Prins reaction template

Acid protonates formaldehyde; 2-methylpropene attacks it to give the tertiary carbocation, which water traps: 3-methylbutane-1,3-diol after loss of a proton.

Mechanism step by step

An alkene adds to protonated formaldehyde; water traps the cation to give a 1,3-diol

1. Protonation – Acid protonates formaldehyde: an oxocarbenium ion
2. C–C bond formation – The alkene attacks through its CH₂ end, giving the more stable tertiary cation (Markovnikov)
3. Water traps the cation – Water adds to the tertiary carbocation
4. Deprotonation – Water takes a proton: the 1,3-diol and the acid catalyst back

Key points

• Electrophilic addition of an alkene to a protonated aldehyde (usually formaldehyde): a new C–C bond and a 1,3-relationship of the two oxygens.
• The product depends on the conditions: with water a 1,3-diol; without a nucleophile loss of H⁺ gives an allylic or homoallylic alcohol (2-methylpropene → 3-methylbut-3-en-1-ol); with excess formaldehyde a 1,3-dioxane.
• Markovnikov: the alkene bonds through the carbon that leaves the more stable carbocation.
• Lewis-acid and intramolecular Prins cyclisations make tetrahydropyrans (the oxa-Prins route in natural-product synthesis).

Drawn with 63 atoms, 12 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 16 Textbook