Aromatic Reactions: Gatterman-Koch Formylation

Benzene molecules react with carbon monoxide (CO) and hydrochloric acid (HCl), catalyzed by aluminum chloride (AlCl3), to undergo a reaction known as the Gatterman–Koch Formylation. In this reactions, AlCl3 helps catalyze the reaction to add a aldehyde group to the benzene molecule. The location of the addition depends on what functional group(s) is present on the benzene molecule prior to the reaction starting. If the specific position is occupied by another functional group, the reaction will not occur as there is not and available location for the addition to happen:

Gatterman–Koch Formylation Electron Withdrawing Group (EWG)

Gatterman–Koch Formylation Electron Donating Group (EDG)

Mechanism

The reaction mechanism for is depicted below:

In the first step, the CO molecule reacts with HCl to prepare it for the next step of the reaction by having the electrons from the negatively charged C atom pull the H atom off of HCl. The free Cl then attaches to the AlCl3 molecule to form AlCl4, which will be used in the foruth step.

In the second step, the CO triple bond is broken and the pi electrons make their way to the O atom. AlCl3 is in close proximity to stabilize the O atom.

In the third step, the benzene bond reacts with the positively charged C atom, attaching the aldehyde moiety to the benzene.

In the fourth step, the Cl atom from the AlCl4 molecule attacks the H atom on the benzene, thereby reforming HCl and re-establishing the benzene ring.

Make sure you learn which groups are considered electron withdrawing and electron donating as professors like to trip students up on this topic. Additionally, check for open positions on the benzene ring; if the para position had been occupied in the mechanism example above, the reaction could proceeded on either ortho position.

Practice this reaction using our Reaction Solver!