Organic Chemistry 2 · Synthesis roadmaps: reagents, protecting groups and multistep planning
Acid hydrolysis of 2-methyl-2-phenyl-1,3-dioxolane releases acetophenone
Conditions: 2-Methyl-2-phenyl-1,3-dioxolane, 1 M HCl (aq, catalytic H₃O⁺), acetone/water, 25 °C
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Drag from the electrons that move to where they go, or tap one and then the other. Every arrow is checked; Hint helps when you're stuck. No account needed.
The mechanism, step by step
Next plays each step's curved arrows and explains them. The written steps are below.
Read all 9 written steps
Hydronium protonates a ring oxygen
A lone pair on one ketal oxygen takes a proton from hydronium, turning that oxygen into a leaving group that can depart as part of a neutral alcohol.
The curved arrows
- The ring oxygen lone pair takes the proton.
- The O–H pair stays on oxygen, leaving water.
The ring opens to an oxocarbenium ion
The other ring oxygen pushes a lone pair toward the ketal carbon as the bond to the protonated oxygen breaks. The five-membered ring opens and the positive charge is shared by carbon and oxygen.
The curved arrows
- The oxygen lone pair forms a C=O⁺ π bond.
- The C–O bond breaks and the pair stays on the hydroxyl oxygen.
What drives the hydrolysis?
Ketal formation and hydrolysis are the same equilibrium run in opposite directions; the amount of water decides the direction.
Water adds to the oxocarbenium carbon
A water molecule from the solvent attacks the electrophilic carbon and the C=O⁺ π pair returns to oxygen.
The curved arrows
- The water lone pair forms the new C–O bond.
- The π pair returns to the ether oxygen, which becomes neutral.
Water removes a proton to give the hemiketal
Another water molecule takes a proton from the oxonium ion, giving the neutral hemiketal and hydronium.
The curved arrows
- A water lone pair takes the proton.
- The O–H pair stays on the new hydroxyl oxygen.
Hydronium protonates the glycol oxygen
The remaining ether oxygen, the one still attached to the CH₂CH₂OH chain, is protonated so that ethane-1,2-diol can leave.
The curved arrows
- The ether oxygen lone pair takes the proton.
- The O–H pair stays on oxygen, leaving water.
Ethane-1,2-diol leaves
The hydroxyl oxygen pushes a lone pair toward carbon as the bond to the protonated glycol oxygen breaks, giving protonated acetophenone and free ethane-1,2-diol.
The curved arrows
- The hydroxyl lone pair forms the C=O⁺ π bond.
- The C–O bond breaks and the diol departs.
Water deprotonates the carbonyl oxygen
Water removes the proton from the protonated ketone, giving acetophenone and regenerating hydronium, so the acid is a catalyst.
The curved arrows
- A water lone pair takes the proton.
- The O–H pair stays on oxygen as the C=O lone pair.
Acetophenone and ethane-1,2-diol
The protecting group is gone: acetophenone is released with ethane-1,2-diol, and hydronium is regenerated. The carbonyl oxygen of the ketone is the oxygen of the water molecule that added.
What you'll practice
Draw the acid-catalysed hydrolysis that removes an ethylene ketal protecting group: ring opening to the oxocarbenium ion, capture by water, and loss of ethane-1,2-diol to give acetophenone.
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