--- title: "Elimination and Choosing Between Two Pathways" description: "The same haloalkane and the same hydroxide can give an alcohol or an alkene. Concentration, solvent and temperature decide whether the hydroxide acts as a nucleophile or as a base." canonical: https://lightmysky.com/learn/science/elimination-and-choosing-between-two-pathways-mt_tRj_kiSS51 source: https://lightmysky.com/learn/science/elimination-and-choosing-between-two-pathways-mt_tRj_kiSS51.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Elimination and Choosing Between Two Pathways The same haloalkane and the same hydroxide can give an alcohol or an alkene. Concentration, solvent and temperature decide whether the hydroxide acts as a nucleophile or as a base. Subject: Science · Area: Chemistry · Ages 16 to 18 Page: https://lightmysky.com/learn/science/elimination-and-choosing-between-two-pathways-mt_tRj_kiSS51 ## Ready when they can - Draws the elimination mechanism, showing hydroxide taking a hydrogen and the double bond forming - States the conditions that favour substitution and those that favour elimination - Predicts the products, including which alkene isomers form when there is a choice - Explains what it means to call one species both a nucleophile and a base ## Lesson: One reagent, two pathways The same haloalkane with the same hydroxide can give an alcohol or an alkene. In substitution the reagent forms a new bond to carbon. In elimination a base pulls a hydrogen off the next carbon while the leaving group leaves, and the two carbons form a double bond instead. Hydroxide can play either role, which is why one species counts as both a nucleophile and a base. Conditions decide the contest. Substitution wins when things are gentle: lower temperature, a good nucleophile that is only a weak base, dilute cool solution, and polar solvents that comfort ions. Elimination wins with a strong base, hot concentrated solution, a bulky base that cannot easily reach carbon, or a crowded starting material. Read the conditions first, then call the pathway. **Example.** Take bromopropane with hydroxide. In dilute cool aqueous solution hydroxide attacks carbon and substitutes, giving propanol. With hot concentrated hydroxide it grabs a hydrogen instead and eliminates, giving propene. Draw both mechanisms side by side: one arrow to carbon versus one arrow to a neighbouring hydrogen. **Tip.** An unsymmetrical haloalkane can give more than one alkene, since hydrogens on different neighbouring carbons lead to double bonds in different places. List every distinct neighbouring hydrogen, draw each resulting alkene with its cis and trans versions, then pick the major product: usually the more substituted alkene, the one with more carbon groups on the double bond, as extra alkyl groups steady it. **Recap.** Read the conditions to call the pathway, then list every alkene before picking the major one. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Alkanes and Alkenes: The First Homologous Series](https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL) - [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW) ## Opens up - [Alcohols: Classification, Oxidation and Dehydration](https://lightmysky.com/learn/science/alcohols-classification-oxidation-and-dehydration-mt_FleYaf40y3) - [E1, E2 and E1cb: Three Ways to Lose Two Groups](https://lightmysky.com/learn/science/e1-e2-and-e1cb-three-ways-to-lose-two-groups-mt_vVq_xeZrKv)