Carbonyl Compounds

### Introduction to Carbonyl Compounds

Carbonyl compounds are a class of organic compounds characterized by the presence of a carbonyl group (C=O), a functional group where a carbon atom is double-bonded to an oxygen atom. This topic focuses on two main families of carbonyl compounds: aldehydes and ketones.

In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom, meaning it is always found at the end of a carbon chain. Their general formula is RCHO, where R can be an alkyl group, aryl group, or a hydrogen atom (in the case of methanal). In a ketone, the carbonyl group is bonded to two carbon atoms, meaning it is always found within a carbon chain. Their general formula is R-CO-R', where R and R' are alkyl or aryl groups.

The geometry around the carbonyl carbon is trigonal planar, with bond angles of approximately 120°. The C=O double bond is polar because oxygen is more electronegative than carbon. This creates a partial positive charge (δ+) on the carbon atom and a partial negative charge (δ-) on the oxygen atom. This polarity is the key to the reactivity of carbonyl compounds.

### Preparation of Carbonyl Compounds

Aldehydes and ketones are typically prepared by the oxidation of alcohols. The specific product depends on the class of the starting alcohol.

* Example: CH₃CH₂OH (Ethanol) + [O] → CH₃CHO (Ethanal) + H₂O

* Example: CH₃CH(OH)CH₃ (Propan-2-ol) + [O] → CH₃COCH₃ (Propanone) + H₂O

### Redox Reactions

1. Oxidation (Distinguishing Aldehydes from Ketones)

Aldehydes are easily oxidized, whereas ketones are resistant to oxidation. This difference forms the basis of simple chemical tests to distinguish between them.

* Tollens' Reagent: This is an aqueous solution of ammoniacal silver nitrate, containing the complex ion [Ag(NH₃)₂]⁺. When warmed with an aldehyde, the aldehyde is oxidized to a carboxylate ion, and the Ag⁺ ions are reduced to metallic silver, forming a characteristic silver mirror on the inside of the test tube. Ketones give no reaction.

* RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O

* Fehling's Solution: This is an alkaline solution containing copper(II) ions complexed with tartrate ions, which gives it a deep blue colour. When heated with an aldehyde, the aldehyde is oxidized to a carboxylate ion, and the blue Cu²⁺ ions are reduced to a red-orange precipitate of copper(I) oxide (Cu₂O). Ketones show no change.

* RCHO + 2Cu²⁺(aq) + 5OH⁻(aq) → RCOO⁻(aq) + Cu₂O(s) + 3H₂O(l)

2. Reduction

Both aldehydes and ketones can be reduced back to their corresponding alcohols. This is a form of nucleophilic addition. A common reducing agent used in the lab is sodium borohydride (NaBH₄) in aqueous or alcoholic solution.

* Reduction of Aldehydes produces primary alcohols.

* CH₃CHO (Ethanal) + 2[H] → CH₃CH₂OH (Ethanol)

* Reduction of Ketones produces secondary alcohols.

* CH₃COCH₃ (Propanone) + 2[H] → CH₃CH(OH)CH₃ (Propan-2-ol)

### Nucleophilic Addition Reactions

The most characteristic reaction of carbonyl compounds is nucleophilic addition. The electron-deficient (δ+) carbonyl carbon is readily attacked by nucleophiles (electron-pair donors).

Mechanism:

Reaction with Hydrogen Cyanide (HCN)

This reaction is a key example of nucleophilic addition and is important as it increases the length of the carbon chain by one carbon atom. Since HCN is a highly toxic gas, it is generated *in situ* by reacting potassium cyanide (KCN) or sodium cyanide with a dilute acid (e.g., H₂SO₄).

The product is a hydroxynitrile (or cyanohydrin).

* Example: CH₃CHO + HCN → CH₃CH(OH)CN (2-hydroxypropanenitrile)

Mechanism with :CN⁻:

### Test for the Carbonyl Group

To test for the presence of a carbonyl group in either an aldehyde or a ketone, 2,4-dinitrophenylhydrazine (2,4-DNPH), also known as Brady's reagent, is used. It reacts with carbonyl compounds to form a bright orange or yellow precipitate, which is a 2,4-dinitrophenylhydrazone derivative. The melting point of these crystalline derivatives is sharp and can be used to identify the specific aldehyde or ketone.