The reagent used in Gattermann Koch aldehyde synthesis is CO + HCl.

Gattermann Koch aldehyde synthesis is a method used to convert aromatic compounds into aldehydes. It involves the reaction of a benzene derivative with carbon monoxide (CO) in the presence of hydrogen chloride (HCl) as a catalyst. The reaction is typically carried out under high pressure and high temperature conditions.

Explanation:

The Gattermann Koch aldehyde synthesis proceeds through a complex mechanism involving several steps. Here is a step-by-step explanation of the reaction:

1. Formation of the electrophile:
- The carbon monoxide (CO) molecule reacts with the hydrogen chloride (HCl) to form an electrophilic species called chloromethylidene (CCl2).
- This reaction is facilitated by the presence of a Lewis acid catalyst such as aluminum chloride (AlCl3).

2. Attack of the electrophile:
- The electrophilic chloromethylidene attacks the aromatic ring of the benzene derivative.
- This attack results in the formation of a resonance-stabilized carbocation intermediate.

3. Rearrangement of the carbocation:
- The carbocation intermediate undergoes rearrangement to form a more stable carbocation.
- This rearrangement can involve shifts of hydrogen or alkyl groups.

4. Nucleophilic attack:
- A nucleophilic species such as chloride ion (Cl-) attacks the carbocation, resulting in the formation of a new carbon-carbon bond.
- This bond formation leads to the conversion of the benzene derivative into an aldehyde.

In summary, the reagents used in Gattermann Koch aldehyde synthesis are carbon monoxide (CO) and hydrogen chloride (HCl). These reagents react to form an electrophilic species that attacks the aromatic ring of the benzene derivative. The reaction proceeds through a series of steps, including the formation of a carbocation intermediate and nucleophilic attack. Ultimately, the reaction results in the conversion of the benzene derivative into an aldehyde.