Q.Give two laboratory methods, with equations, for preparing an alkene, one starting from an alkyl halide and one from an alcohol.

Dehydrohalogenation. Heating an alkyl halide with potassium hydroxide dissolved in ethanol (alcoholic KOH; aqueous KOH instead favours nucleophilic substitution to the alcohol) eliminates a hydrogen and the halogen from two adjacent carbons: $\text{R--CH}_2\text{--CH}_2\text{--X} \xrightarrow{\text{alc. KOH},\Delta} \text{R--CH=CH}_2 + \text{HX}$. Dehydration. Heating the corresponding alcohol with concentrated sulphuric acid (or passing its vapour over hot alumina) instead eliminates a molecule of water from two adjacent carbons: $\text{R--CH}_2\text{--CH}_2\text{--OH} \xrightarrow{\text{conc. H}_2\text{SO}_4,\Delta} \text{R--CH=CH}_2 + \text{H}_2\text{O}$. In both routes, if the starting material could eliminate towards more than one direction (i.e. it is not on a terminal c …

For each starting material, identify the leaving group/atom to be removed (halide ion, or hydroxyl as water after protonation) and the adjacent hydrogen removed alongside it, and quote t …

Using the wrong solvent for the halide route (aqueous instead of alcoholic KOH gives substitution, not elimination); omitting the heat/concentrated-acid condition for dehydration, whic …