyl compounds. Thus, it serves as a test for carbonyl compounds. But it does not distinguish an aldehyde from a ketone.
5 🧫 Oxidation, Reduction & Distinguishing Tests
Reduction properties of aldehydes (Oxidation of aldehydes)
1. Oxidation with Tollens’ reagent
The Tollens’ reagent is ammoniacal silver nitrate solution – [Ag(NH₃)₂]OH.
Tollens’ reagent is prepared by adding ammonium hydroxide solution to silver nitrate solution till a precipitate, once formed, gets dissolved. The reactions involved are:
When an aldehyde is heated with Tollens’ reagent, Tollens’ reagent is reduced to free metallic silver in the form of a mirror, and the aldehyde is oxidised to a carboxylate salt. The reaction, therefore, is widely known as the silver mirror test.
Ordinary ketones do not undergo these reactions. Therefore, aldehydes can be distinguished from ketones and other organic compounds by the silver mirror test in the laboratory.
2. Oxidation with Fehling’s solution
The alkaline solution of copper sulphate complexed with tartrate ions (Rochelle salt) is called Fehling’s solution.
When an aldehyde is heated with Fehling’s solution, a red precipitate of cuprous oxide (Cu₂O) is obtained.
Fehling’s test is given only by aldehydes, not ketones. Fehling’s solution cannot oxidise aromatic aldehydes such as benzaldehyde.
Benedict’s solution is similar to Fehling’s solution, and is obtained by mixing CuSO₄ solution with Na₂CO₃ solution and sodium citrate. Benedict’s solution reacts similarly to Fehling’s solution only with aldehydes.
Iodoform test (test for CH₃CO— group)
Methyl ketones (R–CO–CH₃) and ethanal (acetaldehyde) react with iodine in the presence of alkali to produce iodoform. Compounds containing the CH₃CO– group give the iodoform test; ethanal is the only aldehyde that gives this test.
Reduction of Aldehydes and Ketones
1. Reduction by H₂/Ni (catalytic reduction / addition of H₂)
Catalytic reduction of aldehydes and ketones gives primary and secondary alcohols.
2. Reduction with LiAlH₄
Aldehydes give primary (1°) alcohol and ketones give secondary (2°) alcohols by reduction with lithium aluminium hydride (LiAlH₄).
3. Clemmensen’s reduction
Aldehydes and ketones are reduced to alkanes by zinc amalgam in concentrated HCl. The reaction is known as Clemmensen’s reduction.
4. Wolff–Kishner reduction
Aldehydes and ketones are reduced to alkanes by a mixture of hydrazine and a strong base in a high-boiling solvent like ethylene glycol.
6 🧵 Other Reactions & Special Compounds
Reaction of aldehydes and ketones with PCl₅
Aldehydes and ketones give geminal dichlorides on treatment with PCl₅. They give terminal and non-terminal gem-dihalides respectively.
Aldol condensation reaction
Aldehydes or ketones having α-hydrogen, on treatment with dilute bases like NaOH, undergo condensation reaction to give β-hydroxy aldehyde or ketone, called Aldol.
Cannizzaro’s reaction
Aldehydes that do not contain α-hydrogen atoms (like formaldehyde, benzaldehyde), when treated with concentrated aqueous solution of alkali (e.g. 50% NaOH), undergo disproportionation, forming an alcohol and a carboxylate salt.
Reaction of methanal with ammonia
Methanal (formaldehyde) reacts with ammonia, giving hexamethylenetetramine, also called Urotropine, which is used as medicine (urinary antiseptic) and in some industries.
Reaction of formaldehyde with phenol
Formaldehyde reacts with phenol in an acidic medium, forming phenol–formaldehyde resin or Bakelite. Bakelite is a thermosetting plastic, so it is used in electrical appliances (switches, plugs), handles of cooking utensils, etc.
Formalin and its uses
Formalin is a 37% (to 40%) formaldehyde solution in water (usually stabilised with a small amount of methanol). It is used to preserve museum specimens in biology laboratories and as an antiseptic.
🗺️ Reaction Map at a glance
📌 Chapter Summary
- Aldehydes and ketones are carbonyl compounds containing the >C=O group. Aldehydes have the general structure R–CHO, while ketones have R–CO–R′.
- Aldehydes and ketones are named using the suffixes -al and -one, respectively.
- They show chain, position and functional isomerism. Aldehydes and ketones with the same molecular formula are functional isomers.
- Aldehydes are commonly prepared from primary alcohols, and ketones from secondary alcohols.
Common oxidising agents: KMnO₄/H⁺; K₂Cr₂O₇/H⁺; KMnO₄/OH⁻ - Other methods to prepare aldehydes and ketones include ozonolysis of alkenes, Rosenmund reduction and alkyne hydration.
- The C=O bond is polar, making the carbonyl carbon partially positive and susceptible to attack by nucleophiles.
- They undergo important nucleophilic addition reactions with HCN, sodium bisulfite and ammonia derivatives.
- Aldehydes are readily oxidised to carboxylic acids, while ketones generally resist oxidation under mild conditions.
- On reduction, aldehydes give primary alcohols and ketones give secondary alcohols. Both can be converted to hydrocarbons by Clemmensen or Wolff–Kishner reduction.
Common reducing agents: H₂/Ni; LiAlH₄ etc. - Compounds with α-hydrogen can undergo the aldol reaction, while aldehydes without α-hydrogen can undergo the Cannizzaro reaction.
- 2,4-DNP detects aldehydes and ketones; Tollens’ reagent gives a silver mirror with aldehydes; and Fehling’s solution generally gives a brick-red precipitate with aliphatic aldehydes (not ketones).
- Methyl ketones and ethanal give the iodoform test, producing a yellow precipitate of CHI₃.
- Methanal is used to prepare formalin and phenol–formaldehyde resin (Bakelite).
