Organic Chemistry 1 · Carbocation rearrangements and ring expansions
Acid-catalysed ethanol addition to 3-methylbut-1-ene
Conditions: CH3CH2OH, H2SO4 (cat.)
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The mechanism, step by step
Next plays each step's curved arrows and explains them. The written steps are below.
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Protonate the alkene
Sulfuric acid protonates the ethanol solvent, and protonated ethanol transfers its proton to the terminal CH2 of the alkene. The secondary cation that forms sits next to the isopropyl CH.
The curved arrows
- The C=C π electrons form the C–H bond on the terminal carbon.
- The O–H bonding pair stays on oxygen, releasing neutral ethanol.
1,2-Hydride shift
The hydrogen on the isopropyl CH moves with its pair to the secondary cation, giving the tertiary 2-methylbutan-2-yl cation.
The curved arrows
- The C–H bond pair carries the hydride to the adjacent cationic carbon.
Ethanol adds to the cation
An ethanol oxygen lone pair bonds to the tertiary cation, forming a protonated ether. Ethanol is the solvent, so it captures the cation far more often than any other nucleophile.
The curved arrows
- An ethanol lone pair forms the new C–O bond to the tertiary carbon.
Deprotonate the oxonium ion
A second ethanol removes the proton from the oxonium oxygen, giving the neutral ether. Protonated ethanol is regenerated, so the acid is catalytic.
The curved arrows
- An ethanol lone pair takes the proton from the oxonium ion.
- The O–H bonding pair stays on the ether oxygen.
2-Ethoxy-2-methylbutane
The ethoxy group is on the tertiary carbon reached by the hydride shift, not on the carbon next to the original alkene. The product is achiral.
What you'll practice
Draw acid-catalysed addition of ethanol to 3-methylbut-1-ene: protonation, the 1,2-hydride shift, ethanol capture and deprotonation to 2-ethoxy-2-methylbutane.
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