Forces
Intermolecular Intermolecular forces take place between molecules.
An understanding of intermolecular forces is crucial to a chemist's ability to successfully carry out reactions in the organic chemistry laboratory.
The Forces Between Neutral Atoms or Molecules
Induced Dipole Induced Dipole Dipole Dipole Dipole Induced Dipole
Induced Dipole Induced Dipole (IDID) Interactions (London Dispersion Force)
= polarizability
I = ionization potential r = distance between atoms or molecules
Induced Dipole Induced Dipole Interactions Between Helium Atoms
Boiling Point Differences of the Noble Gases
Atom bp(C) He -269
Ne Ar Kr Xe Rn -246 -186 -152 -107 -62
I (KJ/mol) 2,372
2,081 1,521 1,351 1,170 1,037
(10-24cm3) 0.20
0.40 1.64 2.48 4.04 5.30
IDID Interactions in Molecules
IDID interactions between linear alkanes increase as the chain length increases (approximately 1 kcal per CH2 group)
Alkane CH4 H3CCH3 H3CCH2CH3 H3CCH2CH2CH3 H3CCH2CH2CH2CH3 H3C(CH2)6CH3 H3C(CH2)16CH3 Boiling Point (C) -162 -89 -42 -0.5 36 125 316 (mp = 26 C)
Isomers of Pentane (FW = 72 g/mol)
available surface for IDID interactions
Isomers of Hexane (FW = 86 g/mol)
available surface for IDID interactions
Boiling Point Differences Between Alkanes and (g/mol) Aldehydes
MW 44 58 72 86 100 114 128
Alkane
bp(C) bp (C) of RH of RC(O)H -42 -0.5 35 20 49 76 103 131 153 171
Aldehyde
H3CCH2CH3 H3CCH2CH2CH3 H3CCH2CH2CH2CH3
H3CC(O)H H3CCH2C(O)H H3CCH2CH2C(O)H H3CCH2CH2CH2C(O)H H3C(CH2)4C(O)H H3C(CH2)5C(O)H H3C(CH2)6C(O)H
H3CCH2CH2CH2CH2CH3 69 H3C(CH2)5CH3 H3C(CH2)6CH3 H3C(CH2)7CH3 98 126 151
Dipole Dipole (DD) Interactions
= dipole moment K = Boltzman's constant T = temperature r = distance between atoms or molecules
If induced dipoles can induce dipoles (IDID); then permanent bond dipoles should be able to induce dipoles!
Dipole Induced Dipole (DID) Interactions
polarizability is back!
van der Waals Interactions in Heptanal
MW (g/mol) 44 58 72 86 100 114 128
Alkane
bp(C) bp (C) bp (C) of RH of RC(O)H -42 -0.5 35 69 98 126 151 62 49.5 41 34 33 27 20 20 49 76 103 131 153 171
Aldehyde
H3CCH2CH3 H3CCH2CH2CH3 H3CCH2CH2CH2CH3 H3CCH2CH2CH2CH2CH3 H3C(CH2)5CH3 H3C(CH2)6CH3 H3C(CH2)7CH3
H3CC(O)H H3CCH2C(O)H H3CCH2CH2C(O)H H3CCH2CH2CH2C(O)H H3C(CH2)4C(O)H H3C(CH2)5C(O)H H3C(CH2)6C(O)H
A Special Dipole Dipole Interaction: Hydrogen Bonding
Proton Exchange
Hydrogen Bonding Results in Unusually Strong DD Interactions
Classification of Organic Solvents
Nonpolar
Polar Protic
Polar Aprotic