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Lecture 6 Chemical Bondings II : Intermolecular Forces
Human Biology and Organic Chemistry I HS 37-006-31 (41)
Dr. Thomas Lui ([email protected])
1. Dipole-dipole interaction
2. Dipole-induced dipole interaction
3. Instantaneous dipole-induced dipole interaction
4. Hydrogen bonding
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Intermolecular vs. Intramolecular Force
Intermolecular force Attractive force between one molecule and a neighboring molecule
Intramolecular force
Attractive force which hold an individual molecule together
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Intermolecular Force
Attractive force between covalent molecules
Much weaker than intramolecular force
Determining the physical properties of the substance, such as the state of substance, melting and boiling point, viscosity, density, etc.
2 types Van der Waals force Hydrogen bonding
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Van der Waals Force
Dipole-dipole interaction The molecules are polar in nature (e.g. H-Cl) Attraction between + and - of the permanent dipoles of the neighboring
molecules
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Van der Waals Force
Dipole-induced dipole interaction When a non-polar molecule approaches a polar molecule, a dipole will be
induced in the non-polar molecules Attraction between polar and non-polar molecules Generally weaker than dipole-dipole interaction
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Van der Waals Force
Instantaneous dipole-induced dipole interaction (also known as dispersion forces or London forces) Instantaneous dipole is a temporary
dipole resulted from the fluctuation in the electron cloud, and this will induce a dipole moment in the neighboring atom or molecule
Exists between noble gas and non-polar molecules and last for very short time interval
Much weaker than dipole-dipole interaction
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Van der Waals Force
Strength of instantaneous dipole-induced dipole interaction when : no. of electron in a molecule
Easy to set up instantaneous dipole, as the electrons are weakly held by nucleus e.g. Boiling point : F2 -188
oC Cl2 -34.7
oC Br2 58.8
oC I2 184
oC
surface area
Extension area for contact with other molecules, and a large dipole can be induced e.g. Boiling point :
n-pentane (C5H12) 36oC
2,2-dimethylpropane (C5H12) 9oC
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Hydrogen Bonding
Specific dipole-dipole interaction which occurs when a hydrogen atom covalently bounded to a highly electronegative atom containing lone pair electrons (involving N, O and F) Stronger than dipole-dipole interaction
The attractive force between the lone pair electron of the
electronegative atom and slightly positive charged hydrogen atom in another molecule
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Evidence of Hydrogen Bonding
In general, the boiling points of hydrogen-containing compounds when moving down the group in molecular size and van der Waals force
However, for Group 15, 16 and 17, the general trend is broken by an abnormally high boiling point of the first member of each group
These 3 hydrogen-containing compounds (NH3, H2O and HF) form intermolecular hydrogen bonds, which are stronger than van der Waals force
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Examples of Inter- and Intramolecular Hydrogen Bonding
Intermolecular H-bond Intramolecular H-bond
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A Comparison between Inter- and Intramolecular Force
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Intermolecular Force in Macromolecules Stabilizing and Maintaining the Structure
DNA
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Intermolecular Force in Macromolecules Stabilizing and Maintaining the Structure
Protein
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Classwork
Which of the following substances is/are able to form hydrogen bond? 1) CH3NH2
2) HBr
3) CH2O
4) CH3OH
5) CH3Cl
6) (CH3)3N
H
C
H
O
H3C
N
CH3
CH3
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Reference
1. Bettelheim FA, Brown WH, Campbell MK & Farrell SO (2010). Introduction to general, organic & biochemistry, 9th edition: Thomson Brooks/Cole.
2. McMurry J, Castellion ME, Ballantine DS, Hoeger CA & Peterson VE (2010). Fundamentals of general, organic & biological chemistry, 6th edition: Pearson Prentice Hall.
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