Dipoledipole attraction Van der waals force

1
Intermolecular force

• Dipole-dipole attraction (Van der waals force)
• Dipole permanent dipole
• or instantaneous dipole
• or induced dipole
• Strength depends on
• Size of electron cloud (molecular mass)
• Polarity of dipole (e.g b.p. H-F gt Ne , both mr
  20)
• Surface area of molecules (e.g. b.p.
  1,1-dimethyl propane lt pentane, both C5H12)

2
Intermolecular force van der Waals and hydrogen
bonding
van der Waals radii (fixed distance between
molecules)
3
Intermolecular force van der Waals and hydrogen
bonding
Molecular crystal (due to strong van der Waals
force)
Iodine crystal
4
Intermolecular force van der Waals and hydrogen
bonding Comparing the strength of van der Waals
force, covalent bond and hydrogen bonding
5
Intermolecular force van der Waals and hydrogen
bonding
Hydrogen bonding (in HF, H2O, NH3, CH3OH, CH3Cl
and ethyl ethanoate)
6
Intermolecular force van der Waals and hydrogen
bonding
Hydrogen bonding (in HF, H2O, NH3, CH3OH, CCl3H
and ethyl ethanoate)
7
Intermolecular force van der Waals and hydrogen
bonding
Essential requirements for the formation of a
hydrogen bond
1. A hydrogen atom must be directly bonded to a
highly electronegative atom (F, O, N).
2. An unshared pair of electrons (lone pair
electrons) on the electronegative atom.
x x
Class work Q.2 p.275
8
Specials in Hydrogen bonding 1. Intermolecular
and intramolecular hydrogen bonding
cis- butenedioic acid m.p. 130 oC
trans- butenedioic acid m.p. 290 oC
Class work Q.4 p.275
9
Specials in Hydrogen bonding 2. Abnormal of HF,
H2O, NH3
Hydrogen bonding in HF, H2O and NH3 rises their
boiling points.
Molecular mass increases, Van der Waals force
increases, b.p. increases
Class work Q.5 p.275
10
Specials in Hydrogen bonding 3. High boiling
point and water solubility of C2H5OH
RSH has higher van der Waals force but ROH has
hydrogen bonding,
11
Specials in Hydrogen bonding 4. Dimer in
carboxylic acids
Class work Q.1 p.275
2CH3COOH(l) ? (CH3COOH)2(g)
12
Specials in Hydrogen bonding 5. Anormal
behaviour of ice

• Volume of ice (solid water) is greater than that
  of liquid water.
• Density of ice is lower than that of water.

13
Specials in Hydrogen bonding 6. Hydrogen bonding
in protein and DNA
Hydrogen bonding causes the polypeptide chain to
become twisted into tightly coiled helices.
Hydrogen bonding causes formation of double helix
of DNA.
Class work Q.9 p.276