Alkene addition reactions are reactions in which atoms add across the carbon-carbon double bond of an alkene. Students need this cheat sheet because many organic chemistry problems ask for the major product, the reagent type, and the regiochemistry. Markovnikov's rule is especially important because it predicts where hydrogen and the other group attach in unsymmetrical alkenes.
This reference helps organize the most common patterns in a clear, formula-forward way.
The key idea is that the alkene bond is electron-rich and reacts with electrophiles. In many acid-catalyzed additions, the reaction forms the more stable carbocation intermediate, so the major product follows Markovnikov orientation. Some reactions, such as hydroboration-oxidation, give anti-Markovnikov hydration without a carbocation.
Stereochemistry also matters because halogenation usually gives anti addition, while catalytic hydrogenation gives syn addition.
Key Facts
- In Markovnikov addition of to an unsymmetrical alkene, adds to the alkene carbon that already has more hydrogens.
- The general hydrohalogenation pattern is when the Markovnikov product is favored.
- Markovnikov's rule works because the pathway forms the more stable carbocation intermediate before the nucleophile attacks.
- Carbocation stability generally increases in the order because alkyl groups stabilize positive charge.
- Acid-catalyzed hydration converts an alkene to an alcohol using and acid, with the pattern .
- Hydroboration-oxidation adds water across an alkene with anti-Markovnikov regiochemistry using then .
- Halogenation adds or across the double bond to form a vicinal dihalide, such as .
- Catalytic hydrogenation adds across the double bond using a metal catalyst such as , , or .
Vocabulary
- Alkene
- An alkene is a hydrocarbon that contains at least one carbon-carbon double bond, written as .
- Addition reaction
- An addition reaction is a reaction in which atoms or groups add across a multiple bond to form a more saturated product.
- Markovnikov's rule
- Markovnikov's rule states that in many additions of or to an unsymmetrical alkene, adds to the carbon with more hydrogens.
- Carbocation
- A carbocation is an organic intermediate with a positively charged carbon, usually written as .
- Regiochemistry
- Regiochemistry describes which atom or group attaches to which carbon when more than one constitutional product is possible.
- Anti-Markovnikov addition
- Anti-Markovnikov addition places on the more substituted alkene carbon and the other group on the less substituted carbon.
Common Mistakes to Avoid
- Putting the halogen on the carbon with more hydrogens in Markovnikov hydrohalogenation is wrong because adds to that carbon and goes to the more substituted carbon.
- Ignoring carbocation stability is wrong because the major product usually comes from the pathway with the more stable carbocation, often over over .
- Treating hydroboration-oxidation as Markovnikov hydration is wrong because followed by gives anti-Markovnikov alcohol placement.
- Forgetting that halogenation adds two halogens is wrong because or adds one halogen to each alkene carbon to form a vicinal dihalide.
- Using Markovnikov's rule on a symmetrical alkene as if two different products form is wrong because both alkene carbons are equivalent and give the same product.
Practice Questions
- 1 Predict the major product of under normal conditions.
- 2 How many moles of are required to completely hydrogenate of cyclohexene, if each double bond consumes of ?
- 3 Predict the major alcohol product from treated with and .
- 4 Explain why acid-catalyzed hydration of propene gives as the major product instead of .
Understanding Alkene Addition Reactions and Markovnikov's Rule
A useful way to predict products is to track electrons before tracking atoms. The double bond contains a pi bond that can break and form two new single bonds. In hydrohalogenation, the hydrogen halide first gives a proton to one alkene carbon.
This leaves the other carbon electron-poor and positively charged. Then the halide ion bonds to that carbon. The preferred route is the one with the less unstable positive charge.
A carbon attached to more carbon groups can spread out some of the positive charge through nearby bonds. This is why product prediction is really a stability problem, not just a rule to memorize.
Carbocation rearrangements can change an expected answer. If the first carbocation sits next to a carbon that could form a more stable positive center, a hydrogen atom or an alkyl group may shift with its bonding electrons. The positive charge moves to a new location before the nucleophile attacks.
This can produce an alcohol or halide with a rearranged carbon skeleton. Students should check for this possibility whenever an acid reaction creates a secondary carbocation beside a potential tertiary one. Rearrangements do not occur in every problem, but ignoring them can lead to the wrong major product.
The reaction conditions tell you which mechanism to use. Water with a strong acid forms an alcohol through a carbocation, so rearrangement is possible and the addition is not stereospecific. Hydroboration followed by oxidation takes a different route.
Boron and hydrogen add in one connected step, with boron reaching the less substituted alkene carbon because that path has less crowding. Oxidation later replaces boron with oxygen without changing the carbon framework. Halogenation forms a bridged halonium ion rather than a free carbocation.
The halide ion then attacks from the opposite side, which explains anti addition. Hydrogenation happens on a metal surface, where both hydrogen atoms are delivered from the same side.
In real chemistry, these patterns help chemists turn simple petroleum based alkenes into alcohols, haloalkanes, plastics, medicines, and chemical intermediates. In class, draw the starting alkene clearly and label the two double bond carbons before adding anything. Decide whether the alkene is symmetrical, since orientation does not matter when both carbons are equivalent.
Next identify the reagent family, then predict the intermediate and any rearrangement. Finally check both regiochemistry and stereochemistry. A correct list of atoms is not enough if they are attached to the wrong carbon or placed on the wrong side of the former double bond.