[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 i