The Functional Map

0 / 19 explored

Intermolecular forces

What holds the molecules together

Physical properties — boiling point, melting point, solubility — come from the forces between molecules, not the covalent bonds inside them. Walk the ladder. Then test it on the atlas.

A three-question test

Use this on any new structure. You do not need a lecture — just these questions.

  1. 1

    Is there a hydrogen attached to N, O, or F?

    Yes → hydrogen bonding (plus dipole plus London). No → question 2.

  2. 2

    Is the molecule polar — uneven electron density?

    Yes → dipole–dipole (plus London). No → question 3.

  3. 3

    Does it have electrons?

    Yes. Everything has London dispersion. That is the whole list for alkanes.

  1. 01

    London dispersion

    Induced dipole–induced dipole, van der Waals

    Every molecule. The only force in alkanes, alkenes, alkynes and most aromatics.

    Grows with more electrons and more surface contact. That is why boiling points climb a homologous series, and why branched isomers boil lower.

    AlkanesAlkenesAlkynesAromatics

    The straight chain has more surface area for temporary dipoles to line up. The spherical isomer has less contact, so weaker London forces.

    See these on the atlas

  2. 02

    Dipole–dipole

    Permanent dipole attractions

    Polar molecules: alkyl halides, aldehydes, ketones, ethers, esters, nitriles, nitro compounds, acyl chlorides.

    Raises boiling point above the matched alkane. Still much weaker than hydrogen bonding. Nitriles have especially large dipoles.

    Alkyl halidesEthersAldehydesKetonesEstersAcyl chloridesAcid anhydridesNitriles

    The C=O is polar, so propanone molecules attract by dipole–dipole as well as London forces. Butane has only London forces.

    See these on the atlas

  3. 03

    Hydrogen bonding

    H attached to N, O or F, attracted to a lone pair on N, O or F

    Alcohols, phenols, carboxylic acids, 1°/2° amines, 1°/2° amides. Ethers, ketones and esters can accept but not donate.

    The reason small alcohols are liquids, mix with water, and boil far above their ether isomers. Acids dimerise. Amides can be solids.

    AlcoholsPhenolsCarboxylic acidsAminesAmides

    No. There is no H on the oxygen. Hydrogen bonding between like molecules needs a donor and an acceptor. Ketones only have the acceptor.

    See these on the atlas

  4. 04

    Ion–dipole

    Ions interacting with polar molecules (usually water)

    Salts: carboxylates (RCOO⁻ Na⁺), ammonium salts (RNH₃⁺ Cl⁻), not the parent neutrals.

    Why sodium ethanoate dissolves readily, and why amines and acids become water-soluble when they are protonated or deprotonated.

    Carboxylate saltsAmmonium salts

    OH⁻ deprotonates it to benzoate. The anion is solvated by ion–dipole attractions to water — much stronger than the H-bonds the parent acid could offer.

Similar mass, different boiling points

These small molecules sit between 44 and 60 g mol⁻¹. The climb is the intermolecular-force ladder — London, then dipole, then hydrogen bonding, then the amide network.