Properties of Carbonyl Compounds

Physical properties of carbonyl compounds

In comparison to alcohols and carboxylic acids, carbonyl compounds have lower boiling points due to the limited hydrogen bonding present. The hydrogen bonding between molecules of carbonyl compounds is weaker compared to alcohols and carboxylic acids.

Among carbonyl compounds themselves, the boiling point increases with the increase in the number of carbon atoms. Carbonyl compounds are slightly soluble in water due to their ability to form hydrogen bonds with water molecules, but they are more soluble in organic solvents.

Chemical reactions of carbonyl compounds

Nucleophilic addition reactions

In a carbonyl group, oxygen is more electronegative than carbon, making oxygen negatively polarized and carbon positively polarized. This makes the carbonyl carbon electrophilic, making it a good site for incoming nucleophiles. Carbonyl compounds are more likely to undergo nucleophilic addition reactions because of the π-bond present in the carbonyl group.

The ability of a carbonyl compound to undergo nucleophilic addition reactions depends on the amount of partial positive charge on the carbonyl carbon. If the carbon is more positively polarized, the compound will be more likely to undergo nucleophilic addition reactions.

In general:

1. Aldehydes are more reactive than ketones due to less steric hindrance and stronger inductive effects in ketones.
2. Higher members of aldehydes are less reactive due to the increasing length of alkyl chains that exert a positive inductive effect.

Examples of nucleophilic addition reactions:

Reaction with hydrogen cyanide (HCN)

Carbonyl compounds react with hydrogen cyanide in the presence of sodium cyanide and a strong acidic medium like HCl, yielding cyanohydrins.

R–CHO + HCN → R–C(OH)CN

Formation of bisulphite adducts

Carbonyl compounds react with sodium bisulphite (NaHSO₃) to form bisulphite adducts. This reaction is hindered in aromatic ketones due to large steric hindrance.

Reaction with alcohols

Aldehydes react with alcohols to form hemiacetals, which are unstable. When alcohol is in excess, stable acetals are formed.

RCHO + R'OH → RCH(OH)OR' → Acetal

Nucleophilic substitution reactions

Carbonyl compounds can undergo nucleophilic substitution with halogenating agents, where the oxygen in the carbonyl group is replaced by halogens.

RCOOH + X₂ → RX + HX

Formation of alcohols (reduction)

Carbonyl compounds react with reducing agents like lithium aluminum hydride (LiAlH₄) to form alcohols:

• Aldehydes form primary alcohols.
• Ketones form secondary alcohols.

Example

RCHO + [H] → RCH₂OH (Primary Alcohol)

Oxidation of aldehydes

Aldehydes are good reducing agents and can be oxidized to carboxylic acids:

RCHO + [O] → RCOOH

Reaction with hydrazine

Carbonyl compounds react with hydrazine to form hydrazones:

RCHO + NH₂NH₂ → RCH=NNH₂

Chemical tests to distinguish between aldehydes and ketones

Fehling's / Benedict's solution

Aldehydes react with Fehling's or Benedict's solution to give a brick-red precipitate of copper (I) oxide, while ketones do not react and give a negative test.

RCHO + Cu(OH)₂ → Cu₂O (Brick-red ppt)

Tollen's reagent (silver mirror test)

Aldehydes reduce Tollen's reagent to form a silver mirror, while ketones do not give this reaction.

RCHO + [Ag(NH₃)₂]⁺ → Ag (Silver Mirror)

Iodoform test

The iodoform test detects the presence of a terminal methyl group attached to a carbonyl group, forming a yellow precipitate of iodoform.

RCOCH₃ + I₂ → CHI₃ (Yellow ppt)