Intermolecular forces – dipole – dipole forces Lesson Objectives: To describe the interaction of molecules by permanent dipole – dipole To compare dipole.

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Intermolecular forces – dipole – dipole forces

Lesson Objectives:To describe the interaction of molecules by

permanent dipole – dipoleTo compare dipole - dipole forces

(permanent and induced)To explain the interactions between

permanent dipoles, and implications for boiling points

KeywordsDipoles, permanent dipole moment,

intermolecular, flip, attraction, repulsion, rotation

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Intermolecular forces - dipole – dipole forces

Types of intermolecular forces:Induced dipole-dipole (van der Waals)

(weakest)Permanent dipole-dipoleHydrogen bonding

(strongest)

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Types of intermolecular forces:Induced dipole-dipole/van der

Waals/dispersion forces/temporary dipole

Permanent dipole-dipoleHydrogen

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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.

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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.

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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 2.0

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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.

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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:

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CO2 BF3 CCl4,

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CHCl3 is a polar molecule but CCl4 is a nonpolar molecule.

CCl4CHCl3

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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.

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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.

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NH3CHCl3 SO2

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Polarity of moleculesHexaneCyclohexane, Ethanol Propan-1-ol, Propanone Butanone Water.

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Polarity of molecules18/04/2023

F

AB C

E

G

D

Name the compounds above and state if they are polar or non polar.

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Polarity of molecules18/04/2023

F

AB C

E

G

D

Cyclohexane, nonpolar

Water - polarEthanol - polar

Hexane – non polarPropanone - polar

Butanone - polarPropan -1 – ol - polar

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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.

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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.

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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, CH3CH3, and fluoromethane, CH3F.

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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°.

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http://www.chemguide.co.uk/atoms/bonding/c2h6vch3f.GIF

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Boiling points of ethane, CH3CH3, and fluoromethane, CH3F,

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The higher boiling point of fluoromethane is due to the large permanent dipole on the molecule because of the high electronegativity of fluorine.

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Substance Cl2 HBr CH3CH(CH3)CH3 CH3COCH3

Number of electrons

Van der waal forces

Permanent dipole

Melting point(oC)

-101 -88 -159 -95

Boiling point(oC)

-45 -67 -73 -44

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Number of electrons

Van der waal forces

Permanent dipole

Melting point(oC)

Boiling point(oC)

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• 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.

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Number of electrons

34 36 34 32

Van der waal forces

YES YES YES YES

Permanent dipole

NO YES NO YES

Melting point(oC)

-101 -88 -159 -95

Boiling point(oC)

-45 -67 -73 -44

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• 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.

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Substance

Cl2 HBr CH3CH(CH3)CH3 CH3COCH3

Number of electrons

34 36 34 32

Van der waal forces

YES YES YES YES

Permanent dipole

NO YES NO YES

Melting point(oC)

-101 -88 -159 -95

Boiling point(oC)

-45 -67 -73 -44

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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?

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CHCl3 Vs. CCl4Here is another example showing the dominance of the dispersion forces: Trichloromethane, CHCl3, 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, CCl4, is non-polar. The outside of the molecule is uniform - in all directions. CCl4 has to rely only on dispersion forces. So which has the higher boiling point?

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http://www.chemguide.co.uk/atoms/bonding/chcl3.GIF

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CHCl3 Vs. CCl4

Here is another example showing the dominance of the dispersion forces Trichloromethane, CHCl3, 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, CCl4, is non-polar.

The outside of the molecule is uniform - in all directions. CCl4 has to rely only on dispersion forces.

So which has the higher boiling point? CCl4 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:◦ CHCl3 61.2°C

◦ CCl4 76.8°C

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http://www.chemguide.co.uk/atoms/bonding/chcl3.GIF

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• CCl4 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:

CCl4 76.8°CCHCl3

61.2°CNumber of e-

58

74

Polar molecule Non Polar molecule

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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, CH4, with tetrachloromethane, CCl4, shows this most strongly.

Both are non-polar, but CH4 is a gas at room Temperature and CCl4 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, CH4 into the polar compounds, CH3Cl, CH2Cl2 and CHCl3.

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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:

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CH3Cl Chloromethane – Bpt = -24.2°C

CH2Cl2 dichloromethane – Bpt = 39.6°C

CHCl3 Trichloromethane Bpt = 61.2°C

CCl4Tetrachloromethane Bpt = 78.6°C

CCl4 Methane,Bpt = -161.5 °C

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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.

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Intermolecular forces – dipole – dipole forces Lesson Objectives: To describe the interaction of molecules by permanent dipole – dipole To compare dipole.

Documents

permanent dipole dipole

dipoledipole interactions

overall dipole

permanent dipole moment

dipole moment bonds

induced dipoledipolevan

permanent dipoles

waal forces