Question

ОН ОН R) H2SO4 (cat.)

I need help working through the mech. for this problem. Thank you!

Answer 1

Acetal Formation:

The given reaction is a simple acetal formation reaction. The diol compound reacts with acetone in the presence of catalytic acid gives acetal compound. Important note: In the acetal formation reaction we have to use the catalytic amount of mineral acid like HCl, H₂SO₄ or organic acid like PTSA. Excess amount of mineral acid leads to acetal hydrolysis into diol (starting material).

Mechanism:

The mechanism is shown in the below image please go through it.

In the first step, acid protonates the carbonyl oxygen of acetone. Now the acetone (carbonyl carbon) become more electrophilic. The electrophilic carbon attacked by anyone of the diol oxygen. Now alcohol becomes positive to nullify the charge proton shift takes place. Further protonation leads to oxonium ion intermediate. (This oxonium ion intermediate is very important because OH group substitution follows the S_N1 mechanism). The formed oxonium ion attacked by adjacent OH group. To nullify the formed positive charge on oxygen proton shift takes place which will give the desired acetal compound.

Acetal Formation, HO H H₂SO4 (cat) Dior Acetal Mechanism Но le H te -OH HO OH Diol Hot - HO OOH COH - HD شان oxoniumion Jntermidiate H 0 0 Acetal