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Organic Chemistry - Alkenes (Addition reactions)

Grade 11IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond (C=CC=C), making them more reactive than alkanes.

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Addition reactions occur when the double bond breaks, and new atoms add to the carbon atoms that were part of the C=CC=C bond.

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Hydrogenation: The addition of hydrogen (H2H_2) to an alkene to form an alkane. This requires a Nickel (NiNi) catalyst and a temperature of 150∘C150^\circ C.

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Halogenation: The addition of halogens (e.g., Br2Br_2, Cl2Cl_2). Adding bromine water (Br2(aq)Br_2(aq)) is a standard test for unsaturation; the color changes from orange/brown to colorless.

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Hydration: The addition of steam (H2O(g)H_2O(g)) to an alkene to produce an alcohol. Conditions: 300∘C300^\circ C, 6060 atm pressure, and a phosphoric(V) acid (H3PO4H_3PO_4) catalyst.

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Addition Polymerization: Small alkene molecules (monomers) join together by opening their double bonds to form long-chain molecules (polymers) like poly(ethene).

📐Formulae

CnH2nC_nH_{2n}

C2H4+H2→Ni,150∘CC2H6C_2H_4 + H_2 \xrightarrow{Ni, 150^\circ C} C_2H_6

C2H4+Br2→CH2BrCH2BrC_2H_4 + Br_2 \rightarrow CH_2BrCH_2Br

C2H4+H2O(g)→H3PO4,300∘C,60atmC2H5OHC_2H_4 + H_2O(g) \xrightarrow{H_3PO_4, 300^\circ C, 60 atm} C_2H_5OH

n(CH2=CH2)→−(CH2−CH2)n−n(CH_2=CH_2) \rightarrow -(CH_2-CH_2)_n-

💡Examples

Problem 1:

Identify the product formed when propene (C3H6C_3H_6) reacts with steam in the presence of a catalyst and state the type of reaction.

Solution:

The product is propanol (C3H7OHC_3H_7OH). The type of reaction is an addition reaction (specifically hydration).

Explanation:

In an addition reaction, the HH and OHOH from the water molecule (H2OH_2O) add across the C=CC=C double bond of propene, converting the unsaturated alkene into a saturated alcohol.

Problem 2:

Explain the observation when ethene (C2H4C_2H_4) is bubbled through bromine water (Br2(aq)Br_2(aq)).

Solution:

The orange-brown bromine water turns colorless.

Explanation:

Ethene undergoes an addition reaction with bromine to form 1,21,2-dibromoethane (C2H4Br2C_2H_4Br_2). Because the molecular bromine (Br2Br_2) is consumed to form a colorless product, the solution loses its distinct orange-brown color.

Problem 3:

Write the balanced chemical equation for the hydrogenation of butene (C4H8C_4H_8).

Solution:

C4H8+H2→C4H10C_4H_8 + H_2 \rightarrow C_4H_{10}

Explanation:

Hydrogenation adds two hydrogen atoms to the alkene, breaking the double bond to form the corresponding alkane, which in this case is butane (C4H10C_4H_{10}).

Alkenes (Addition reactions) - Revision Notes & Key Formulas | IGCSE Grade 11 Chemistry