[CHEM] Chapter 11.2 - Alkanes

1 DARRELL ER (COPYRIGHTED) © TOPIC 11.2: ALKANES DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT MASTERY EXAM WEIGHTAGE • Important topic • Take note of ‘isomerism’ • Alkanes are tested lightly • Explanation for physical properties is applicable to all other hydrocarbon compounds as well* • Heavy overall weightage • Entire Organic Chemistry portion accounts for 15- 20% of each year’s Chemistry paper DARRELL ER (COPYRIGHTED) ©

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

4 Alkanes Alkanes are hydrocarbons with the general formula CnH2n+2. Alkanes contain only C-C single bonds and C-H single bonds . Alkanes are ‘saturated’ as each carbon atom is covalently bonded to a maximum of four other atoms. Functional group Alkanes have no functional group. (Take note that C-C single bond is not a functional group as it does not have any chemical properties!) DARRELL ER (COPYRIGHTED) © Name Carbon atoms Molecular Formula Full Structural Formula Condensed structural formula Methane 1 CH4 CH4 Ethane 2 C2H6 CH3CH3 Propane 3 C3H8 CH3CH2CH3 Butane 4 C4H10 CH3CH2CH2CH3 *Need to know how to draw full structural formula and name the alkane.

5 ALKANES PHYSICAL PROPERTIES CHEMICAL PROPERTIES ISOMERISM 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 alkane increases, the melting and boiling points of alkanes increases as well. When the number of carbon atoms in an alkane increases, the molecules are bigger and have stronger intermolecular forces of attraction between the alkane molecules. As such, more heat energy is needed to overcome the intermolecular forces of attraction between the alkane molecules. Hence, larger alkanes containing more carbon atoms will have higher melting and boiling points. Volatility When the number of carbon atoms in an alkane increases, the alkane becomes less volatile it is. (similar to m.p. & b.p.) With a higher relative molecular mass, there would be stronger intermolecular forces of attraction between the alkane molecules. As such, more energy is needed to overcome the intermolecular forces of attraction between the alkane molecules. Hence, larger alkane molecules are less likely to evaporate. Density When the number of carbon atoms in an alkane increases, the density will increase. Viscosity When the number of carbon atoms in an alkane increases, the viscosity will increase. (more difficult to flow) Alkanes with longer hydrocarbon chains flow less smoothly as they tend to get stuck together. Flammability The higher the relative molecular mass of an alkane, the lower the flammability. (more difficult to burn) The larger alkanes contain a higher percentage by mass of carbon atoms and undergo incomplete combustion to produce a smokier flame. Solubility Alkanes are insoluble in water but are soluble in organic solvents like ethanol.

7 SUBSTITUTION (Free Radical Substitution) During substitution, alkanes can react with halogens In the presence of ultraviolet (UV) light. For example, CH4 (g) + Br2 (g)  CH3Br (g) + HBr (g) It is also possible for Br atoms to replace all the H atoms to become CBr4. COMBUSTION During complete combustion, an alkane burns in excess oxygen to produce carbon dioxide and water. For instance, methane undergoes complete combustion in excess oxygen: 2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (l) Incomplete combustion of the alkane occurs when there is an insufficient oxygen. In this case, water and carbon monoxide are produced. 2 C2H6 (g) + 5 O2 (g)  4 CO (g) + 6 H2O (l) If there is even lesser amounts of oxygen, there could only be just carbon (soot) and water that are produced. CHEMICAL REACTIONS DARRELL ER (COPYRIGHTED) © SUBSTITUTION COMBUSTION

8 ISOMERISM (ALKANE) DARRELL ER (COPYRIGHTED) © ISOMERISM Isomers are compounds with the same molecular formula but different structural formula. In order to display isomerism, alkanes would have to contain at least four carbon atoms. Isomers have similar chemical properties but slightly different physical properties such as different melting and boiling points & density. Alkane Isomers Structural formula Butane 2 Pentane 3 Dimethylpropane

ALL ORGANIC COMPOUNDS Complete Combustion Incomplete Combustion ALKANE ALKENE ALCOHOL CARBOXYLIC ACID C - C Prefix Meth- 1 Eth- 2 Prop- 3 But- 4 Pent- 5 Hex- 6 Hep- 7 Oct- 8 Non- 9 Dec- 10 Catalytic Cracking (Al2O3 & SiO2, 600 °C) X + O2 (g) → CO2 (g) + 2H2O (l) CH4 (g) + Br2 (g) → CH3Br (g) + HBr (g) Substitution (UV light) Hydrogenation (200 °C & nickel) Bromination (Test for C=C bonds) Hydration (300 °C & 60 atm, Phosphoric(V) acid) LONG CHAIN ALKANE Addition Polymerisation (High temp & pressure) H2 gas (For Haber process) Fermentation (37°C, yeast & no O2) SUGAR C6H12O6 → 2 C2H5OH + 2 CO2 C = C -OH Oxidation (acidified aqueous potassium manganate(VII) / exposed to air) Esterification (warm, sulfuric acid) ESTER + H2O -COOH -COO- Condensation Polymerisation (elimination of water) C2H4 (g) + Br2 (aq) → C2H4Br2 (aq) DARRELL ER (COPYRIGHTED) © DARRELL ER (COPYRIGHTED) © X + O2 (g) → CO (g) + 2H2O (l) POLYMER O

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