Organic Chemistry 1 · Substitution pathways
SN1 of tert-butyl bromide in aqueous ethanol: two products
Conditions: 80% EtOH / 20% H2O, 25 °C
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The mechanism, step by step
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Two molecules ionize to the same cation
Each tert-butyl bromide loses bromide in the slow, rate-determining step, independent of which solvent molecule will trap it. Every molecule gives the identical planar tert-butyl cation; two molecules are drawn so both outcomes can be shown.
The curved arrows
- The C–Br bond pair of the first molecule leaves with bromine as bromide.
- The C–Br bond pair of the second molecule also leaves with bromine, giving a second tert-butyl cation.
Water and ethanol each capture a cation
The cation is so reactive that it bonds to whichever solvent oxygen is next to it. Water traps one cation and ethanol traps the other, so the product ratio reflects the solvent composition and the two nucleophiles’ similar reactivity, not the rate of ionization.
The curved arrows
- A water lone pair forms a C–O bond to the first cation.
- An ethanol lone pair forms a C–O bond to the second cation.
Water removes both protons
Both oxonium ions are strong acids, and water is the best base in the mixture. Each loses its O–H proton to a water molecule, giving the neutral alcohol and the neutral ether.
The curved arrows
- A water lone pair takes the proton on the alcohol-forming oxonium ion.
- That O–H bond pair stays on oxygen, giving neutral tert-butyl alcohol.
- A second water takes the proton on the ether-forming oxonium ion.
- That O–H bond pair stays on the ether oxygen, which becomes neutral.
tert-Butyl alcohol and tert-butyl ethyl ether
The solvolysis gives both tert-butyl alcohol and tert-butyl ethyl ether from one common cation, plus hydronium bromide. Changing the water content changes the product ratio but barely changes which step is rate-determining.
What you'll practice
Draw the SN1 solvolysis of tert-butyl bromide in aqueous ethanol for two substrate molecules: one cation captured by water gives tert-butyl alcohol, the other captured by ethanol gives tert-butyl ethyl ether.
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