Unit 7: Elimination Reactions
The same alkyl halides from the last unit, pushed a different way: a base rips off a proton, a pi bond forms, and the leaving group leaves.
5 topics · 19 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The E2 Mechanism and Anti-Periplanar Geometry
- How it worksHow does the proton and the leaving group need to line up anti-periplanar before an E2 elimination can happen?
- Worked exampleFind the E2 product formed when trans-1-bromo-4-tert-butylcyclohexane reacts with a strong, bulky base.
- Common mistakeWhy doesn't E2 proceed through a syn arrangement of proton and leaving group when an anti-periplanar path exists?
- Going furtherWhat happens to an E2 reaction's rate when a locked ring conformation has no proton anti-periplanar to the leaving group?
Your turn1 of 2
In trans-1-bromo-4-tert-butylcyclohexane, the bulky tert-butyl group locks the ring so it is always equatorial. Because of the trans-1,4 relationship, where does that leave the bromine, and what does this mean for a normal, fast E2 elimination?The E1 Mechanism and Carbocation Intermediates
- How it worksHow does a base pull a proton off the carbon next to a carbocation to form the new π bond of an E1 alkene?
- Worked exampleFind the major alkene product when tert-butyl bromide undergoes E1 in heated water and ethanol.
- Common mistakeWhy doesn't E1 need the departing proton to be anti-periplanar to the leaving group the way E2 does?
Your turn1 of 2
Tert-butyl bromide is heated in a water/ethanol mixture, favoring E1 elimination over substitution. What is the major alkene product?Zaitsev's Rule and the More Substituted Alkene
- How it worksHow does extra alkyl substitution around a double bond stabilize it through hyperconjugation into the empty p orbital?
- Worked exampleFind the Zaitsev product when 2-bromobutane undergoes E2 with a small, unhindered base like ethoxide.
- Common mistakeWhy doesn't Zaitsev's rule mean the more substituted alkene is the only product that can ever form?
- Going furtherWhat happens to the product ratio when one elimination path gives a trisubstituted alkene and the other only a disubstituted one?
Your turn1 of 2
2-bromobutane reacts with ethoxide, a small, relatively unhindered base, under E2 conditions. What is the Zaitsev, major, product?Hofmann Product with Bulky Bases
- How it worksHow does a bulky base like tert-butoxide reach past the more crowded proton to grab the more accessible one instead?
- Worked exampleFind the Hofmann product when 2-bromo-2-methylbutane undergoes E2 with the bulky base potassium tert-butoxide.
- Ties togetherHow does choosing between a small base and a bulky one give a chemist deliberate control over which alkene forms?
Your turn1 of 2
2-bromo-2-methylbutane undergoes E2 with the bulky base potassium tert-butoxide. What is the major, Hofmann, product?Predicting Substitution vs Elimination: SN1, SN2, E1, E2 Head to Head
- How it worksHow does substrate structure, primary versus secondary versus tertiary, rule out entire mechanisms before you check the reagent?
- How it worksHow does a reagent's strength and bulk decide between SN2 and E2, or between SN1 and E1, once the substrate allows both?
- Worked exampleFind the dominant mechanism and product when 1-bromobutane reacts with sodium ethoxide, a strong small base and nucleophile.
- Worked exampleFind the dominant mechanism and product when 2-bromo-2-methylbutane reacts with hot ethanol, a weak nucleophile and base.
- Ties togetherHow does laying SN1, SN2, E1, and E2 out on one decision tree turn four separate mechanisms into a single strategic choice?
Your turn1 of 2
1-bromobutane, a primary alkyl halide, reacts with sodium ethoxide, a strong, small base and nucleophile. What is the dominant mechanism and product?