CHT213 Week 6 Properties of Alkanes PDF

This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24...

This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 CHT213 – ORGANIC CHEMISTRY 1: Week 6: Fundamentals of Organic Chemistry Properties and Chemical Reactions of Alkanes Week 6: At the end of the lesson, you should be able PROPERTIES AND to: CHEMICAL REACTIONS identify the physical properties of alkanes; OF ALKANES identify the chemical properties of alkanes; describe the reactivity of alkanes; and familiarize with the methods of preparation of alkanes. BONIFACIO M. JUBAY III, RChT, MENRM, MRSC Faculty Member Chemical Engineering Technology Department Technological University of the Philippines Visayas 1 2 INTRODUCTION PHYSICAL PROPERTIES Alkanes can exist as gases, liquids, or solids at room temperature. Alkanes are the simplest and least reactive hydrocarbon Alkanes contain only C-C and C-H sigma bonds which are almost non- species containing only C and H atoms. polar due to very small difference of electronegativity between carbon Its structural feature is the singly bonded C – C and C – H or and hydrogen atoms. Therefore, alkanes possess weak van der Waals’ a lack of unsaturation. forces as the intermolecular forces. It releases large amount of energy upon oxidation Due to the weak interparticle forces, the first four members (C1 to C4 (combustion). are gases; the next thirteen members, (C5 to C17) are liquids while the higher members are waxy solids. Straight-chain alkanes and branched alkanes have different physical and chemical properties. Solid alkanes are normally soft, with low melting points. These characteristics are due to strong repulsive forces generated between electrons on neighboring atoms. The strong repulsive forces counterbalance the weak van der Waals forces of attraction. 3 4 1 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 PHYSICAL PROPERTIES PHYSICAL PROPERTIES Boiling point and Melting point Boiling point and Melting point A plot of melting and boiling points versus number of carbon atoms for the C1–C14 straight-chain alkanes. There is a regular increase with molecular size. The first 10 Straight-chain Alkanes 5 6 PHYSICAL PROPERTIES PHYSICAL PROPERTIES Boiling point and Melting point Volatility A branched hydrocarbon generally has a smaller surface area Due to their low boiling points, lower alkanes are highly than its corresponding straight-chain isomer, and therefore, volatile. Among alkanes, volatility decreases with increase in branching causes a decrease in boiling point. This trend can be chain length. Among isomeric alkanes, the more the seen by comparing the following constitutional isomers of C5H12: branching, the greater is the volatility. Density The densities of alkanes increase with increasing molecular masses but become constant at about 0.8 g /cm3. This means that all alkanes are lighter than water. (36.1oC) (27.9oC) (9.5oC) 7 8 2 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 PHYSICAL PROPERTIES CHEMICAL PROPERTIES Solubility Alkanes are less reactive than most classes of organic Alkanes are insoluble in water but soluble in organic solvents. compounds. No reaction is observed with acidic or basic solutions When a molecular substance dissolves in water, the following must including oxidizing agents such as KMnO4. To bring about a occur: reaction, some conditions such as high temperature and breaking of the intermolecular forces within the substance. In electrophilic reagents are needed. the case of the alkanes, these are the Van der Waals dispersion Moreover, alkanes react with oxygen, halogens, and a few forces. other substances under appropriate conditions. breaking of the intermolecular forces in the water so that the 1. Oxidation (Combustion) substance can fit between the water molecules. In water, the primary intermolecular attractions are hydrogen bonds. 9 10 CHEMICAL PROPERTIES CHEMICAL PROPERTIES 2. Halogenation 2. Halogenation Halogenation is a substitution reaction wherein one or more Halogenation is a substitution reaction wherein one or more hydrogen atoms is/are replaced by halogen atom/s. hydrogen atoms is/are replaced by halogen atom/s. 11 12 3 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 CHEMICAL PROPERTIES CHEMICAL PROPERTIES 2. Halogenation 2. Halogenation The following facts must be accommodated by any reasonable mechanism for the halogenation reaction. Light-induced gas phase chlorination at 25ºC gives 45% 1-Chloropropane and 55% 2-Chloropropane. The reactivity of the halogens decreases in the following order: F2 > Cl2 > Br2 > I2 too reactive unreactive Light-induced bromination at 25ºC yields 2-Bromopropane (97%): Chlorinations and brominations are normally exothermic. Energy input in the form of heat or light is necessary to initiate these halogenations. 13 14 CHEMICAL PROPERTIES CHEMICAL PROPERTIES 2. Halogenation 2. Halogenation The light-induced chlorination of 2-Methylpropane gave Chlorination of Ethylbenzene at the benzylic location in the presence of predominantly (65%) 2-Chloro-2-methylpropane: a peroxide (a free radical initiator): Benzylic hydrogens H2O2 30 H atom Chlorinated product Benzylic carbon (at benzylic position) 15 16 4 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 CHEMICAL PROPERTIES CHEMICAL PROPERTIES 3. Nitration 4. Pyrolysis (Cracking) This reaction is done by heating the HC with the vapors of Nitric The complex alkanes with high molecular weights that are acid at 420ºC. The hydrogen atom is replaced by the Nitro group found in crude oil are frequently broken into smaller, more useful (NO2). alkanes by thermal cracking; alkenes and hydrogen gas are also 420ºC produced by using this method. Thermal cracking is typically CH3CH3 + HNO3 CH3CH2NO2 + CO2 + H2O performed at high temperatures, and often in the presence of a name? name? catalyst. A mixture of products results, and these alkanes and alkenes can be separated by fractional distillation 20% yield of nitrated product The cleavage of C–C or C–H bonds under the influence of heat is called pyrolysis. 17 18 CHEMICAL PROPERTIES CHEMICAL PROPERTIES 4. Pyrolysis (Cracking) 4. Pyrolysis (Cracking) Pyrolysis in petroleum industry is known as cracking. This process is important in modifying the carbon chain length of higher hydrocarbons obtained by the distillation of crude oil. 19 20 5 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 CHEMICAL PROPERTIES PREPARATION OF ALKANES 5. Catalytic reforming I. Reactions with no change in carbon skeleton The conversion of long chain aliphatic hydrocarbons into 1. From unsaturated hydrocarbons by catalytic hydrogenation aromatic hydrocarbons with the same number of carbon atoms is Alkanes can be prepared by the catalytic hydrogenation (or called reforming. Sabatier–Sendern’s reaction or S–S reaction) of unsaturated hydrocarbons in the presence of Ni, Pt, or Pd catalyst at 200⁰C Catalytic reforming of n-heptane into toluene. to 300⁰C. H2 , 200–300oC CH2=CH2 Ni or Pd, or Pt (room temp) CH3CH3 500oC, 200 atm, H2 CH≡CH 2H2 , 200–300oC Ni or Pd, or Pt (room temp) 21 22 PREPARATION OF ALKANES PREPARATION OF ALKANES 2. From haloalkanes 3. From aldehydes and ketones by Clemmensen reduction a. by reduction Aldehydes and ketones can be reduced into alkanes in the When treated with reducing agents like LiAlH4/ether or presence of reducing agents such as amalgamated zinc in Zn/dilute HCl producing atomic hydrogen, haloalkanes are concentrated HCl. reduced to its corresponding alkanes. Zn(Hg), conc. HCl LiAlH4 /ether General reaction: R – X + 2[H] or Zn/HCl R – H + HX Propanal LiAlH4 /ether Example: CH3I + 2[H] or Zn/HCl CH3 – H + HI Propanone Propane Methane Zn(Hg), conc. HCl 23 24 6 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 PREPARATION OF ALKANES PREPARATION OF ALKANES 4. From Grignard’s Reagent 5. From salts of Carboxylic acid Hydrolysis of Grignard’s reagent in the presence of ether gives a. By Kolbe’s electrolytic method alkanes. Electrolysis of aqueous conc. solution of sodium or potassium dry ether General reaction: R – X + Mg or THF RMgX (Grignard reagent) salt of carboxylic acid gives alkanes. C2H6 , CO2 RMgX + H2O R – H + Mg(OH)X C2H6 Alkane ? Example: ? dry ether CH3CH2Br + Mg or THF CH3CH2MgBr CH3CH2MgBr + H2O CH3CH3+ Mg(OH)Br Ethane NaC2H3O2 (aq) KOH (aq) 25 26 PREPARATION OF ALKANES PREPARATION OF ALKANES 6. From metal carbides II. Reactions with change/s in carbon skeleton Metal carbide like aluminum carbide (Al4C3) and beryllium 1. From haloalkanes carbide (Be2C) reacts with pure water to produce methane. a. Coupling of RX by Wurtz reaction When alkyl halides are heated with sodium metal in the presence of dry ether, alkanes are obtained (generally having Al4C3 + 12 H2O 4 Al(OH)3 + 3 CH4 double number of C-atoms than in alkyl halides). This reaction is known as Wurtz reaction and used for the preparation of Be2C + 4 H2O 2 Be(OH)2 + CH4 symmetrical alkanes. dry ether General reaction: 2 R – X + 2 Na R – R + 2 NaX dry ether Example: 2 CH3I + 2 Na CH3 – CH3 + 2 NaI Ethane 27 28 7 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 PREPARATION OF ALKANES PREPARATION OF ALKANES II. Reactions with change/s in carbon skeleton 1. From haloalkanes 1. From haloalkanes b. Coupling of RX by Corey-House synthesis b. Coupling of RX by Corey-House synthesis Two alkyl groups can be coupled by indirectly reacting two molecules of RX, or RX with R’X, to give R – R or R – R’, respectively, using organometallic lithium dialkylcuprates (R2CuLi), as intermediates. General reactions: dry ether (most R groups 2 R – Li + CuI R2CuLi + LiI are possible) dry ether R2CuLi + R’ – X R – R’ + RCu + LiX (except 3o R) 29 30 PREPARATION OF ALKANES PRACTICE TASK 2. From salts of Carboxylic acid 1. Prepare heptane using Lithium dipropylcopper and a suitable a. By heating Sodium salt of carboxylic acid organohalide When Na-salt of carboxylic acid is heated with soda lime (NaOH & CaO), alkane is obtained having one carbon less than salt by 2. Prepare 2-Pentane via catalytic hydrogenation removal of a molecule of CO2. This reaction is also known as 3. Prepare Hexane via hydrolysis of a suitable Grignard reagent decarboxylation. 31 32 8 This lecture material is a property of ChemTech Department intended for academic purposes only and is not for sale nor for reproduction. 10/3/24 REFERENCES Balacuit, G. G. (2020). CHT213: Organic Chemistry 1. Learning Module. Chemical Engineering Technology, TUP Visayas. Jubay III, B. M. (2024). CHT213: Organic Chemistry 1. Learning Module. Chemical Engineering Technology, TUP Visayas. Klein, D. (2015). Organic Chemistry 2nd Edition. New York: John Wiley & Sons, Inc. McMurry, J. (2008). Organic Chemistry 7th Edition. Thomson Learning, Inc. Solomons, T. W. G., Fryhle, C. B., & Snyder, S. A. (2014). Organic Chemistry 11th Edition. New York: John Wiley & Sons, Inc. 33 9