Reaction In Msc. II Sem (Kumaun University)

Name Reactions

1. Vilsmeier reaction

2. Gattermann-Koch reaction

3. Sandmeyer reaction

4. Hunsdiecker reaction

5. Michael reaction

6. Sharpless asymmetric epoxidation

7. Aldol

8. Knoevenagel

9. Claisen

10. Mannich

11. Benzoin

12. Perkin

13.  Benzoin and Stobbe reactions (pdf)

14. Wittig reaction

15. Heck reaction

16. Still reaction

17. Sonogarishira

18. Negishi coupling

19. Grubbs Catalyst

Gattermann Koch Reaction

Gattermann Koch Reaction Mechanism begins with the formation of the reactive species with the help of the acid. The overall aim of the reaction is to attach a formyl group (-CHO group) to an aromatic system. An example of the Gattermann – Koch reaction is given below.

The Gattermann – Koch reaction is not applicable to phenol and phenol ether substrates. If zinc chloride is used as a catalyst in the Gattermann – Koch reaction, traces of copper(I) chloride is often necessary since it acts as a co-catalyst.

Gattermann – Koch Reaction Mechanism

Step 1

The first step of the Gattermann Koch reaction mechanism is the generation of the reactive species which can later be used to react on the aromatic ring. Since carbon monoxide acts as a lewis base, it can accept a proton from the hydrochloric acid. This results in a positively charged molecule which has different resonance structures. One such resonance structure displays a positive charge on the carbon, explaining the reactivity of the hybrid. This species can act as an electrophile while reacting with the aromatic ring. However, it is more likely to be the target of a nucleophilic attack from the chloride ion in the hydrochloric acid.

Step 2

When a Lewis acid (aluminium chloride) is added, it easily removes a chloride ion from the species. The species now reverts back to the reactive formyl cation.

Step 3

An electrophilic aromatic substitution occurs at the aromatic ring. The aromatic ring acts as a nucleophile and donates an electron pair to the formyl cation. The temporary loss of aromaticity is quickly solved by the expulsion of a proton.

The Vilsmeier-Haack reaction is an organic reaction used to convert an electron rich aromatic ring to an aryl aldehyde using DMF, an acid chloride, and aqueous work-up. The mechanism begins with the reaction of DMF with the acid chloride to form an iminium salt known as the "Vilsmeier reagent". The electron rich aromatic ring then attacks the iminium ion with loss of aromaticity. A deprotonation step restores aromati-city, which is followed by the release of a chloride ion to form another iminium intermediate. Aqueous work-up then leads to the aryl aldehyde final product.^[1]