Type Of Chemical Reactions In Carbonyl Compounds

(i) Addition across C = O bond.

(ii) Replacement of carbonyl oxygen by other groups.

(iii) Oxidation

(iv) Reduction

(v) Reaction with alkalies

(vi) Miscellaneous reactions

1. Addition across C = O bond

2. Replacement of carbonyl oxygen atom with other groups

(a) Reaction with ammonia derivatives

Aldehydes & ketones react with a number of NH3 derivatives such as hydroxyl amine, hydrazine, semicarbazide etc, in weak acidic medium. In general, if we represent these derivatives by NH2 ⎯G, then their reaction with aldehydes & ketones can be represented as follows:

Ammonia derivatives & their products with carbonyl compounds

(b) Reaction with ammonia

Like ammonia derivatives, ammonia also reacts with aldehyde (except formaldehyde) & ketones to form the products, called imines.

However, formaldehyde reacts with NH₃ to form hexamethylene tetramine, (CH₂)₆N₄ also known as urotropine as shown below:

Acetone reacts with NH₃ to form diacetonamine

(c) Reaction with primary amines

Aldehydes & ketones react with 10 amines to form Schiff's bases. These compounds are also called imines.

(d) Reaction with PCl₅ or SOCl₂ (thionyl chloride)

Aldehydes or ketones with PCl₅ or thionyl chloride to form geminal dihalides.

3. Oxidation

Aldehydes are easily oxidised to carboxylic acids containing the same number of carbon atoms, as in parent aldehyde.

The reason for this easy oxidation is the presence of a hydrogen atom on the carbonyl carbon, which can be converted into ⎯OH group without involving the cleavage of any other bond. Hence, aldehydes are oxidised not only by strong oxidizing agent but also by weak oxidizing agents. As a result, aldehydes act as strong reducing agents.

• Aldehydes reduce Tollen’s reagent to Ag & appear in the form of silver mirror. This test is called silver mirror test. It is given by all aldehydes & reducing sugars.

• Aldehydes (except benzaldehyde) reduce Fehling’s solution (Cu⁺² reduced to Cu+) which is an alkaline solution of cupric (Cu²⁺) ion complexed with tartarate ion.

• Aldehydes also reduced Benedict’s solution (Cu2+ complexed with citrate ion) to Cu+.

• Aldehydes & ketones with a methyl or methylene group adjacent to the carbonyl group are oxidised by SeO₂

• Ketones are also oxidised by caro’s acid (H₂SO₅) or peroxybenzoic acid (C₆H₅CO₃H) to esters.

It is called Bayer villiger oxidation.

It is exactly oxygen insertion between carbonyl carbon & the larger of two groups attached to it.

Haloform Reaction

Due to the formation of yellow ppt. of iodoform in this reaction, it is known as iodoform test & used in for characterizing compound containing CH3CO⎯ or a group like CH3CH2OH which can be easily oxidised to CH₃CO⎯ group by halogens.

4. Reduction

Carbonyl compounds can be reduced to 1° or 2° alcohol, by LiAlH4, NaBH4 or direct reduction with H2/Ni.

(a)

with LiAlH₄ ⎯CHO group is reduced to ⎯CH₂OH (1° alcohol) and C = C bond is also reduced when it is in conjugation with carbonyl groups.

• LiAlH4 also reduces ester & acid chloride to alcohols.

(b) NaBH4 has similar function. But this reagent does not affects (C = C) double bond.

NaBH₄ does not reduce ester & acid chloride

(c) Amalgamated zinc, Zn(Hg) & conc. HCl (Clemmensen reduction) & hydrazine (NH₂ –NH₂) followed by reaction with strong base like KOH in alkaline glycol (Wolf Kishner reduction) reduces carbonyl group to alkyl group.

(d) Reduction to pinacol

5. Reaction with Alkalies

(A) Aldol Condensation

Two molecules of an aldehydes or a ketone having atleast one α - hydrogen atom, condense in presence of a dilute alkali to give a β - hydroxyaldehyde or β - hydroxy ketone.

The products of aldol condensation when heated with dilute acids undergo dehydration to form α, β - unsaturated aldehydes or ketones.

In general all aldehydes & ketones which contain α - hydrogen can undergo this reaction. Those which do not contain α - hydrogen like HCHO, C₆H₅CHO etc, do not undergo this reaction.

Mechanism

Mechanism involves formation of carbanions (i) a nucleophile form first molecules which is condensed with second molecule.

Aldol product on dehydration give α, β - unsaturated ketones.

(B) Cannizzaro’s reaction

Aldehydes that have no α-hydrogen atom (or acidic hydrogen) undergo cannizzaro reaction (CR) in which disproportionation reaction takes place one being reduced to alcohol & other being oxidised to salt of the corresponding acid. The reaction lakes place with 50% aqueous or ethanolic alkali solution.

When an aldehyde (showing CR) is treated with HCHO & 50% base, then HCHO undergo oxidation (rather than any other aldehyde). This reaction is called crossed CR.

CR involving different aldehydes or same aldehydes is proton (H+) hydride (H−) transfer reaction.

Mechanism

Step I

Step II

When the reaction is carried out in D₂O instead of in H₂O, it is found that there is no new C – D bond formation. This indicate that the hydrogen must come from aldehyde & not from the solvent.

(C) Perkin reaction

In this reaction aromatic aldehyde is heated with an acid anhydride & its corresponding sodium salt to form condensation products which on hydrolysis gives α, β - unsaturated acids. Acetic anhydride & sodium acetate are commonly used in this reaction.

6. Miscellaneous reactions

(i) Formation of phorone

Three moles of acetone condense in the presence of dry HCl to form phorone.

(ii) Formation of mesitylene

Three moles of acetone on refluxing with conc. Sulphuric acid produces mesitylene as one of the products.

(iii) Reaction with alc. KCN

On heating with ethanolic solution of KCN, two molecules of aromatic aldehyde undergo condensation to form benzoin. It is called benzoin condensation.

(iv) Reaction with chloroform

Ketones condenses with chloroform in presence of alkali to form chloretone.