Organic Addition Reactions

An addition reaction is one in which two molecules combine to form a single product, and it is the characteristic reaction of alkenes and alkynes, whose π bond is broken to give two new σ bonds. In brief: ethene adds bromine to give 1,2-dibromoethane (the bromine water test), and propene adds HBr mainly at the carbon with more hydrogens to give 2-bromopropane, according to Markovnikov’s rule.

Addition reactions are the way to turn unsaturated hydrocarbons, the cheap products of cracking, into alcohols, haloalkanes and polymers. Their mechanisms explain a lot about reactivity, and the rule that predicts their products is one of the most useful in organic chemistry. This article presents the main addition reactions of alkenes, explains the electrophilic mechanism and Markovnikov’s rule, gives a summary table and works through examples, including a calculation for hydrogenation.

What are the main addition reactions?

• Hydrogenation: H₂ adds with a Ni, Pd or Pt catalyst, to give an alkane: CH₂=CH₂ + H₂ → CH₃CH₃ (ΔH ≈ −137 kJ/mol).
• Halogenation: Br₂ or Cl₂ adds at room temperature to give a 1,2-dihaloalkane. The decolorization of orange bromine water is the standard test for unsaturation.
• Hydrohalogenation: HCl, HBr or HI add to give a haloalkane.
• Hydration: water adds in the presence of a strong acid catalyst (H₃PO₄ or H₂SO₄) to give an alcohol. This is the industrial route to ethanol from ethene (at about 300 °C and 60–70 atm).
• Polymerization: many alkene molecules add to each other to form a polymer (see the article on polymerization).

What is the mechanism of electrophilic addition?

The π bond of the alkene is a region of high electron density, and so alkenes are attacked by electrophiles (electron-pair acceptors). For the addition of HBr:

1. The π electrons attack the partially positive hydrogen of HBr. A bond forms between carbon and hydrogen, the H–Br bond breaks, and a bromide ion leaves. The other carbon now has a positive charge: a carbocation intermediate.
2. The bromide ion attacks the carbocation and forms the C–Br bond.

The first step is slow and determines the rate. For bromine addition, the Br₂ molecule is polarized as it approaches the π bond, and a cyclic bromonium ion forms. The bromide ion then attacks from the opposite side, so the two bromine atoms add on opposite faces of the double bond (anti addition).

What is Markovnikov’s rule?

When HX adds to an unsymmetrical alkene, the hydrogen attaches to the carbon that already has more hydrogen atoms, and the halogen to the carbon with fewer. The reason is the stability of the carbocation intermediate: the order is tertiary > secondary > primary > methyl, since alkyl groups donate electrons and stabilize the positive charge. For propene, protonation of the terminal CH₂ gives a secondary carbocation (CH₃CH⁺CH₃), which is more stable than the primary carbocation formed by protonation of the central carbon, so 2-bromopropane is the major product and 1-bromopropane the minor one.

In the presence of peroxides, HBr (but not HCl or HI) adds by a free-radical mechanism with the opposite orientation, called the anti-Markovnikov or peroxide effect, to give 1-bromopropane.

Summary of addition reactions of propene

How do alkynes react?

Alkynes undergo the same additions, but they can add twice since they have two π bonds. With 1 mol of H₂ over a poisoned catalyst (Lindlar) an alkyne gives a cis-alkene, and with 2 mol of H₂ and Ni gives the alkane. HC≡CH + 2Br₂ → CHBr₂CHBr₂. The hydration of ethyne with Hg²⁺ and acid gives ethanal, via an unstable enol that rearranges to the carbonyl compound. Alkynes are less reactive than alkenes toward electrophiles, although their π bonds are more numerous, because the carbocation formed (a vinyl cation) is less stable.

Worked examples

Example 1: predicting the major product

• But-1-ene + HBr → 2-bromobutane (hydrogen goes to the CH₂ carbon).
• 2-Methylpropene + H₂O (acid) → 2-methylpropan-2-ol, through the tertiary carbocation.
• Ethene + Br₂ → 1,2-dibromoethane.
• Cyclohexene + H₂ (Ni) → cyclohexane.

Example 2: a bromine water test

A colorless liquid decolorizes orange bromine water at once. This shows that it is unsaturated (an alkene or alkyne). A liquid alkane such as hexane does not decolorize the bromine water in the dark; it merely forms a colored upper layer.

Example 3: hydrogen needed to harden oil

Vegetable oils have an iodine value, the mass of iodine in grams that adds to 100 g of oil. An oil with an iodine value of 130 takes up 130 / 253.8 = 0.512 mol of I₂ per 100 g, which equals the number of moles of C=C bonds. Complete hydrogenation needs 0.512 mol of H₂, a mass of 0.512 × 2.016 = 1.03 g of hydrogen per 100 g of oil. Margarine is made by partial hydrogenation, to give a solid fat.

Example 4: the volume of ethene

What volume of ethene at 25 °C and 1 atm reacts with 10.0 g of bromine (M = 159.8 g/mol)? n(Br₂) = 10.0 / 159.8 = 0.0626 mol, so n(C₂H₄) = 0.0626 mol and V = 0.0626 × 24.47 = 1.53 L.

Where are addition reactions used?

• Fats and oils: partial hydrogenation converts liquid oils to solid fats, although trans fats formed as by-products are now avoided.
• Industrial alcohols: hydration of ethene and propene.
• Plastics: addition polymerization of alkenes.
• Fine chemicals: haloalkanes and epoxides as intermediates.
• Analysis: the iodine value and bromine number measure unsaturation in fats and fuels.

Who discovered the rule?

Vladimir Markovnikov, a Russian chemist at the University of Kazan, published the rule in 1869 on the basis of his observations of the addition of hydrogen halides to alkenes. The electronic explanation, in terms of carbocation stability, came in the 1920s and 1930s with the work of Christopher Ingold and others. Morris Kharasch and Frank Mayo showed in 1933 that the abnormal orientation observed with HBr in the presence of peroxides was caused by a free-radical chain mechanism. Paul Sabatier developed catalytic hydrogenation with finely divided nickel around 1897, for which he shared the 1912 Nobel Prize in Chemistry.

Which mistakes are common?

• Reversing Markovnikov’s rule: hydrogen goes to the carbon that already has more hydrogens.
• Forgetting the catalyst for hydrogenation: without Ni or Pt, it does not go.
• Expecting an alkane to react with bromine water: it needs UV light and a substitution mechanism.
• Ignoring rearrangements: carbocations may rearrange to more stable ones.
• Drawing the wrong number of products: alkynes can add two molecules.