Alkene Reactions Practice- Organic Chemistry Exercises

Alkene Reactions Practice: The Only Guide You Need

Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds. They're reactive, predictable, and show up constantly on exams. If you're struggling with alkene reactions, you probably need more practice, not more theory.

Let's fix that.

Why Alkenes Behave the Way They Do

The C=C double bond is electron-rich. Those π electrons hang out above and below the plane, exposed and ready to react. Electrophiles love them. That's why addition reactions dominate alkene chemistry.

Markovnikov's rule, carbocation stability, and stereochemistry—these aren't optional extras. They're the rules that let you predict products. Know them cold.

The Six Reactions You Must Master

Most exam questions come from this short list. Memorize the mechanism, the regioselectivity, and the stereochemistry for each.

1. Hydrogenation

Add H₂ across the double bond. You need a metal catalyst—Pd/C, Pt, or Ni. The reaction is syn addition, meaning both hydrogens land on the same face of the double bond.

Example:

CH₂=CH₂ + H₂ → CH₃-CH₃

Straightforward. No regioselectivity issues with symmetric alkenes. With asymmetric alkenes, both faces are equally accessible, so you get a racemic mixture if stereochemistry matters.

2. Halogenation (Bromination or Chlorination)

Add Br₂ or Cl₂ across the double bond. This is anti addition—the two halogens end up on opposite faces. The mechanism goes through a cyclic bromonium ion intermediate.

Example:

CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-Br

The reddish-brown color of Br₂ disappears when it reacts with an alkene. This is a classic test for unsaturation in organic chemistry labs.

3. Hydrohalogenation (HX Addition)

Add H-X where X = Cl, Br, or I. Markovnikov's rule applies here. The hydrogen attaches to the carbon with more hydrogens already attached. The halogen lands on the more substituted carbon.

Example:

CH₃-CH=CH₂ + HBr → CH₃-CH(Br)-CH₃

With HBr and peroxides, you get anti-Markovnikov addition. The mechanism changes—it's a free radical process, not electrophilic addition.

4. Hydration (Acid-Catalyzed Water Addition)

Add H₂O across the double bond with acid catalysis. Markovnikov orientation applies. The OH ends up on the more substituted carbon.

Example:

CH₃-CH=CH₂ + H₂O → CH₃-CH(OH)-CH₃

You can also do this with oxymercuration-demercuration or hydroboration-oxidation. These give different regioselectivity and stereochemistry.

5. Oxymercuration-Demercuration

Add water with Hg(OAc)₂ followed by NaBH₄. This gives Markovnikov hydration without rearrangement. The OH lands on the more substituted carbon, no carbocation rearrangement occurs.

Example:

CH₃-CH=CH₂ → CH₃-CH(OH)-CH₃

This is useful when you need Markovnikov addition but can't risk carbocation rearrangements.

6. Hydroboration-Oxidation

Add BH₃ followed by H₂O₂/NaOH. This gives anti-Markovnikov hydration. The OH lands on the less substituted carbon. It's also syn addition—both H and OH add to the same face.

Example:

CH₃-CH=CH₂ → CH₃-CH₂-CH₂OH

This is the go-to method when you need to put an OH on the terminal carbon.

Oxidation Reactions

Alkenes get oxidized by various reagents. The product depends on the oxidant and the substitution pattern of the alkene.

• OsO₄ (Osmium tetroxide): Dihydroxylation. Adds two OH groups syn. Gives vicinal diols.
• O₃ / (CH₃)₂S: Ozonolysis. Breaks the double bond. Symmetrical alkenes give two identical carbonyl compounds. Unsymmetrical alkenes give two different products.
• Hot KMnO₄: Cleaves the double bond completely. Terminal alkenes give CO₂. Internal alkenes give carboxylic acids or ketones.
• mCPBA: Epoxidation. Forms an epoxide (three-membered ring with oxygen). Syn addition stereochemistry.

Quick Reference: Alkene Reaction Products

How to Solve Alkene Reaction Problems

Most students freeze when they see a reaction scheme. Here's the process that works:

Step 1: Identify the Alkene

Find the C=C double bond. Count the substituents on each carbon of the double bond. This tells you about carbocation stability if rearrangement is possible.

Step 2: Identify the Reagent

Match the reagent to the reaction type. If you see H₂ with a metal catalyst, it's hydrogenation. If you see HBr, it's hydrohalogenation. If you see acid and water, it's hydration.

Step 3: Apply the Selectivity Rules

Ask these questions:

• Does Markovnikov's rule apply? If yes, H goes to the less substituted carbon.
• Is there a carbocation intermediate? If yes, expect rearrangement to more stable carbocations.
• What stereochemistry is required? Syn or anti?

Step 4: Draw the Product

Break the double bond and form new bonds according to the mechanism. Add hydrogens where needed. Check your answer against the rules.

Practice Problem

Draw the product of this reaction and name it:

2-butene + Br₂ → ?

Solution:

2-butene is CH₃-CH=CH-CH₃. Adding Br₂ across the double bond gives CH₃-CH(Br)-CH(Br)-CH₃. The product is 2,3-dibromobutane.

Because the reaction is anti addition, you get a meso compound if the starting alkene is cis, or a racemic mixture of enantiomers if the starting alkene is trans.

Common Mistakes to Avoid

• Forgetting Markovnikov's rule on HX addition. H always goes to the carbon with more hydrogens.
• Confusing regioselectivity with stereochemistry. Regioselectivity is about which carbon gets the new group. Stereochemistry is about spatial arrangement.
• Missing carbocation rearrangements. Hydride and methyl shifts happen when they form a more stable carbocation.
• Drawing the wrong stereochemistry. Syn means same side. Anti means opposite sides. Know which reagents give which.
• Forgetting that hydroboration-oxidation is anti-Markovnikov. Students mix this up with acid-catalyzed hydration.

Study Strategy That Actually Works

You don't need to read more. You need to practice. Here's what to do:

1. Draw the six basic reactions from memory. Check your work.
2. Do 20 synthesis problems where you start with an alkene and build a target molecule.
3. Work backwards from products to starting materials for ozonolysis problems.
4. Time yourself on mechanism problems until you can do them in under 3 minutes.

Organic chemistry is a skill. You build it by doing, not by reading summaries.

When You're Stuck

If a problem asks for a product and you don't know where to start:

1. Identify all functional groups in the starting material and product.
2. Figure out what bonds formed or broke.
3. Match the reagent to a reaction type.
4. Apply the selectivity rules for that reaction.

That's it. No magic. Just follow the process.