Why do some organic reactions need anhydrous conditions?

Some organic reactions require anhydrous conditions to prevent unwanted side reactions with water.

In organic chemistry, many reactions are sensitive to the presence of water. This is because water can act as a nucleophile or a base, participating in the reaction and leading to different products than intended. Therefore, to ensure the reaction proceeds as planned, it is often necessary to carry out the reaction under anhydrous (water-free) conditions.

Anhydrous conditions are particularly important for reactions involving strong acids or bases, or reactive metal species. For example, in the Grignard reaction, an organomagnesium compound reacts with a carbonyl compound to form an alcohol. However, if water is present, the Grignard reagent will react with it to form a hydrocarbon and magnesium hydroxide, preventing the desired reaction from taking place.

Similarly, in the Friedel-Crafts alkylation, an alkyl halide reacts with an aromatic compound in the presence of a strong Lewis acid catalyst. If water is present, it can react with the catalyst, reducing its effectiveness and potentially leading to unwanted side reactions.

In addition to preventing unwanted reactions, anhydrous conditions can also help to drive reactions to completion. Many organic reactions involve the elimination of water as a by-product. If water is present in the reaction mixture, it can shift the equilibrium back towards the reactants, slowing down the reaction or preventing it from going to completion.

Therefore, to ensure that organic reactions proceed as planned, it is often necessary to use anhydrous solvents and reagents, and to carry out the reaction under anhydrous conditions. This can involve using techniques such as drying the reaction vessel and apparatus before use, using molecular sieves to absorb any water present, and carrying out the reaction under an inert atmosphere to prevent moisture from the air from entering the reaction mixture.