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.
Understanding Chemistry: Addition Reactions of Alkenes
The first step in many alkene reactions is easier to understand by following electrons. Electron-rich parts of a molecule are drawn with curved arrows toward an electron-poor atom or ion. When a hydrogen halide approaches an unsymmetrical alkene, the hydrogen receives the electron pair first.
This leaves one carbon temporarily electron-deficient. That species is a carbocation. A negative ion then bonds to the positively charged carbon.
The order matters because the two alkene carbons are not always equivalent. Adding hydrogen to one carbon can place the positive charge on either carbon, but one choice usually gives a more stable carbocation.
Carbocation stability is a useful prediction tool. A carbon attached to more carbon groups can spread out some of its positive charge through nearby bonds. For this reason, tertiary carbocations are usually more stable than secondary ones, which are more stable than primary ones.
This trend explains the usual Markovnikov product. It has an important limitation. Carbocations can rearrange before the second particle attaches.
A hydrogen atom with its bonding electrons may shift to the charged carbon. A carbon group may shift in a similar way.
These shifts produce a new, more stable carbocation, so the final product can have a carbon skeleton that looks unexpected. Students should draw every carbon and hydrogen carefully before deciding whether a rearrangement is possible.
Not every addition reaction forms a free carbocation. When bromine reacts with an alkene, one bromine atom can bond to both alkene carbons at once. This short-lived structure is called a bromonium ion.
It blocks one face of the molecule. The bromide ion must attack from the opposite face, so the two new carbon-bromine bonds tend to end up on opposite sides. This is called anti addition.
If water is present during bromination, water can open the bromonium ion instead of bromide. The product then has a bromine atom and an alcohol group on neighboring carbons.
In contrast, hydroboration followed by oxidation gives an alcohol with the opposite regional pattern from acid-catalyzed hydration. It usually places the alcohol group on the less substituted carbon and adds its new groups to the same face.
Hydrogenation works differently again. The alkene and hydrogen molecules attach to a metal surface, often palladium, platinum, or nickel. The metal weakens the hydrogen-hydrogen bond and holds the alkene in position.
Both hydrogen atoms transfer from the surface to the same face of the double bond. This detail matters in ring-shaped molecules, where products with groups pointing up or down can be different substances. Addition reactions appear outside textbook reaction schemes.
They are used to harden some unsaturated oils, make starting materials for plastics, and build complex drug molecules in stages. When solving reaction problems, first identify the reagent, then decide whether the pathway involves a carbocation, a cyclic ion, or a metal surface. That choice usually tells you where groups attach and how they are arranged in space.
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.