Markovnikov's rule predicts the major product when a hydrogen halide such as HCl, HBr, or HI adds across an unsymmetrical alkene. It matters because the two carbons of the double bond are not equivalent, so two different constitutional products are often possible. The rule says that hydrogen adds to the alkene carbon that already has more hydrogens, while the halide adds to the more substituted carbon.
This shortcut helps chemists predict reaction products quickly and draw correct mechanisms.
Key Facts
- Markovnikov's rule: in HX addition to an unsymmetrical alkene, H adds to the carbon with more H atoms, and X adds to the more substituted carbon.
- General reaction: RCH=CH2 + HX gives RCHX-CH3 as the major Markovnikov product.
- The reaction usually proceeds through protonation of the alkene followed by attack by X- on a carbocation.
- Carbocation stability trend: 3° > 2° > 1° > methyl.
- For propene + HBr, the major product is 2-bromopropane: CH3-CH=CH2 + HBr gives CH3-CHBr-CH3.
- Anti-Markovnikov HBr addition can occur with peroxides, where Br adds to the less substituted carbon by a radical mechanism.
Vocabulary
- Markovnikov's rule
- A rule predicting that during HX addition to an unsymmetrical alkene, hydrogen attaches to the double-bond carbon with more hydrogens.
- Unsymmetrical alkene
- An alkene in which the two carbons of the C=C double bond have different groups attached.
- Carbocation
- An organic ion with a positively charged carbon atom that is electron deficient.
- More substituted carbon
- A carbon atom bonded to a larger number of carbon groups and fewer hydrogen atoms.
- Anti-Markovnikov product
- The product in which the added hydrogen and halogen appear in the opposite positions from the Markovnikov prediction.
Common Mistakes to Avoid
- Putting the halide on the carbon with more hydrogens, which reverses the rule and usually gives the minor product for normal HX addition.
- Ignoring carbocation stability, which is wrong because the major pathway forms the more stable carbocation intermediate before halide attack.
- Applying the peroxide effect to HCl or HI, which is wrong because the common radical anti-Markovnikov pathway is reliable mainly for HBr.
- Forgetting possible rearrangements, which is wrong because carbocations can sometimes shift hydride or alkyl groups to form a more stable carbocation before the final product forms.
Practice Questions
- 1 Predict the major product when 1.00 mol of propene reacts with 1.00 mol of HCl under normal Markovnikov conditions. Name the product and draw its condensed structure.
- 2 2-methylpropene reacts with HBr without peroxides. How many distinct constitutional products are expected from simple addition, and which one is the major product? Draw the major product.
- 3 Explain why protonation of 1-butene at the terminal carbon gives the major Markovnikov product, using carbocation stability in your reasoning.