Organic Chemistry 1 · Stereocenters and configuration
Base closes (2S,3R)-3-bromobutan-2-ol to a trans epoxide
Conditions: NaOH, H2O
Draw this problem yourself
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 3 written steps
Hydroxide deprotonates the alcohol
Hydroxide removes the O–H proton of the bromohydrin in a fast, reversible acid–base step. The small amount of alkoxide formed is a far stronger nucleophile than the neutral alcohol and sits on the carbon next to the C–Br bond.
The curved arrows
- A lone pair on hydroxide bonds to the hydroxyl proton of the bromohydrin.
- The O–H bond pair stays on the oxygen, turning it into a negatively charged alkoxide.
The alkoxide displaces bromide from behind
The alkoxide attacks the neighbouring C–Br carbon in an intramolecular SN2 reaction, which needs the oxygen anti-periplanar to bromine. Only the bromine-bearing carbon inverts; in the reacting conformer the two methyl groups sit anti, so they end up on opposite faces of the three-membered ring.
The curved arrows
- A lone pair on the alkoxide oxygen forms the new C–O ring bond from the side opposite bromine.
- The C–Br bond pair leaves with bromine as bromide as the ring closes, inverting that carbon.
(2S,3S)-2,3-Dimethyloxirane
The methyls are trans on the ring, so the product is the chiral trans epoxide, not the meso cis isomer. The alcohol carbon keeps its S configuration, and the inverted carbon is now S as well, giving (2S,3S)-2,3-dimethyloxirane; the (2S,3S) bromohydrin would instead give the cis epoxide.
What you'll practice
Draw the base-promoted ring closure of a bromohydrin and use the anti-periplanar requirement to assign the trans epoxide and its (2S,3S) configuration.
You just pushed every arrow of this one.
Every one of the 5349 Orgo 1 and Orgo 2 problems works like this: feedback on each arrow, hints when you're stuck, a no-hint quiz mode. Free with an account, no card.
Stuck on your own homework?
Snap it — first scan free, no sign-up
Scanning every week? Scan Standard: for about problems