What a learner can do afterwards
- 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
1 · Read
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 such as water that stabilise ions. Elimination wins with a strong base dissolved in ethanol, hot concentrated solution, a bulky base that cannot easily reach carbon, or a crowded starting material. Read the conditions first, then call the pathway.
Take bromopropane with hydroxide. In dilute cool aqueous solution hydroxide attacks carbon and substitutes, giving propanol. With hot concentrated hydroxide in ethanol it grabs a hydrogen instead and eliminates, giving propene. Draw both side by side. In substitution, hydroxide's lone pair goes to carbon and the C-Br bond pair goes to bromine. In elimination, the lone pair goes to a hydrogen on the next carbon, that C-H bond pair swings in to make the C=C, and the C-Br bond pair goes to bromine.
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 E and Z 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.
Read the conditions to call the pathway, then list every alkene before picking the major one.
Readings 'Two alkenes from 2-bromobutane' and 'Read the conditions first' are original to this dataset.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.