Intermolecular forces – dipole – dipole forces Lesson Objectives: To describe the interaction of molecules by permanent dipole – dipole To compare dipole - dipole forces (permanent and induced) To explain the interactions between permanent dipoles, and implications for boiling points Keywords Dipoles, permanent dipole moment, intermolecular, flip, attraction, repulsion, rotation 30/11/2015 1

Intermolecular forces - dipole – dipole forces Types of intermolecular forces: Induced dipole-dipole (van der Waals) (weakest) Permanent dipole-dipole Hydrogen bonding (strongest) 30/11/2015 2

Intermolecular forces - dipole – dipole forces Types of intermolecular forces: Induced dipole-dipole/van der Waals/dispersion forces/temporary dipole Permanent dipole-dipole Hydrogen 30/11/2015 3

Intermolecular forces - dipole – dipole forces Temporary dipoles exist in all molecules, but in some molecules there is also a permanent dipole. Most covalent bonds have a degree of ionic character resulting from a difference in electronegativity between the atoms. This results in a polar bond and a dipole. 30/11/2015 4

Intermolecular forces - dipole – dipole forces A molecule like HCl has a permanent dipole because chlorine is more electronegative than hydrogen. The permanent, in-built dipoles will cause the molecules to attract each other. Dipole-dipole interactions are not an alternative to van der waal forces. Molecules which have permanent dipoles will therefore have boiling points rather higher than molecules which only have temporary fluctuating dipoles. 30/11/2015 5

Polar bonds - polarity and dipole Non-polar bonds occur when the difference in electronegativity between the two atoms is less than 0.5 Polar bonds occur when the difference in electronegativity between the two atoms is between 0.4 and 2.0 Ionic bonds occur when the difference in electronegativity between the two atoms is greater than /11/2015

Polarity and dipole moment Bonds between species with an electronegativity difference of more than about 0.5, are considered to be polar. For example, C (2.5) or Hydrogen (2.1) bonded to Fluorine (4.0), Oxygen (3.5) or Nitrogen (3.0) will be left carrying a partial positive charge because of the difference in electronegativities. This charge separation within a molecule is called a dipole and the vector describing it is the dipole moment of the molecule. 30/11/2015 7

Intermolecular forces - dipole – dipole forces In many cases, the presence of polar bonds (dipoles) does not result in a permanent dipole on the molecule. This is because other polar bonds (dipoles) in the same molecule cancelling each other out. This effect can be seen in a number of linear, trigonal planar and tetrahedral substances: 30/11/ CO 2 BF 3 CCl 4,

Intermolecular forces - dipole – dipole forces 30/11/ CHCl 3 is a polar molecule but CCl 4 is a nonpolar molecule. CCl 4 CHCl 3

Intermolecular forces - dipole – dipole forces There are dipoles resulting from polar bonds but the vector sum of these dipoles is zero, the dipoles cancel each other out. The molecule thus has no overall dipole and is said to be non-polar. Non-polar molecules are those in which there are no polar bonds or in which the dipoles resulting from the polar bonds all cancel each other out. The only intermolecular forces that exist between non- polar molecules are temporary-induced dipole attractions, or Van der Waal’s forces. 30/11/

Intermolecular forces - dipole – dipole forces In the molecules below, there are dipoles on the molecule which do not cancel each other out: The dipoles resulting from polar bonds whose vector sum is not zero. The molecules have permanent dipole and are said to be polar. 30/11/ NH 3 CHCl 3 SO 2

Polarity of molecules Hexane Cyclohexane, Ethanol Propan-1-ol, Propanone Butanone Water. 30/11/

Polarity of molecules 30/11/ F A B C E G D Name the compounds above and state if they are polar or non polar.

Polarity of molecules 30/11/ F A B C E G D Cyclohexane, nonpolar Water - polar Ethanol - polar Hexane – non polar Propanone - polar Butanone - polar Propan -1 – ol - polar

Intermolecular forces - dipole – dipole forces In addition to the Van der Waal's forces caused by temporary dipoles, molecules with permanent dipoles are also attracted to each other by dipole- dipole bonding. This is an attraction between a permanent dipole on one molecule and a permanent dipole on another. 30/11/

Intermolecular forces - dipole – dipole forces Dipole-dipole bonding usually results in the boiling points of the compounds being slightly higher than expected from temporary dipoles alone. It slightly increases the strength of the intermolecular bonding. The effect of dipole-dipole bonding can be seen by comparing the melting and boiling points of different substances which should have Van der Waal's forces of similar strength. Suggest two substances that will have VDW forces of similar strengths. 30/11/

Van der waals Vs. dipole-dipole A molecule like HCl has a permanent dipole because chlorine is more electronegative than hydrogen. These permanent, in-built dipoles will cause the molecules to attract each other rather more than they otherwise would if they had to rely only on dispersion forces. All molecules experience dispersion forces. Dipole-dipole interactions occur in addition to van der waals forces. Molecules which have permanent dipoles will therefore have boiling points rather higher than molecules which only have temporary fluctuating dipoles. Dipole-dipole attractions are fairly minor compared with dispersion forces. Their effect can only really be seen if you compare two molecules with the same number of electrons and the same size. For example, the boiling points of ethane, CH 3 CH 3, and fluoromethane, CH 3 F. 30/11/

Ethane Vs. Fluoromethane Why choose these two molecules to compare Both have identical numbers of electrons, and if you made models you would find that the sizes were similar - as you can see in the diagrams. That means that the dispersion forces in both molecules should be much the same. The higher boiling point of fluoromethane is due to the large permanent dipole on the molecule because of the high electronegativity of fluorine. However, even given the large permanent polarity of the molecule, the boiling point has only been increased by some 10°. 30/11/

Intermolecular forces - dipole – dipole forces Boiling points of ethane, CH 3 CH 3, and fluoromethane, CH 3 F, 30/11/ The higher boiling point of fluoromethane is due to the large permanent dipole on the molecule because of the high electronegativity of fluorine.

Intermolecular forces - dipole – dipole forces SubstanceCl 2 HBrCH 3 CH(CH 3 )CH 3 CH 3 COCH 3 Number of electrons Van der waal forces Permanent dipole Melting point ( o C) Boiling point( o C) /11/

Intermolecular forces - dipole – dipole forces SubstanceCl 2 HBrCH 3 CH(CH 3 )CH 3 CH 3 COCH 3 Number of electrons Van der waal forces Permanent dipole Melting point ( o C) Boiling point( o C) 30/11/ Complete the table above. Based on your understanding of intermolecular forces, arrange these molecules in terms of increasing boiling point. Give reasons for your arrangement.

Intermolecular forces - dipole – dipole forces SubstanceCl 2 HBrCH 3 CH(CH 3 )CH 3 CH 3 COCH 3 Number of electrons Van der waal forces YES Permanent dipole NOYESNOYES Melting point ( o C) Boiling point( o C) /11/ Complete the table above. Based on your understanding of intermolecular forces, arrange these molecules in terms of increasing boiling point. Give reasons for your arrangement.

Intermolecular forces - dipole – dipole forces SubstanceCl 2 HBrCH 3 CH(CH 3 )CH 3 CH 3 COCH 3 Number of electrons Van der waal forces YES Permanent dipole NOYESNOYES Melting point ( o C) Boiling point( o C) /11/ Compare: Chlorine and butane why is the boiling point of chlorine bigger, consider surface area HBr and Chlorine Chlorine and acetone(propanone). Acetone has a higher bpt, but not with a significant difference, despite having both vdw and dipole – dipole. Why?

CHCl 3 Vs. CCl 4 Here is another example showing the dominance of the dispersion forces: Trichloromethane, CHCl 3, is a highly polar molecule because of the electronegativity of the three chlorines. There will be quite strong dipole-dipole attractions between one molecule and its neighbours. On the other hand, tetrachloromethane, CCl 4, is non-polar. The outside of the molecule is uniform - in all directions. CCl 4 has to rely only on dispersion forces. So which has the higher boiling point? 30/11/

CHCl 3 Vs. CCl 4 Here is another example showing the dominance of the dispersion forces Trichloromethane, CHCl 3, is a highly polar molecule because of the electronegativity of the three chlorines. There will be quite strong dipole-dipole attractions between one molecule and its neighbours. On the other hand, tetrachloromethane, CCl 4, is non-polar. The outside of the molecule is uniform - in all directions. CCl 4 has to rely only on dispersion forces. So which has the higher boiling point? CCl 4 does, because it is a bigger molecule with more electrons. The increase in the dispersion forces more than compensates for the loss of dipole-dipole interactions. The boiling points are: ◦ CHCl °C ◦ CCl °C 30/11/

Intermolecular forces - dipole – dipole forces 30/11/ CCl 4 has higher boiling point because it is a bigger molecule with more electrons. Hence the van der waal forces contribute more to intermolecular forces. Bpts: CCl °CCHCl ° C Number of e Polar moleculeNon Polar molecule

Intermolecular forces - dipole – dipole forces Methane is a gas, but all four substituted methanes are liquids. Substitution of Cl for H has obviously changed the nature of the Intermolecular Forces. Chlorine has 17 electrons, compared to one electron on Hydrogen, so the van der waal London Forces will be higher in the substituted hydrocarbons. A comparison of methane, CH 4, with tetrachloromethane, CCl 4, shows this most strongly. Both are non-polar, but CH 4 is a gas at room Temperature and CCl 4 is a liquid. Substitution of 1, 2 or 3 Chlorines for Hydrogen increases the London Forces in the chloro-, dichloro-, and trichloro-methanes, as well, but it has an even more dramatic effect. It turns non-polar methane, CH 4 into the polar compounds, CH 3 Cl, CH 2 Cl 2 and CHCl 3. 30/11/

Intermolecular forces - dipole – dipole forces Each of the C-Cl bonds is highly polarized, because chlorine is much more electronegative than carbon. The substituents are arranged tetrahedrally around the central carbon atom. The chloro-, dichloro- and trichloro- isomers all have net dipole moments as shown in the figure below: 30/11/201528

Intermolecular forces - dipole – dipole forces 30/11/ CH 3 Cl Chloromethane – Bpt = -24.2°C CH 2 Cl 2 dichloromethane – Bpt = 39.6°C CHCl 3 Trichloromethane Bpt = 61.2°C CCl 4 Tetrachloromethane Bpt = 78.6°C CCl 4 Methane, Bpt = °C

Intermolecular forces - dipole – dipole forces The tetrachloromethane is non-polar, because the four equivalent dipoles cancel each other. Tetrachloromethane is not soluble in water. The other three halogenated methanes are water soluble because of their polarity. 30/11/