LA.3.1 Intermolecular Interactions

An Introduction

Intermolecular interactions are weak non-covalent bonding effects between functional groups on

different molecules. Their strengths depend on the electron density around the interacting areas. The most important interactions include:

 Hydrogen Bonding – The attraction between a hydrogen on one molecule and an electronegative atom on another (Figure 1).  Van der Waals’ – Temporary or permanent dipole-dipole Figure 1 The intermolecular interactions in water (hydrogen bonding) interactions You can guess how two molecules will interact in a crystal by considering where the electron density on each molecule is. What to Look For Bond Polarity More electronegative atoms will draw electrons away from less electronegative ones. This creates a partial positive charge (δ+) on one molecule, which can then act as an attractive area for a partial negative charge (δ-) on another molecule. Lone Pairs Lone pairs are high density negative charge, so will be attracted to a positively charged area. Sterics Very bulky parts of a molecule can block other parts which would usually interact with other molecules. Also, if you have INTRAmolecular (interactions within the same molecule) interactions, these can stop INTERmolecular interactions.

Intermolecular interactions have a large influence on molecule shapes, crystal structures and physical properties of solid-state compounds. Melting and boiling points are strongly affected by intermolecular interactions. Boiling points of Hydrogen Halides:

HCl > HBr > HI

The stronger intermolecular forces, the higher the boiling point. This is due to there being an increased bond polarity in the H-X bond which results in a greater dipole-dipole interaction (Van der Waals’).

Relative Boiling Points:

AMIDE > CARBOXYLIC ACID > NITRILE >> ACYL CHLORIDE ~ ALDEHYDE ~ KETONE

Hydrogen Bonding Hydrogen bonding is responsible for the structure of proteins and DNA and allows us to engineer materials with particular structural features. You will already have encountered simple molecules which have predominantly hydrogen bonds as intermolecular forces: H2O, HF, and NH3 all have very strong H-bonding, as well as alcohols, acids, Figure 2 The hydrogen bonding between water and ammonia amines, and amino acids .The illustration above shows hydrogen bonding between water and ammonia.

Requirements for a Hydrogen Bond:  An H atom covalently bonded to another electronegative atom (e.g. O, Cl, N, etc.) i.e a polar

bond where the H has a partial positive charge.  A second electronegative atom* on a nearby which can easily access the H atom, and thus can donate electron density to it.

*Halides are not very good H-Bond donors – they are dominated by van der Waals’ interactions.

H-bonds range in strength, from almost covalent – Hydrogen bond properties Strong Moderate Weak i.e. very strong - to essentially electrostatic. The -1 Bond Energy (kJ mol ) 60-120 16-60 <12 strongest bonds have the shortest bond lengths

Bond Length (Å) H · · · A 1.2-1.5 1.5-2.2 2.2-3.2 and bond angles closet to 180°, resulting in higher bond energies. The H-A and D-A measurements Bond Length (Å) D · · · A 2.2-2.5 2.5-3.2 3.2-4.0 are included as not all the structures in the CSD Bond Angle (degrees) 175-180 130-180 90-150 have H atoms present; due to the difficulty in locating them using X-ray diffraction.

MALEIC ACID (MALIAC12) displays both inter- The isomer of maleic acid is fumaric acid and intra-molecular H-bonding. One C-O bond (FUMAAC01). Interconversion of these two binds to the accessible O-H, effectively creating forms is difficult because of the C=C bond; this a ring; the other bonds to the O-H of a nearby feature forces the carboxylic acid groups away molecule. In the pure compound, the molecules from each other so they cannot interact. This form chains, as seen below. means both carboxylic acid groups are fully linked to other molecules, also forming chains in the pure compound.

Solubility

Molecules that can form hydrogen bonds are generally soluble in other molecules that can form hydrogen bonds. Ethanol is completely miscible in water as the –OH group can be involved in H bonding with water. Propane is INSOLUBLE in water because it’s not polar, nor capable of forming H bonds.