1. 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
4/27/2017

2. What is Organic Chemistry?
4/27/2017
Dr Seemal Jelani

3. Organic Chemistry
Chemistry of the compounds present in living organisms.
They all contain carbon.
Organic Chemistry is the Chemistry of Carbon.
4/27/2017
Dr Seemal Jelani

4. A variety of organic products obtained from living things
Natural Sources of Organic Compounds
Living things
Carbohydrates / Proteins / Fats / Vitamins / Antibiotics
A variety of organic products obtained from living things
4/27/2017
Dr Seemal Jelani

5. A variety of useful products derived from crude oil and coal
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
4/27/2017
Dr Seemal Jelani

6. Destructive distillation
Decomposition of a solid by heating it in a closed container and collecting the volatile constituents given off.
4/27/2017

7. Development of Organic Chemistry as a Science
In the past …,
Chemistry
Organic compounds
obtained from living organisms
Inorganic compounds
obtained from non-living sources
4/27/2017
Dr Seemal Jelani

8. Development of Organic Chemistry as a Science
In 1828, Wohler (a German chemist)
(Inorganic compound)
(Organic compound)
4/27/2017
Dr Seemal Jelani

9. 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.
4/27/2017
Dr Seemal Jelani

10. The Unique Nature of Carbon
4/27/2017
Dr Seemal Jelani

11. Electronic configuration of carbon (ground state) : 1s22s22p2
Ability to form four strong covalent bonds
Electronic configuration of carbon (ground state) : 1s22s22p2
Carbon (ground state)
4/27/2017
Dr Seemal Jelani

12. Hund’s Rule
Every orbital in a sub shell is singly occupied one electron before any one orbital is doubly occupied
All electrons in singly occupied orbitals have the same spin
Dr seemal Jelani
4/27/2017

13. Each carbon atom has four unpaired electrons when excited
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)
4/27/2017
Dr Seemal Jelani

14. 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
4/27/2017
Dr Seemal Jelani

15. Ability to Catenate C – C > Si – Si > Ge – Ge > Sn – Sn
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 
Decrease as bond length increase
C – C > N – N > O – O
Bond strength  (decrease)
as the number of lone pairs (increase)
4/27/2017
Dr Seemal Jelani

16. Ability to Catenate CnH2n+2 n = 1,2,3,…(no limit for n)
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
4/27/2017
Dr Seemal Jelani

17. Ability to Form Multiple Bonds
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)
4/27/2017
Dr Seemal Jelani

18. * X = halogens 21.2 The Unique Nature of Carbon (SB p.6) Single bond
Double bond
Triple bond
* X = halogens
4/27/2017
Dr Seemal Jelani

19. Classification of Organic Compounds
4/27/2017
Dr Seemal Jelani

20. 21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
Organic compounds are classified by the the presence of characteristic functional groups.
4/27/2017
Dr Seemal Jelani

21. Functional Groups
4/27/2017
Dr Seemal Jelani

22. Functional Groups Propane does not react with sodium
Ethanol and propan-1-ol react with sodium to give hydrogen gas
4/27/2017
Dr Seemal Jelani

23. Functional Groups and have similar chemical properties
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
4/27/2017
Dr Seemal Jelani

24. 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
4/27/2017
Dr Seemal Jelani

25. 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
4/27/2017
Dr Seemal Jelani
The first four members of straight-chain alkanes

26. 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
4/27/2017
Dr Seemal Jelani
The first four members of straight-chain alcohols

27. 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
4/27/2017
Dr Seemal Jelani

28. 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
4/27/2017
Dr Seemal Jelani

29. 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
4/27/2017
Dr Seemal Jelani

30. 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
4/27/2017
Dr Seemal Jelani

31. 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
4/27/2017
Dr Seemal Jelani

32. 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
4/27/2017
Dr Seemal Jelani

33. Factors Affecting the Physical Properties of Organic Compounds
4/27/2017
Dr Seemal Jelani

34. 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)
4/27/2017
Dr Seemal Jelani

35. 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
4/27/2017
Dr Seemal Jelani

36. 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
4/27/2017
Dr Seemal Jelani

37. 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
4/27/2017
Dr Seemal Jelani

38. Examples of Polar Molecules with Net Dipole Moment
Electronegativity
4/27/2017
Dr Seemal Jelani

39. Examples of Non-polar Molecules with No Net Dipole Moment
4/27/2017
Dr Seemal Jelani

40. 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
4/27/2017
Dr Seemal Jelani

41. Hexane in tetrachloromethane
Solubility of Organic Molecules
Hexane in tetrachloromethane
Hexane in water
4/27/2017
Dr Seemal Jelani

42. Intermolecular forces between hexane and Tetrachloromethane molecules
Why does Hexane Dissolve Readily in Tetrachloromethane?
Intermolecular forces among hexane molecules and those among Tetrachloromethane molecules 
Intermolecular forces between hexane and Tetrachloromethane molecules
4/27/2017
Dr Seemal Jelani

43. Why is Hexane Insoluble in Water?
4/27/2017
Dr Seemal Jelani

44. 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
4/27/2017
Dr Seemal Jelani

45. Why does Propan-1-ol have a Higher Boiling Point?
4/27/2017
Dr Seemal Jelani

46. 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
4/27/2017
Dr Seemal Jelani

47. 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
4/27/2017
Dr Seemal Jelani

48. 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
4/27/2017
Dr Seemal Jelani

49. 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
4/27/2017
Dr Seemal Jelani

50. R = CnH2n+1 – 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 –
4/27/2017
Dr Seemal Jelani

51. R = CnH2n+1 – 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 –
4/27/2017
Dr Seemal Jelani

52. R = CnH2n+1 – 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 –
4/27/2017
Dr Seemal Jelani

53. R = CnH2n+1 – 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
4/27/2017
Dr Seemal Jelani
R = CnH2n+1 –

54. R = CnH2n+1 – Family General formula Functional group Example Formula
IUPAC name
Acid anhydride
acid anhydride group
Ethanoic anhydride
R = CnH2n+1 –
4/27/2017
Dr Seemal Jelani

55. The END
4/27/2017
Dr Seemal Jelani

56. 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.
4/27/2017
Dr Seemal Jelani

57. Check Point 21-1 Answer Back
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.
4/27/2017
Dr Seemal Jelani

58. Why is carbon able to catenate?
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
4/27/2017
Dr Seemal Jelani

59. 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
4/27/2017
Dr Seemal Jelani

60. Check Point 21-2 Answer Back
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.
4/27/2017
Dr Seemal Jelani

61. 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)
4/27/2017
Dr Seemal Jelani

62. 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 ( )
4/27/2017
Dr Seemal Jelani

63. 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 ( )
4/27/2017
Dr Seemal Jelani

64. 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
4/27/2017
Dr Seemal Jelani

65. 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
4/27/2017
Dr Seemal Jelani

66. 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
4/27/2017
Dr Seemal Jelani

67. 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.
4/27/2017
Dr Seemal Jelani

68. 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.
4/27/2017
Dr Seemal Jelani

69. 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.
4/27/2017
Dr Seemal Jelani

70. 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
4/27/2017
Dr Seemal Jelani

71. 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
4/27/2017
Dr Seemal Jelani

72. Check Point 21-3 Answer Back
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
4/27/2017
Dr Seemal Jelani

73. Why is oil immiscible with water?
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
4/27/2017
Dr Seemal Jelani

74. 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
4/27/2017
Dr Seemal Jelani

75. 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
4/27/2017
Dr Seemal Jelani

76. 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.
4/27/2017
Dr Seemal Jelani

77. 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.
4/27/2017
Dr Seemal Jelani

78. 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
4/27/2017
Dr Seemal Jelani

79. 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
4/27/2017
Dr Seemal Jelani

80. 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.
4/27/2017
Dr Seemal Jelani

81. 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.
4/27/2017
Dr Seemal Jelani

82. 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
4/27/2017
Dr Seemal Jelani

83. 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.
4/27/2017
Dr Seemal Jelani

84. 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
4/27/2017
Dr Seemal Jelani

85. 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.
4/27/2017
Dr Seemal Jelani

86. 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
4/27/2017
Dr Seemal Jelani

87. 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.
4/27/2017
Dr Seemal Jelani