IMF - Intermolecular Forces of Attraction Also called van...

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1 Intermolecular Forces of Attraction Also called van der Waal’s Forces Responsible for many physical properties of molecular cpds, e.g., BP, MP, density, solubility Two Main categories: London Dispersion Forces Permanent dipole-dipole forces Perhaps Hydrogen Bonding
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2 Molecular temporary dipoles responsible for London Forces
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3 London Dispersion Forces Instantaneous dipoles present in ALL molecules Non-polar molecules have only London London increases with increasing molecular mass London decreases as molecular surface area decreases
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4 He Ne Ar Kr Xe Molecular Mass, amu 4.0 20.2 39.9 83.8 131.3 Boiling Point (K) 4.6 27.3 87.5 120.9 166.1 Noble Gas atoms Inc London
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5 Molecular Mass, amu 38.0 71.0 159.8 253.8 Boiling Point (K) 85.1 238.6 332.0 457.6 Halogen Molecules Nonpolar Molecules Inc London
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6 Nonpolar Hydrocarbon Molecules BP, °C -161 -89 -44 -1 36 68 98 125 151 174 Inc London
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7 CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 C CH 3 CH 3 CH 3 n-pentane neopentane Greater surface area… Greater London forces More compact… Less London 72.15 g/mol 72.15 g/mol BP 36.1°C BP 9.5°C Structural isomers, C 5 H 12
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8 Permanent Dipole-Dipole Forces Present only in polar molecules in addition to London Forces Strength increases with molecular polarity
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9 Permanent Dipole-Dipole Forces Present only in polar molecules in addition to London Forces Strength increases with molecular polarity For molecules of comparable molecular mass, permanent dipole forces can significantly differentiate physical properties MW, g/mol μ, Debye BP, °C F 2 HCl (CH 3 ) 2 C=CH 2 (CH 3 ) 2 C=O 38.0 36.5 56.1 58.1 0 1.08 0 2.8 -188 -85 -6.9 56
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10 H 3 CH 2 CH 3 3 OCH 3 3 Cl 3 CHO 3 CN
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11 For molecules of rather different mass, London forces are likely of most impact to physical properties MW, g/mol μ, Debye BP, °C HCl HBr HI 36.5 80.9 127.9 1.08 0.78 0.38 -85 -67 -35 Inc London Inc Polarity
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12 London only
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13 Most polar Greatest London London only
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14
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15 H 2 O NH 3 HF
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16
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17 Hydrogen Bonding A special case of permanent dipole-dipole interaction To hydrogen bond, a molecule must contain a very polar N-H, O-H, F-H bond. δ+ δ- δ+ δ- δ+ δ- N H O H F H
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18 H X: Hydrogen Bonding A special case of permanent dipole-dipole interaction To hydrogen bond, a molecule must contain a very polar N-H, O-H, F-H bond.
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