Ethylene glycol

One of the most common acetal formation reactions you see in organic synthesis is the use of ethylene glycol. It is widely used as automotive antifreeze. A sweet-tasting, colorless, odourless liquid, which will kill pets or humans if they drink enough of it.

The TsOH is para-Toluenesulfonic acid. It is as strong as sulphuric acid, but exists as a stable white solid which makes it convenient to use. It is also non-oxidizing. And it dissolves in organic solvents.

Mechanism:

Don't miss out the formation of the oxonium ion.

A  five-membered cyclic acetal is known as a dioxolane, as created in this example.

This forms readily even from ketones. 6-ring diols work too. I don't know why ethylene glycol is more common, presumably it is cheaper to make.

An example of the use of this protecting group is below:

The reaction works if we first protect the carbonyl. Since acetals are stable to a base (cyclic acetals moreso), the grignard reagent will only attack the Br. Once we have used the Grignard reagent to attack an electrophile, we can then hydrolyse the acetal off using acid and water.

Notice that water needs to be distilled off for the protecting group to be fully added. Thankfully, ethylene glycol has a boiling point of 197 C. If the reactants or solvent had a boiling point lower than water, then we could remove water using a Dean Stark Head with benzene.