Introduction to Organic Chemistry

Introduction to Organic Chemistry
21 Introduction to Organic Chemistry 21.1 What is Organic Chemistry? 21.2 The Unique Nature of Carbon 21.3 Classification of Organic Compounds 21.4 Factors Affecting the Physical Properties of Organic Compounds

What is Organic Chemistry?
21.1 What is Organic Chemistry? New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A
21.1 What is Organic Chemistry (SB p.2) Organic Chemistry Chemistry of the compounds present in living organisms. They all contain carbon. Organic Chemistry is the Chemistry of Carbon. New Way Chemistry for Hong Kong A-Level 3A

Natural Sources of Organic Compounds
21.1 What is Organic Chemistry (SB p.4) Natural Sources of Organic Compounds Living things Carbohydrates / Proteins / Fats / Vitamins / Antibiotics A variety of organic products obtained from living things New Way Chemistry for Hong Kong A-Level 3A

Natural Sources of Organic Compounds
21.1 What is Organic Chemistry (SB p.4) Natural Sources of Organic Compounds Crude oil or coal Fractional distillation / destructive distillation Alkanes / Alkenes / Alkynes / Aromatic hydrocarbons A variety of useful products derived from crude oil and coal Check Point 21-1 New Way Chemistry for Hong Kong A-Level 3A

Development of Organic Chemistry as a Science
21.1 What is Organic Chemistry (SB p.3) Development of Organic Chemistry as a Science In the past …, Chemistry Organic compounds obtained from living organisms Inorganic compounds obtained from non-living sources New Way Chemistry for Hong Kong A-Level 3A

21.1 What is Organic Chemistry (SB p.3) Development of Organic Chemistry as a Science In 1828, Wohler (a German chemist) (Inorganic compound) (Organic compound) New Way Chemistry for Hong Kong A-Level 3A

21.1 What is Organic Chemistry (SB p.3) Development of Organic Chemistry as a Science Redefining … ... Organic chemistry is the study of carbon compounds (except CO, CO2, carbonates, hydrogencarbonates, carbides and cyanides) obtained from natural sources or synthesized in the laboratories. New Way Chemistry for Hong Kong A-Level 3A

The Unique Nature of Carbon
21.2 The Unique Nature of Carbon New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to form four strong covalent bonds Electronic configuration of carbon (ground state) : 1s22s22p2 Carbon (ground state) New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to form four strong covalent bonds Each carbon atom has four unpaired electrons when excited Tend to form four strong covalent bonds Carbon (excited state) New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to Catenate Carbon atoms link together to form chains of varying length, branched chains and rings of different sizes Catenation:  Ability of atoms in forming stable bonds with itself, hence joining up into chains or rings New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to Catenate C – C > Si – Si > Ge – Ge > Sn – Sn Bond strength  as bond length  New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to Catenate C – C > N – N > O – O Bond strength  as the number of lone pairs  New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to Catenate CnH2n+2 n = 1,2,3,…(no limit for n) SinH2n+2 n = 1 to 6 only  silanes GenH2n+2 n = 1 to 3 only  germanes SnnH2n+2 Only SnH4 (stannane) exists New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.5) Ability to Form Multiple Bonds sp3 4 bonds sp2 1 bond, 3 bonds sp 2 bonds, 2 bonds Carbon (excited state) New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.6) Single bond Double bond Triple bond * X = halogens New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.6) Example 21-2 Check Point 21-2 New Way Chemistry for Hong Kong A-Level 3A

Classification of Organic Compounds
21.3 Classification of Organic Compounds New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.7) Functional Groups Organic compounds are classified by the the presence of characteristic functional groups. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.7) Functional Groups A functional group is defined as an atom or a group of atoms that effectively determines the chemical properties of an organic compound. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.7) Functional Groups New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.7) Functional Groups Propane does not react with sodium Ethanol and propan-1-ol react with sodium to give hydrogen gas New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.7) Functional Groups and have similar chemical properties  they contain the same functional group –OH  they are classified into the same homologous series — alcohols New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.12) Homologous Series A homologous series is a series of compounds that have the same functional group, and each member differs from the next member by a – CH2 – unit in their formulae. CH4 C2H6 C3H8 C4H10 CH CH CH2 New Way Chemistry for Hong Kong A-Level 3A

Number of carbon atom(s) Condensed structural formula
21.3 Classification of Organic Compounds (SB p.12) Number of carbon atom(s) IUPAC name Molecular formula Condensed structural formula Structural formula 1 Methane CH4 2 Ethane C2H6 CH3CH3 3 Propane C3H8 CH3CH2CH3 4 Butane C4H10 CH3CH2CH2CH3 New Way Chemistry for Hong Kong A-Level 3A The first four members of straight-chain alkanes

Number of carbon atom(s) Condensed structural formula
21.3 Classification of Organic Compounds (SB p.12) Number of carbon atom(s) IUPAC name Molecular formula Condensed structural formula Structural formula 1 Methanol CH3OH 2 Ethanol C2H5OH CH3CH2OH 3 Propan-1-ol C3H7OH CH3CH2CH2OH 4 Butan-1-ol C4H9OH CH3CH2CH2CH2OH New Way Chemistry for Hong Kong A-Level 3A The first four members of straight-chain alcohols

21.3 Classification of Organic Compounds (SB p.13) Homologous Series Members in the same series can be represented by a general formula. e.g. alkanes: CnH2n+2 alkenes: CnH2n alkynes: CnH2n-2 New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.13) Homologous Series Members in the same series can be represented by a general formula. e.g. alkanols: CnH2n+1OH alkanals: CnH2n+1CHO alkanoic acids: CnH2n+1COOH New Way Chemistry for Hong Kong A-Level 3A

Homologous Series Functional group of an organic compound Chemical
21.3 Classification of Organic Compounds (SB p.13) Homologous Series Functional group of an organic compound Chemical properties Members of a homologous series have similar chemical properties New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.13) Homologous Series The physical properties change gradually along the homologous series e.g. the longer the carbon chain in the molecule ( or the greater the molecular mass)  the greater the attractive force between molecules  the higher the melting point, boiling point and density New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.13)
Some physical properties of the first 20 members of straight-chain alkanes Number of carbon atom(s) Molecular formula State (at room temperature and pressure) Melting point (°C) Boiling point (°C) Density of solid / liquid at 20°C (g cm–3) 1 2 3 4 5 6 7 8 9 10 CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 Gas Liquid –183 –172 –188 –135 –130 –95 –91 –57 –54 –30 –161 –89 –42 36 69 98 126 151 174 – 0.626 0.657 0.684 0.703 0.718 0.730 New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.13)
Some physical properties of the first 20 members of straight-chain alkanes Number of carbon atom(s) Molecular formula State (at room temperature and pressure) Melting point (°C) Boiling point (°C) Density of solid / liquid at 20°C (g cm–3) 11 12 13 14 15 16 17 18 19 20 C11H24 C12H26 C13H28 C14H30 C15H32 C16H34 C17H36 C18H38 C19H40 C20H42 Liquid Solid –26 –10 –7 –3 10 22 28 32 37 196 216 233 260 271 287 302 316 330 344 0.740 0.749 0.753 0.761 0.769 0.773 0.778 0.777 0.785 New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.13) Example 21-3A Example 21-3B Example 21-3C Check Point 21-3 New Way Chemistry for Hong Kong A-Level 3A

Factors Affecting the Physical Properties of Organic Compounds
21.4 Factors Affecting the Physical Properties of Organic Compounds Refer to notes on ‘Bonding and Structure’ pp – intermolecular forces New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17) Main Factors Affecting the Physical Properties of Organic Compounds 1. Structure of the functional group 1.1 Dipole moment of the molecule 1.2 Formation of hydrogen bonding 2. Length of carbon chains (London dispersion forces) New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17) Structure of Functional Group Molecules having a polar functional group have a higher b.p. than others with a non-polar functional group of similar molecular masses  Stronger intermolecular attraction among molecules New Way Chemistry for Hong Kong A-Level 3A

Structure of Functional Group
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17) Structure of Functional Group Molecule Relative molecular mass Boiling point (oC) Molecules with polar functional groups CH3CH2CH2OH 60 97.2 CH3CH2CH2NH2 59 48.6 CH3CH2Cl 64.5 12.5 CH3CH2COOH 141 Molecules with non-polar functional groups CH3CH2CH2CH3 58 -0.5 CH3CH2CH=CH2 56 -6.2 CH3CH2CCH 54 8.1 New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Dipole Moment of Molecule Tetrachloromethane has 4 polar bonds in the molecule M.p. and b.p. are very low  the molecule is non-polar  the molecule is tetrahedrally symmetrical  the dipole moments of the C  Cl bond cancel each other New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Examples of Polar Molecules with Net Dipole Moment New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Examples of Non-polar Molecules with No Net Dipole Moment New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Solubility of Organic Molecules Depends on the polarity of organic molecules and the solvent Non-polar or weakly polar compounds dissolve readily in non-polar or weakly polar solvents Highly polar compounds dissolve readily in highly polar solvents “Like dissolves like” Let's Think 2 New Way Chemistry for Hong Kong A-Level 3A

Solubility of Organic Molecules
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Solubility of Organic Molecules Hexane in tetrachloromethane Hexane in water New Way Chemistry for Hong Kong A-Level 3A

Why does Hexane Dissolve Readily in Tetrachloromethane?
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.19) Why does Hexane Dissolve Readily in Tetrachloromethane? Intermolecular forces among hexane molecules and those among tetrachloromethane molecules  Intermolecular forces between hexane and tetrachloromethane molecules New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.19) Why is Hexane Insoluble in Water? New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.19) Formation of Hydrogen Bonding Molecules having OH or  NH2 groups are able to form hydrogen bonds Hydrogen bonds affect the physical properties of alcohols and amines with low molecular masses New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Why does Propan-1-ol have a Higher Boiling Point? New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Formation of Hydrogen Bonding Also affect the solubility of a molecule Molecules with OH groups are able to form hydrogen bonds with surrounding water molecules  Soluble in water New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Example 21-4A Let's Think 3 Example 21-4B New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Length of Carbon Chains Molecules with higher molecular masses have higher m.p., b.p. and density  Higher molecular masses  Large molecular sizes  Stronger London dispersion forces among molecules New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Length of Carbon Chains Molecules with branched chains  b.p. and density lower than its straight-chain isomer Straight-chain isomers have greater surface area in contact with each other  Greater attractive force among the molecules New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Length of Carbon Chains Molecules with branched chains  m.p. higher than its straight-chain isomer Branched-chain isomers are more spherical  Packed more efficiently in solid state  Extra energy is needed to break down the efficient packing New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Let's Think 4 Example 21-4C Check Point 21-4 New Way Chemistry for Hong Kong A-Level 3A

R = CnH2n+1 – 21.3 Classification of Organic Compounds (SB p.8) Family
General formula Functional group Example Formula IUPAC name Alkane RH (Nil) CH3CH3 Ethane Alkene RCH = CH2 RCH = CHR R2C = CHR R2C = CR2 Carbon-carbon double bond CH2 = CH2 Ethene Alkyne RC  CH RC  CR – C  C – Carbon-carbon triple bond HC  CH Ethyne Aromatic hydrocarbon ArH Phenyl group Benzene R = CnH2n+1 – New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.8) Family General formula Functional group Example Formula IUPAC name Haloalkane RX X halo group CH3Cl Chloromethane Alcohol ROH  OH hydroxyl group CH3OH Methanol Ether RO  R  O  oxy group CH3  O  CH3 Methoxymethane Aldehyde carbonyl group Methanal R = CnH2n+1 – New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.8) Family General formula Functional group Example Formula IUPAC name Ketone carbonyl group Propanone Carboxylic acid carboxyl group Ethanoic acid Amine RNH2 R2NH R3N amino group CH3NH2 Methylamine Nitrile RCN  C  N nitrile group CH3CN Ethanenitrile R = CnH2n+1 – New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.8) Family General formula Functional group Example Formula IUPAC name Ester ester group Methyl ethanoate Acyl halide acyl halide group Ethanoyl chloride Amide amide group Ethanamide New Way Chemistry for Hong Kong A-Level 3A R = CnH2n+1 –

21.3 Classification of Organic Compounds (SB p.9) Family General formula Functional group Example Formula IUPAC name Acid anhydride acid anhydride group Ethanoic anhydride R = CnH2n+1 – New Way Chemistry for Hong Kong A-Level 3A

The END New Way Chemistry for Hong Kong A-Level 3A

21.1 What is Organic Chemistry (SB p.4) Check Point 21-1 How was organic chemistry defined before 1800s? Answer (a) The knowledge of organic and inorganic compounds was raised during the 1780s. Scientists defined organic chemistry as the study of compounds that could be obtained from living organisms. They believed that the synthesis of organic compounds took place in living organisms only. New Way Chemistry for Hong Kong A-Level 3A

21.1 What is Organic Chemistry (SB p.4) Back Check Point 21-1 (b) How is organic chemistry defined nowadays? Answer (b) Nowadays, scientists have discovered that many organic compounds can be synthesized from inorganic substances. The updated definition of organic chemistry is the study of carbon compounds, except for carbon monoxide, carbon dioxide, carbonates, hydrogencarbonates, carbides and cyanides. These compounds have been traditionally classified under inorganic chemistry. New Way Chemistry for Hong Kong A-Level 3A

Answer Let's Think 1 Why is carbon able to catenate? Back
21.2 The Unique Nature of Carbon (SB p.5) Let's Think 1 Why is carbon able to catenate? Answer The ability to catenate of carbon is chiefly due to the high strength of the CC single bond (bond enthalpy of C  C single bond is 356 kJ mol-1). Back New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.6) Example 21-2 Would you expect silicon, which is just below carbon in the Periodic Table, to catenate to form diverse molecular structures? Explain your answer. Answer Silicon, unlike carbon, does not catenate to form diverse molecular structures. Carbon is able to catenate because carbon atoms have a relatively small atomic size. This enables a carbon atom to form strong covalent bonds with other carbon atoms. However, due to the greater atomic size of silicon, its ability to catenate is much lower than that of carbon. Back New Way Chemistry for Hong Kong A-Level 3A

21.2 The Unique Nature of Carbon (SB p.7) Back Check Point 21-2 Would you expect sulphur, which has an electronegativity value very close to carbon, to catenate? Why? Answer The electronic configuration of sulphur is 1s22s22p63s23p4. It has only two unpaired electrons. Its atomic size is larger than that of carbon. So it has a much lower tendency to catenate than carbon. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.14) Example 21-3A Identify the functional group(s) in the following compounds: (a) Answer Carbon-carbon double bond ( ) and chloro group (Cl) New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.14) Example 21-3A Identify the functional group(s) in the following compounds: (b) Answer (b) Carbonyl group ( ) New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.14) Example 21-3A Back Identify the functional group(s) in the following compounds: (c) Answer (c) Amino group ( ) and carboxyl group ( ) New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Example 21-3B To which homologous series does each of the following compounds belong? (a) (a) Ester Answer New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Example 21-3B To which homologous series does each of the following compounds belong? (b) (b) Amide Answer New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Back Example 21-3B To which homologous series does each of the following compounds belong? (c) (c) Acid anhydride Answer New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Example 21-3C State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer. (a) Answer (a) No, the first one is a carboxylic acid and the second one is an ester. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Example 21-3C State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer. (b) Answer (b) Yes, both of them are alcohols. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.15) Back Example 21-3C State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer. (c) Answer (c) No, the first one is an amide and the second one is an amine. New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.16) Check Point 21-3 (a) Name the homologous series of organic compounds that contain oxygen atoms in their functional groups. Answer (a) Alcohol, ether, aldehyde, ketone, carboxylic acid, ester, acyl halide, amide and acid anhydride New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.16) Check Point 21-3 Identify and name the functional groups in glucose which has the following structure. (b) OH (hydroxyl group) and  O  (oxy group) Answer New Way Chemistry for Hong Kong A-Level 3A

21.3 Classification of Organic Compounds (SB p.16) Back Check Point 21-3 (c) Identify and name the functional groups in the following compounds: (c) Br (bromo), (aldehyde), (acyl chloride), (carbon-carbon double bond) groups Answer New Way Chemistry for Hong Kong A-Level 3A

Answer Let's Think 2 Why is oil immiscible with water? Back
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18) Let's Think 2 Why is oil immiscible with water? Answer Oil molecules do not have free OH groups, so they cannot form hydrogen bonds with water molecules. Back New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Let's Think 3 The relative molecular mass of glucose is 180.0, but it is soluble in water. Why? Answer Glucose molecules have OH groups, so they are able to form hydrogen bonds with water molecules. Therefore, glucose is soluble in water despite it has a high molecular mass. Back New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Example 21-4A Despite the fact that butan-1-ol and ethoxyethane have the same relative molecular mass, they have very different boiling points. The boiling points of butan-1-ol and ethoxyethane are 117oC and 35oC respectively. Explain the difference. Answer New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20) Back Example 21-4A There is an OH group in butan-1-ol. Thus, butan-1-ol molecules are able to form hydrogen bonds with one another and the energy required to separate butan-1-ol molecules would be much greater. Whereas for ethoxyethane, the attraction among the molecules is weak van der Waals’ forces only. The amount of energy required to break the forces would not be great. Therefore, the boiling point of ethoxyethane is lower than that of butan-1-ol. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Back Example 21-4B Explain why propan-1-ol is soluble in water but 1-chloropropane is insoluble in water. Answer The  OH group of propan-1-ol molecules enables it to form hydrogen bonds with water molecules. Thus it is soluble in water. Although 1-chloropropane is a polar molecule, it does not form hydrogen bonds with water molecules. So it is insoluble in water. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21) Let's Think 4 Which molecule would have a higher boiling point, 1-bromobutane or 2-bromobutane? Why? Answer 1-bromobutane would have a higher boiling point. 1-bromobutane is a straight-chain molecule while 2-bromobutane is a branched-chain molecule. Straight-chain molecules have a greater surface area in contact with each other, so greater intermolecular forces exist among the molecules. Higher energy is required to break down the intermolecular forces among the molecules of 1-bromobutane. Back New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.22) Example 21-4C 1-Chlorobutane and 2-chloro-2-methylpropane have the same molecular mass, yet their melting points differ. The melting point of 1-chlorobutane is –123oC while that of 2-chloro-2-methylpropane is –27.1oC. Explain the difference. Answer New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.22) Back Example 21-4C Melting point is a measure of how efficient the molecules are packed together in the solid state instead of just comparing the van der Waals’ forces among molecules. Hence melting point is a function of the efficient packing of molecules but not the contact surface area. 1-Chlorobutane is a straight-chain molecule while 2-chloro-2-methylpropane is a branched-chain molecule. As 2-chloro-2-methylpropane is more spherical and symmetrical, its molecules are packed more efficiently in the solid state. 1-Chlorobutane is linear in shape and flattened, its packing in the solid state is not so efficient. Hence, it has a lower melting point than 2-chloro-2-methylpropane. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 (a) What are the major factors that affect the physical properties of organic compounds? Answer (a) The physical properties of organic compounds are mainly affected by the structure of the functional groups, dipole moment of the molecule, the formation of hydrogen bonding between molecules, and the length of carbon chains of the molecule. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 The melting point and boiling point of pentane are –130oC and 36.3oC respectively while the melting point and boiling point of 2,2-dimethylpropane are –15.9oC and 9.5oC respectively. Account for the difference in melting point and boiling point between the two isomers. Answer New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 (b) Pentane is a straight-chain molecule, while 2,2-dimethylpropane is a branched-chain molecule. Straight-chain molecules have a greater surface area in contact with each other than branched-chain molecules. Straight-chain molecules are held together by stronger intermolecular forces. Therefore, pentane has a higher boiling point than 2,2-dimethylpropane. Molecules of 2,2-dimethylpropane are more spherical in shape and are packed more efficiently in the solid state. Molecules of pentane are linear in shape and flattened, so their packing in the solid state is not efficient. Since extra energy is required to break down the efficient packing of 2,2-dimethylpropane, 2,2-dimethylpropane has a higher melting point than pentane. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 (c) Which molecule, hexane or cyclohexane, would have a higher melting point? Explain your answer. Answer New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 (c) Cyclohexane has a higher melting point than hexane. Molecules of cyclohexane are more spherical in shape and are packed more eff iciently in the solid state. Molecules of hexane are linear in shape and flattened, so their packing in the solid state is not efficient. Since extra energy is required to break down the efficient packing of cyclohexane, cyclohexane has a higher melting point than hexane. New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 (d) Arrange the following molecules in increasing order of boiling points. Explain your answer. Answer New Way Chemistry for Hong Kong A-Level 3A

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24) Check Point 21-4 Back (d) The boiling points increase in the order: butane < propanal < propan-1-ol Molecules of butane are non-polar. Their molecules are held together by weak instantaneous dipole-induced dipole interactions. A relatively small amount of energy is required to separate the molecules in the process of boiling. Both propanal and propan-1-ol are polar molecules. Molecules of propanal are held together by relatively weak dipole-dipole interactions, while molecules of propan-1-ol are held together by intermolecular hydrogen bonds. Since the intermolecular forces present in molecules of propan-1-ol are stronger than those present in molecules of propanal, a larger amount of energy is required to separate the propan-1-ol molecules in the process of boiling. New Way Chemistry for Hong Kong A-Level 3A

Introduction to Organic Chemistry

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