Recap – Last Lecture A molecule is polar if it contains polar bonds and is of asymmetric shape. σ-bonds have electron density along the bond axis. π-bonds are formed by overlap of p orbitals and have electron density above and below bond axis. Multiple bonds contain one σ- and one or two π- bonds. Hybridisation of an atom may be determined by: Counting σ bonds + lone pairs, give no of hybrid orbitals If two hybrid orbitals sp hybridisation If three hybrid orbitals sp 2 hybridisation If four hybrid orbitals sp 3 hybridisation 1

2 Intramolecular Forces We have examined the bonding forces that hold atoms together, these are intramolecular forces. The forces are generally strong and to break them you perform a chemical reaction. ForceAttractionEnergy/kJ mol -1 Example IonicCation - Anion NaCl CovalentNuclei sharing e-pair H-H MetallicDelocalised electrons Fe

3 Intermolecular Forces What determines if a substance is a solid, liquid or gas at room temp? Boiling points are determined by intermolecular forces. Melting points are determined by intermolecular forces and crystal packing forces. Intermolecular forces are attractions between molecules.

4 Types of Intermolecular Force Primarily focus on covalently bonded molecules. (Ionic compounds already are 3-D structures.) Intermolecular forces are much weaker than covalent bonds. Three forces: –London Dispersion Forces: occur between all molecules. –Dipole – dipole Forces: occur between polar molecules. –Hydrogen bonds are a special case of dipole-dipole forces.

5 Dispersion Forces Always present; arise from fluctuations in the electron clouds surrounding molecules. They are instantaneous dipole – induced dipole attractions. The greater the dispersion forces the higher the boiling point.

6 Dispersion Forces Total number of electrons directly related to strength of the dispersion forces. The greater the surface area of the molecules the greater the dispersion forces.

7 Dispersion Forces The greater the number of atoms in a molecule, the greater the number of electrons and hence the greater the dispersion forces.

8 Dipole – Dipole Forces A covalent bond between atoms of different electronegativity will be polar. If a molecule containing polar bonds is asymmetric then the molecule will possess a permanent dipole. Dipoles of adjacent molecules align with one another resulting is an attractive force. Typical energy 5 – 25 kJ mol –1 (much less than a covalent bond).

9 Hydrogen Bonding If hydrogen is bonded to a very electronegative atom, the charge on the hydrogen nucleus is partially exposed. This is attracted to the high electron density of another very electronegative atom. The small size of the hydrogen atom allows close approach of the molecules and an unusually strong and directed dipole-dipole force results. Typical energy 10 – 40 kJ mol –1 (still much less than a covalent bond).

10 Hydrogen Bonding N, O and F are all 2 nd Period elements that: –They are the most electronegative elements. –They are small (only 2s, 2p in outer shell). –They have lone pairs.

11 Applications The structure and function of DNA has been widely studied. Its phosphate ion ‘backbone’ provides water solubility and the hydrogen bonded base pairs balance stability (and hence retention of genetic information) with a facile enzyme catalysed cleavage at body temperature allowing replication. Many anti-viral drugs aim to disrupt either the structure or the replication of viral DNA.

By the end of this lecture, you should: describe the different kinds of intermolecular forces that exist. identify which intermolecular forces are present and which are more important between different molecules. relate variations in boiling points in related compounds to their intermolecular forces. −be able to complete the worksheet (if you haven’t already done so…) 12 Learning Outcomes:

13 Questions to complete for next lecture: 1.Complete the table and explain the trend in enthalpies of vaporisation (the heat required to boil a liquid). 2.Which of the following substances show H-bonding? Draw the structure for those that do. a) C 2 H 6 b) CH 3 OHc) CH 3 CONH 2 d) H 2 S e) HCl f) NH 3 SubstanceΔH vap /kJ mol –1 Intermolecular forcesMolar Mass Butane, CH 3 CH 2 CH 2 CH 3 22 Diethyl ether, (C 2 H 5 )-O- (C 2 H 5 ) 27 Methanol, CH 3 OH38 Ethanol, CH 3 CH 2 OH43 Water, H 2 O44 Decane, CH 3 (CH 2 ) 8 CH 3 51