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Addition Reactions

One molecule combines with another to form a larger molecule with no other products.

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Addition Reactions

What is this yellow stuff?

There is big debate these days about using butter or margarine on your toast (or pancakes or muffin). Margarine is less expensive than butter and is lower in fat and cholesterol. Margarine is made from animal or vegetable fats using hydrogenation to reduce the double bonds in the fatty acids. Hydrogen gas is bubbled through the liquid oil and reacts with the carbon-carbon double bonds present in the long-chain fatty acids. The product is less likely to spoil than butter.

Addition Reactions

An addition reaction is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product . An addition reaction can be thought of as adding a molecule across the double or triple bond of an alkene or alkyne. Addition reactions are useful ways to introduce a new functional group into an organic molecule.

One type of addition reaction is called hydrogenation. Hydrogenation is a reaction that occurs when molecular hydrogen is added to an alkene to produce an alkane. The reaction is typically performed with the use of a platinum catalyst. Ethene reacts with hydrogen to form ethane.

\text{CH}_2\text{=CH}_2 (g) + \text{H}_2 (g) \xrightarrow{\text{Pt}} \text{CH}_3 \text{CH}_3 (g)

Alkyl halides can be produced from an alkene by the addition of either the elemental halogen or the hydrogen halide. When the reactant is the halogen, the product is a disubstituted alkyl halide as in the addition of bromine to ethene.

\text{CH}_2\text{=CH}_2 (g) + \text{Br}_2 (l) \rightarrow & \ \text{CH}_2 \text{BrCH}_2 \text{Br} (g)\\& \ 1, 2\text{-dibromoethane}

The addition of bromine to an unknown organic compound is indeed a test for saturation in the compound. Bromine has a distinctive brownish-orange color, while most bromoalkanes are colorless. When bromine is slowly added to the compound, the orange color will fade if it undergoes the addition reaction to the hydrocarbon. If the orange color remains, then the original compound was already saturated and no reaction occurred.

A monosubstituted alkyl halide can be produced by the addition of a hydrogen halide to an alkene. Shown below is the formation of chloroethane.

\text{CH}_2\text{=CH}_2 (g) + \text{HCl} (g) \rightarrow \text{CH}_3\text{CH}_2\text{Cl}(g)

A hydration reaction is a reaction in which water is added to an alkene. Hydration reactions can take place when the alkene and water are heated to near 100°C in the presence of a strong acid, which acts as a catalyst. Shown below is the hydration of ethene to produce ethanol.

\text{CH}_2\text{=CH}_2 + \text{H}_2 \text{O} \rightarrow \text{CH}_3 \text{CH}_2 \text{OH}

Under modest reaction conditions, benzene resists addition reactions because adding a molecule across a double bond in a benzene ring disrupts the ring of delocalized electrons. This greatly destabilizes the molecule. However, under conditions of high temperature and pressure, and with an appropriate catalyst, benzene will slowly react with three molecules of hydrogen to produce cyclohexane.

\text{C}_6 \text{H}_6 + 3\text{H}_2 \xrightarrow{\text{Pt}} \text{C}_6 \text{H}_{12}


  • Addition reaction is defined.
  • Examples of addition reactions are given.


Draw structures of the products of the reactions in the “It is a helpful exercise to predict...” box partway down the page.


  1. What type of compound is needed for an addition reaction?
  2. Name the compound formed in the reaction of CH 3 CH 2 CH=CH 2 and HCl.
  3. What is the product of a hydration reaction?

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