14.2.1—Preparation of alkenes
- Syllabus
- 9701–2028–2029
- Objective
- 14.2.1
- Level
- AS
Alkenes can be prepared by elimination, dehydration or cracking
Elimination removes atoms from neighbouring carbons to create C=C. Halogenoalkanes eliminate HX with hot ethanolic NaOH; alcohols dehydrate with heat and an acid or heated alumina; cracking also forms alkenes.
The reagent and conditions determine the route. Balance the equation and identify the small molecule removed—HX, H₂O or a fragment from a larger alkane.
CH₃CH₂Br + OH⁻(ethanol) → CH₂=CH₂ + H₂O + Br⁻. Ethanol can also dehydrate to ethene with concentrated sulfuric acid and heat.
Aqueous OH⁻ favours substitution more than hot ethanolic OH⁻, so do not ignore the solvent and temperature.
ConceptA-Level CAIE Chemistry AS