[CHEM] Chapter 11.3 - Alkenes

1 DARRELL ER (COPYRIGHTED) © TOPIC 11.3: ALKENES DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT MASTERY EXAM WEIGHTAGE • Important topic • Take note of alkene’s various chemical reactions • Alkenes are always tested • Understand the difference between ‘saturated’ & ‘unsaturated’ compounds • Heavy overall weightage • Entire Organic Chemistry portion accounts for 15- 20% of each year’s Chemistry paper DARRELL ER (COPYRIGHTED) ©

3 ALKENES HOMOLOGOUS SERIES FUNCTIONAL GROUP GENERAL FORMULA KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 Alkenes Alkenes are unsaturated hydrocarbons with a general formula of CnH2n. As the carbon atoms are not bonded to the maximum of four atoms, alkenes are considered to be ‘unsaturated’. Unsaturated hydrocarbons are hydrocarbons that contain one or more C=C double bond. (For eg: Vegetable oil) Functional group Alkenes contain C=C double covalent bonds. DARRELL ER (COPYRIGHTED) © Name Carbon atoms Molecular Formula Full Structural Formula Condensed structural formula Ethene 2 C2H4 CH2CH2 Propene 3 C3H6 CH2CHCH3 Butene 4 C4H8 CH3CH2CHCH2

5 ALKENES PHYSICAL PROPERTIES CHEMICAL PROPERTIES CRACKING KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

6 PHYSICAL PROPERTIES DARRELL ER (COPYRIGHTED) © Physical property Reasoning Melting and boiling points As the number of carbon atoms in the alkene increases, the melting and boiling points of alkenes increases as well. As the number of carbon atoms in an alkene increases, the size of the molecules are bigger and have stronger intermolecular forces of attraction between each other. As such, more heat energy is needed to overcome the intermolecular forces of attraction between the alkene molecules. Hence, larger alkenes containing more carbon atoms will have higher melting and boiling points. Volatility As the number of carbon atoms in the alkene increases, the volatility of alkenes decreases. (similar to m.p. & b.p.) With a higher relative molecular mass, there would be stronger intermolecular forces of attraction between the alkene molecules. As such, more energy is needed to overcome the intermolecular forces of attraction between the alkene molecules. Hence, larger alkene molecules are less likely to evaporate in room temperature. Density As the number of carbon atoms in the alkene increases, the density of alkenes increases. Viscosity As the number of carbon atoms in the alkene increases, the viscosity of alkenes decreases. (more difficult to flow) Alkenes with longer hydrocarbon chains flow less easily as they tend to get stuck together. Flammability As the number of carbon atoms in the alkene increases, the flammability of alkenes decreases. (more difficult to burn) The larger alkenes contain a higher percentage mass of carbon atoms and would undergo incomplete combustion which results in a smokier flame. Solubility All alkenes are insoluble in water but are soluble in organic solvents like ethanol.

7 1) COMBUSTION In the presence of excess oxygen, an alkene would undergo complete combustion, producing carbon dioxide and water. 2 C3H6 + 9 O2  6 CO2 + 6 H2O If there is insufficient oxygen for complete combustion, the alkene would undergo incomplete combustion, producing water and carbon monoxide. (similar to alkanes) C3H6 + 3 O2  3 CO + 3 H2O 2) HYDROGENATION (addition of hydrogen) Condition: 200°C and nickel catalyst. Through the process of hydrogenation, alkenes are converted to alkanes. (E.g. vegetable oils can be converted to margarine.) Margarine is considered a saturated compound as it undergoes hydrogenation process and contains no C=C bonds. CHEMICAL REACTIONS DARRELL ER (COPYRIGHTED) © CHEMICAL REACTIONS OF ALKENES 1) Combustion 1) Hydrogenation (add hydrogen gas) 1) Bromination (add aqueous bromine) 1) Hydration (add water) 1) Polymerisation (combined alkenes to form long chain) Addition reactions

8 3) BROMINATION (addition of aqueous bromine) Conditions: Aqueous bromine, room temperature When aqueous bromine reacts with alkenes occurs, the reddish-brown solution decolourises to become colourless. Aqueous bromine is used as a test to distinguish alkanes and alkenes as only alkenes would react with aqueous bromine in the absence of ultraviolet light. C2H4 (g) + Br2 (aq)  C2H4Br2 (l) 4) HYDRATION (addition of steam) Addition of steam under the right conditions can cause a reaction with the C=C double bond to produce an alcohol containing the −OH functional group. Conditions: Temperature of 300°C and at a pressure of 60 atm, Phosphoric(V) acid as a catalyst. CHEMICAL REACTIONS DARRELL ER (COPYRIGHTED) © CHEMICAL REACTIONS OF ALKENES 1) Combustion 1) Hydrogenation (add hydrogen gas) 1) Bromination (add aqueous bromine) 1) Hydration (add water) 1) Polymerisation (combined alkenes to form long chain) Addition reactions

9 5) POLYMERISATION Polymers are formed when multiple identical alkene molecules (monomers) are joined together to form a large molecule. For example, ethene molecules can be joined together through polymerisation to form the polymer: poly(ethene). CHEMICAL REACTIONS DARRELL ER (COPYRIGHTED) © CHEMICAL REACTIONS OF ALKENES 1) Combustion 1) Hydrogenation (add hydrogen gas) 1) Bromination (add aqueous bromine) 1) Hydration (add water) 1) Polymerisation (combined alkenes to form long chain) Addition reactions

10 CRACKING DARRELL ER (COPYRIGHTED) © Catalytic cracking Catalytic cracking is a process where a long-chain hydrocarbons from petroleum are broken down into shorter-chain hydrocarbons in a presence of a catalyst. The process is used for producing fuels for vehicles, production of alkenes & production of hydrogen. Smaller chain alkanes and alkenes are more useful than longer chain alkanes as they are in higher demand. Conditions: Aluminium oxide (Al2O3) as catalyst and silicon (IV) oxide (SiO2) at a temperature of 600°C. At least one of the products formed will be an alkene. Alkanes, hydrogen and even carbon can be produced from the cracking process as well. Examples: Long chain alkane → shorter chain alkane + alkene C10H22 → C5H12 + C5H10 Long chain alkane → shorter chain alkene + shorter chain alkene + hydrogen C10H22 → C6H12 + C4H8 + H2