Acetal formation: push the electrons yourself
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You just pushed every arrow of the acetal formation.
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Organic Chemistry 2 · Free interactive demo
Ethanal reacts with two molecules of methanol under acid catalysis to give its dimethyl acetal, 1,1-dimethoxyethane. The first methanol makes a hemiacetal; protonation lets the original carbonyl oxygen leave as water, and the second methanol adds. Both methanol oxygens stay bonded to the former carbonyl carbon.
Draw all 14 arrows across seven steps above. Each arrow is checked as you draw it, and Hint reveals the next move a little at a time.
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The acetal formation mechanism, step by step
Acetal formation is reversible: excess methanol and removal of water favor the acetal rather than the aldehyde. Acid first makes the carbonyl more electrophilic, then turns the hemiacetal OH into water, a much better leaving group than hydroxide. The remaining ether oxygen stabilizes the oxocarbenium intermediate before the second methanol attacks; water transfers protons and hydronium is regenerated at the end.
The 8 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
Protonate the carbonyl
Hydronium gives a proton to the carbonyl oxygen, making the carbonyl easier to attack. Hydronium becomes water.
Add the first methanol
The first alcohol attacks the carbonyl carbon. Its oxygen gains a bond and a positive charge.
Remove a proton
Water removes a proton from the incoming alcohol oxygen. This gives a neutral hemiacetal.
Turn OH into water
Add a proton to the OH group on the former carbonyl carbon. It can now leave as water.
Let water leave
The ether oxygen forms a π bond to carbon as the original carbonyl oxygen leaves in water.
Add the second methanol
The second alcohol attacks carbon. Move the C=O π electrons onto oxygen so carbon keeps eight electrons.
Return the acid catalyst
Water removes the proton from the positively charged oxygen. The acetal becomes neutral and hydronium returns.
Products
Both methanol oxygens are now neutral ether oxygens bonded to the former carbonyl carbon. The original carbonyl oxygen leaves in water with two hydrogens. Hydronium is regenerated.
How the mechanism is drawn
Ethanal in excess methanol with an acid catalyst; removing water favors acetal formation. Draw both methanol additions and use water to transfer protons. Counterions are not drawn; no stereochemistry is assigned without bond markers.