Alkenes contain a carbon-carbon double bond that makes them more reactive than alkanes. The double bond has a sigma bond and a pi bond, and the pi electrons are exposed enough to attack electrophiles. Addition reactions matter because they convert simple alkenes into many useful products, including alcohols, haloalkanes, and saturated hydrocarbons.
These reactions are central in organic synthesis, fuels, polymers, and pharmaceutical chemistry.
In most alkene additions, the pi bond breaks and two new sigma bonds form across the two alkene carbons. Electrophilic addition often begins when the alkene attacks an electron-poor species, forming a carbocation or a cyclic intermediate. The structure of the intermediate controls regioselectivity, stereochemistry, and possible rearrangements.
Common pathways include hydrogenation with a metal catalyst, halogenation with Br2 or Cl2, and hydration to form alcohols.
Key Facts
- An alkene has a C=C double bond made of one sigma bond and one pi bond.
- General addition pattern: C=C + A-B gives A-C-C-B.
- Hydrogenation: alkene + H2 with Pt, Pd, or Ni gives an alkane.
- Halogenation: alkene + Br2 or Cl2 gives a vicinal dihalide.
- Acid-catalyzed hydration: alkene + H2O with H+ gives an alcohol, often following Markovnikov addition.
- Markovnikov rule: in HX or H2O addition, H usually adds to the alkene carbon that already has more H atoms, forming the more stable carbocation.
Vocabulary
- Alkene
- An unsaturated hydrocarbon that contains at least one carbon-carbon double bond.
- Electrophile
- An electron-poor atom, ion, or molecule that accepts an electron pair during a reaction.
- Carbocation
- A positively charged carbon species that often forms as an intermediate in electrophilic addition.
- Regioselectivity
- The preference for a reaction to form one constitutional product over another when more than one product is possible.
- Hydration
- An addition reaction in which H and OH are added across a double bond to form an alcohol.
Common Mistakes to Avoid
- Treating the double bond as two separate single bonds, which is wrong because the pi bond is the reactive part that breaks during addition.
- Putting the electrophile on the wrong carbon in Markovnikov addition, which is wrong because the reaction usually forms the more stable carbocation intermediate.
- Forgetting the catalyst in hydrogenation, which is wrong because H2 does not normally add to an alkene at a useful rate without Pt, Pd, or Ni.
- Ignoring stereochemistry in halogenation, which is wrong because Br2 and Cl2 often add through a cyclic halonium ion and give anti addition.
Practice Questions
- 1 Draw the major product when propene, CH3CH=CH2, reacts with HBr under normal ionic conditions.
- 2 Cyclohexene reacts with H2 in the presence of Pt. Write the product formula and name the product.
- 3 Explain why 2-methylpropene forms 2-bromo-2-methylpropane as the major product when it reacts with HBr.