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.
Understanding Chemistry: Markovnikov's Rule
The double bond contains a pi bond, whose electrons are easier to reach than the electrons in a single bond. In an addition reaction, these electrons form a bond to the hydrogen part of the hydrogen halide. At the same time, the bond between hydrogen and the halogen breaks unevenly.
The halogen leaves with both bonding electrons and becomes a negative ion. The carbon that did not receive hydrogen is left electron poor. This short lived electron poor carbon is the key intermediate that controls much of the reaction.
Carbon atoms next to alkyl groups can help stabilize an electron poor carbon. Nearby carbon carbon and carbon hydrogen bonds share a little electron density through an effect called hyperconjugation. Alkyl groups also push electron density toward the positive center.
A carbocation with three carbon groups around it is therefore usually more stable than one with two, one, or none. This is why the first step favors one direction.
The carbocation is flat, so the halide ion can approach from either face. If this attack creates a new chiral center, the product may form as a mixture of mirror image forms.
Students should watch for carbocation rearrangements before deciding on the final product. A hydrogen atom or an alkyl group on a neighboring carbon can shift with its bonding electrons if the shift produces a more stable carbocation. For example, addition of hydrogen chloride to 3-methyl-1-butene first gives a secondary carbocation.
A hydrogen shift can then produce a tertiary carbocation. Chloride attacks this rearranged intermediate, giving 2-chloro-2-methylbutane.
The original double bond position does not always show the final position of the halogen. Drawing the intermediate makes these changes much easier to spot.
Reaction conditions matter because the usual ionic pathway is not the only possible pathway. Peroxides can start a radical chain reaction with hydrogen bromide. In that pathway, a bromine radical adds first, and the preferred direction is controlled by radical stability rather than carbocation stability.
This reversal is mainly useful for hydrogen bromide, not hydrogen chloride or hydrogen iodide under similar conditions. When solving product questions, first identify the reagent and any peroxide condition.
Then count the hydrogens on the alkene carbons, draw the likely intermediate, check for possible shifts, and finally place the incoming group. Writing each step is more reliable than relying on a memorized shortcut.
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.