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Carbonyl Chemistry


  • The carbon and oxygen are both \(sp^2\) hybridised
  • The bonds between the oxygen and carbon are one \(\sigma\) and one unhybridised p orbital resulting in a \(\pi\) bond
  • The oxygen lone pair electrons are held in the two remaining \(sp^2\) orbitals
  • The whole structure is planar
  • They are formed by the oxidation of alcohols
    • A primary alcohol can form a carboxylic acid or aldehyde
    • Secondary alcohols can only form ketones
  • A strong dipole is formed, with the electron density being pulled away from carbon, causing it to become electrophilic, with a nucleophilic oxygen.

Acid Activation

The reactivity of carbonyl groups can be increased by adding an acid. The protonation of the oxygen will give it a formal positive charge, making it more electronegative and causing a larger dipole to form between it and the carbonyl carbon. This makes it even more reactive to a nucleophilic attack.


There are four primary reactions of carbonyl groups as listed below.

  • Nucleophilic Addition
  • Nucleophilic Acyl Substitution
  • α-Substitution
  • Carbonyl Condensation

Nucleophilic Addition

General Mechanism


  • Grignard allow for the nucleophile to be a carbanion, allowing for carbon-carbon bond formation.
  • See Grignards in Organic Synthesis

Reducing Agents

  • Reducing agents can be used to convert ketones and aldehydes into alcohols or alkenes, with varying products
  • Some of these agents are:
Water Hydroxide Alcohols Alkoxides Cyanide
(\(\ce{NaCN, HCN}\)
\(\ce{NaBH4}\) \(\ce{LiAlH4}\)

Aldehydes/Ketones to alcohol + substituents

Depending on the nucleophile, a different alcohol product can be formed

  • Addition of water
  • Germinal diol formation
  • Acid or base catalysed

  • Addition of alkyl group
  • Alcohol formation
  • Acid catalysed

  • Addition of hydride
  • Alcohol formation
  • Acid catalysed

Aldehydes/Ketones to imine

Addition of a \(1^\circ\) amine to corbonyl group gives an imine

Hemiacetal Formation


Nucleophilic Acyl Substitution

Comparative Reactivity of Carboxylic Acid Derivatives

Amide Ester Thioster Anhydride Acid Chloride
Least Reactive Most Reactive

Leaving Groups

Since nucleophilic aryl substitution is a nucleophilic substitution, the suitability of the leaving group in an important factor to consider. Leaving groups also provide stability to the molecule by withdrawing electron density from the carbonyl carbon, causing it to be more electrophilic.

Carboxylic Acid Acid Chloride Ester Amide Anhydride Thioester Acyl Phosphate

This is further facilitated by conjugation mechanisms, due to the oxygen’s higher electronegativity. This can form even more negative charges on the oxygen and even more positive charges on the carbonyl carbon, activating it even further.

Steric hindrance also plays a vital role in the reaction, just like with the SN1 reaction

General Mechanism

Acid Chlorides

The carboxylic acid is usually converted to the more reactive acid chloride before being reacted further

Addition of a \(1^\circ\) or \(2^\circ\) amine

Addition of a \(1^\circ\) alcohol

Addition of carboxylic acid