What is Carbonyl Oxidation?

Aldehyde and Ketone (Formaldehyde and Acetone)

Core Concepts

This article will cover key properties of the carbonyl group and what reactions this group participates in, particularly the oxidation reaction. From nucleophilic addition to carboxylic acid synthesis, carbonyl group oxidation remains a crucial mechanism in organic chemistry.

What Are Carbonyls?

Carbonyl groups are an organic functional group consisting of a carbon-oxygen double bond (C=O). Thus, carbonyl compounds are any molecules containing the carbonyl group, or this carbon-oxygen double bond. Examples of these compounds include aldehydes, carboxylic acids and ketones.

Formaldehyde and Acetone
Formaldehyde and Acetone

Aldehydes are functional groups where the carbonyl carbon is attached to only one carbon and one hydrogen atom, while ketones have two carbon substituents attached to the carbonyl carbon.

Properties of Carbonyl Compounds

The carbonyl carbon and oxygen in the compound is usually sp2 hybridized and has a bond angle of 120 degrees (or trigonal planar). Each carbonyl bond consists of a sigma carbon-oxygen single bond and a pi carbon=oxygen double bond. Most molecules containing this group are polar as the carbon-oxygen bond possess a dipole moment. Compared to other alkene groups (C=C double bonds), the carbonyl double bond is more reactive as the oxygen atom is more electronegative than the carbon atom.

The double bond lengths of C=O is approximately 1.2 angstroms while the bond enthalpy is around 177 kcal/mol.

Carbonyl Group Reactions

As a significant organic chemistry functional group, carbonyl groups are often involved in major reactions – whether these are synthesis mechanisms or reactions with other functional groups.

The main reactions involve nucleophilic additions to the carbon-oxygen double bond, an addition reaction that consists of adding a nucleophile and a hydrogen to the C=O double bond. This reaction results in polarization, where the carbon atom has a partial positive charge and the oxygen atom has a partial negative charge.

Water Addition Reactions

Adding water to an aldehyde group results in a hydrate formation which proceeds via a nucleophilic substitution mechanism. Water attacks the partial positive carbon atom and becomes an oxonium ion, which is then picked up by an oxygen anion in an acid-base reaction. This reaction is catalysed by both acids and bases, leading to a protonation of the oxygen atom.

Addition of Water to Acetaldehyde: Forming Hydrate/Geminal Diol
Addition of Water to Acetaldehyde: Forming Hydrate/Geminal Diol

Nucleophilic Addition Reactions

Carbonyl groups easily undergo nucleophilic addition – as one of the group’s more important reactions. This reaction can occur under two circumstances: base-catalysed (under basic conditions) and acid-catalysed (under acidic conditions). Either set of acidic or basic conditions are viable to conduct the reaction with.

Cyanide Ion Nucleophilic Addition to Aldehyde
Cyanide Ion Nucleophilic Addition to Aldehyde

Oxidation Reactions

With its carbon atom possessing a high oxidation state, carbonyl groups undergoes oxidation with the presence of a catalyst. The main and most common type of oxidation reaction converts the carbonyl group to a carboxylic acid.

Alcohols can undergo oxidation and become ketones or aldehydes – a result based on the alcohol’s substituents and whether the alcohol is primary or secondary.

Carbonyl Group Synthesis from Alcohols

Aldehyde Groups

Aldehydes are formed by oxidising the primary alcohol group in a compound. Based on whether either substituent group is a hydrogen atom, the resulting product is usually an aldehyde. This reaction produces water as the byproduct. Further oxidation of an aldehyde results in a carboxylic acid group due to the reactive C-H bond present.

Oxidation of Methanol to Formaldehyde and Water
Oxidation of Methanol to Formaldehyde and Water

Ketone Groups

Ketones are formed by oxidising the secondary alcohol group in a compound. The resulting product can undergo no further oxidation as ketones lack the reactive C-H bond found in aldehydes.

Oxidation of Propan-2-ol to Propanone and Water
Oxidation of Propan-2-ol to Propanone and Water

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