Chapter 18 · Organic Chemistry
18.9Hydrocarbon Reactions
9 min · two checks
Predict
What does an alkene do with Br₂ that an alkane does not readily do?
The idea
- Contrast addition and substitution.
- Write the product of adding H₂ or Br₂ to a simple alkene.
Alkanes undergo substitution: one H is replaced, often by a halogen under light or heat, and the carbon chain stays saturated. Alkenes and alkynes undergo addition: the multiple bond opens and two new atoms attach, one to each carbon. Adding H₂ (hydrogenation) makes an alkane. Adding Br₂ makes a dibromo compound and decolorizes bromine water, a classic test for unsaturation. Adding H₂O under the right conditions makes an alcohol.
Markovnikov’s rule, for adding H–X to an unsymmetrical alkene, says the H goes to the carbon that already has more hydrogens. You can memorize it as “the rich get richer.” The underlying reason is the stability of the intermediate, which a later course develops. For this course, predict the addition product and do not draw a five-bonded carbon.
Keep these
- Alkanes: substitution. Alkenes: addition across the multiple bond.
- Hydrogenation saturates a double bond.
- Bromine addition is a test for unsaturation.