Intermolecular bonding

Understanding the different types of intermolecular bond is usually straight forward. Describing them in writing is not 😳.

All intermolecular bonds arise from the attraction between dipoles in molecules. There are three types of intermolecular bond:

• hydrogen bonds
• permanent dipole – permanent dipole intermolecular bonds
• instantaneous dipole – induced dipole intermolecular bonds which are also known as London forces

Different exam boards and textbooks use the terminology interchangeably. Some exam boards are more flexible than others so double check your syllabus.

Technically, van der Waals forces are both permanent dipole – permanent dipole intermolecular bonds and instantaneous dipole – induced dipole intermolecular bonds / London forces, but AQA use the term van der Waals forces only to describe instantaneous dipole – induced dipole intermolecular bonds and ‘dipole – dipole forces’ in place of permanent dipole – permanent dipole intermolecular bonds. Please bear this in mind when working through exam questions and answers.

Permanent dipoles occur when two covalently bonded atoms have substantially differing electronegativity. The more electronegative atom attracts the shared electrons in the covalent bond more strongly. The separation of opposite charges is called a dipole.

HCl has a permanent dipole because Cl is more electronegative than H. Molecules with a permanent dipole are often described as polar molecules.

The spatial arrangement of polar covalent bonds determines if a molecule is polar or not – in symmetrical molecules the polar bonds may cancel out.

Tetrachloromethane (CCl₄) is symmetrical – the 4 polar bonds cancel each other out so the molecule does not have an overall dipole – it will not be capable of permanent dipole – permanent dipole bonding.

Chloromethane (CH₃Cl) is asymmetrical with 1 polar bond – it is capable of permanent dipole – permanent dipole bonding.

Ammonia (NH₃) is asymmetrical with 3 polar bonds but a pyramidal structure – it is capable of permanent dipole – permanent dipole bonding.

Polar molecules will form permanent dipole – permanent dipole intermolecular bonds between them as the δ+ end of one molecule will be attracted to the δ- end of a neighbouring molecule.

Instantaneous dipoles form in non-polar molecules or atoms (in the case of noble gases). Electrons are always moving in molecules. Electron movements can lead to an uneven distribution of charge across the molecule, giving rise to an instantaneous dipole.

An instantaneous dipole in one molecule will induce a dipole in a neighbouring molecule. The two molecules are very briefly attracted to each other (an instantaneous dipole – induced dipole i.b.). The electron movement changes, the instantaneous dipole changes and new attractions and repulsions occur.

The strength of i.d. – i.d. bonds or London forces depends on the size of the molecule (bigger molecules = more electrons which means a larger instantaneous and induced dipoles) and how close the molecules can pack together or align (these intermolecular bonds are really very weak in the grand scheme of things and so only act short range).

This means that branched chain isomers will have weaker London forces than straight chain isomers, and this will be reflected in a lower boiling point. Krypton has a higher boiling point than argon because it has far more electrons (which means the potential for greater uneven distributions of charge across the atoms) so it has stronger London forces.

Hydrogen bonds are the strongest intermolecular bond. When hydrogen is covalently bonded to a very electronegative atom (oxygen, nitrogen or fluorine), a very strong partial positive / ẟ+ charge forms on the hydrogen atom. The lone pair of electrons on the electronegative atom in one molecule will be very strongly attracted to the ẟ+ hydrogen atom in a neighbouring molecule.

You must draw hydrogen bonds carefully – they are linear, you should show the ẟ+ and ẟ- charges on relevant atoms and the lone pairs of electrons.

Intermolecular bonding is partially responsible for solubility of molecules
in a particular solvent. If the solute molecule is able to make strong
enough intermolecular bonds with the solvent molecules (as in strong
enough to overcome the intermolecular bonds between the solute molecules themselves, and between the solvent molecules themselves), it will dissolve.

In reality we are talking like for like, so molecules only capable of London forces will dissolve in solvents which are only capable of London forces eg. long chain alkanes dissolving in cyclohexane; propanone dissolving in methanol because the two molecules can form hydrogen bonds between themselves.

In summary:

Practice questions

1. A student incorrectly suggests that bromoethane has a lower boiling point than chloroethane. They explain that as bromine is less electronegative than chlorine, the Br-C bond is less polar and so any permanent dipole – permanent dipole intermolecular bonds will be weaker in bromoethane. Discuss.

2. There are several isomers of butanol. 2-methylpropan-2-ol has the lowest boiling point of of them all. Suggest why. 

3. Predict whether ethene has a lower or higher boiling point than tetrafluoroethene. Explain your reasoning. 

4. Name the strongest type of intermolecular bond present between molecules of pentanol. Explain how they form. 

5. Propanone is soluble in ethanol and methylbenzene is soluble in hexane. Name the strongest type of intermolecular bond between (i) ethanol and propanone and (ii) hexane and methylbenzene. Explain how each of these intermolecular bonds are formed.

6. Propanal, prop-2-en-1-ol and butane have the same molecular mass. Predict the relative boiling points of these molecules and explain your reasoning in terms of the intermolecular bonds present in each substance.

7. Explain what is meant by the term ‘electronegativity’. Explain how permanent dipole – permanent dipole bonds arise in chloromethane. 

8. Cyclohexene is prepared by the dehydration of cyclohexanol using concentrated phosphoric acid. With reference to relevant intermolecular bonds, explain why cyclohexene can be separated from the mixture by distillation. 

Answers

1. The C-Br bond is less polar than the C-Cl bond (make the full comparison), but because bromoethane is a bigger molecule with far more electrons, the instantaneous dipole – induced dipole bonds  / London forces will be far stronger, which must account for the fact that the boiling point of bromoethane is higher, and outweighs the effect of stronger permanent dipole – permanent dipole bonds in chloroethane. 

2. All the isomers will have the same ability to hydrogen bond and the same number of electrons, but the structure of 2-methylpropan-2-ol means that molecules cannot align or pack closely together, so the London forces / instantaneous dipole – induce dipole bonds are weaker. Less energy is needed to break them, hence the lower boiling point.

3. Ethene has a lower boiling point than tetrafluoroethene (fewer electrons in ethene, weaker London forces / instantaneous dipole – induced dipole bonds, less energy needed to break them).

4. Hydrogen bonds. The O-H bond is polar because O is more electronegative than hydrogen. A strong attraction forms between a lone pair of electrons on the O of one molecule and the ẟ+ H of another molecule.

5. (i) hydrogen bonds between propanone and ethanol; the O-H bond in ethanol is polar because O is more electronegative than hydrogen. A strong attraction forms between a lone pair of electrons on the O of a propanone molecule and the ẟ+ H of an ethanol molecule.

         (ii) London forces / instantaneous dipole – induced dipole bonds between methylbenzene and hexane; electron movements create an uneven distribution of charge across a molecule (an instantaneous dipole) which induces a dipole in a neighbouring molecule. The ẟ+ region of one molecule is momentarily attracted to the ẟ- region in the neighbouring molecule. 

6. Order of boiling point is prop-2-en-1-ol > propanal > butane. Prop-2-en-1-ol is capable of hydrogen bonding which is the strongest intermolecular bond and will take the most energy to overcome. Propanal has permanent dipole – permanent dipoles. Butane only forms London forces / instantaneous dipole – induced dipole bonds which are the weakest, hence the lowest boiling point / takes the least amount of energy to overcome. 

7. Electronegativity is the ability of an atom to attract electrons in a covalent bond towards itself. Cl is more electronegative than C so the Cl-C bond is polar. The chloromethane molecule has a permanent dipole because the molecule is asymmetrical / the polar bond creates a permanent uneven distribution of charge across the molecule. The ẟ- region of one molecule is attracted to the ẟ+ region region in another. 

8. Cyclohexene has London forces / instantaneous dipole – induced dipole bonds between molecules; cyclohexanol and phosphoric acid both have hydrogen bonds between molecules. It takes much less energy to break the intermolecular bonds in cyclohexane, so it has the lowest boiling point and will boil off first during distillation. 

If you would like some basic additional questions, then there is also an excellent set on bonding here – you can refresh your understanding of ionic and covalent bonding as well as scroll through for questions on electronegativity, dipoles and intermolecular bonding in general. Just remember to adapt the answers for the terminology that you need for your syllabus (I’m thinking of the use of the term van der Waals forces in particular).