Lecture 6 Chemical Bondings II : Intermolecular Forces

Human Biology and Organic Chemistry I HS 37-006-31 (41)

Dr. Thomas Lui (thomas81hk@gmail.com)

1. Dipole-dipole interaction

2. Dipole-induced dipole interaction

3. Instantaneous dipole-induced dipole interaction

4. Hydrogen bonding

1

Intermolecular vs. Intramolecular Force

Intermolecular force Attractive force between one molecule and a neighboring molecule

Intramolecular force

Attractive force which hold an individual molecule together

2

Intermolecular Force

Attractive force between covalent molecules

Much weaker than intramolecular force

Determining the physical properties of the substance, such as the state of substance, melting and boiling point, viscosity, density, etc.

2 types Van der Waals force Hydrogen bonding

3

Van der Waals Force

Dipole-dipole interaction The molecules are polar in nature (e.g. H-Cl) Attraction between + and - of the permanent dipoles of the neighboring

molecules

4

Van der Waals Force

Dipole-induced dipole interaction When a non-polar molecule approaches a polar molecule, a dipole will be

induced in the non-polar molecules Attraction between polar and non-polar molecules Generally weaker than dipole-dipole interaction

5

Van der Waals Force

Instantaneous dipole-induced dipole interaction (also known as dispersion forces or London forces) Instantaneous dipole is a temporary

dipole resulted from the fluctuation in the electron cloud, and this will induce a dipole moment in the neighboring atom or molecule

Exists between noble gas and non-polar molecules and last for very short time interval

Much weaker than dipole-dipole interaction

6

Van der Waals Force

Strength of instantaneous dipole-induced dipole interaction when : no. of electron in a molecule

Easy to set up instantaneous dipole, as the electrons are weakly held by nucleus e.g. Boiling point : F2 -188

oC Cl2 -34.7

oC Br2 58.8

oC I2 184

oC

surface area

Extension area for contact with other molecules, and a large dipole can be induced e.g. Boiling point :

n-pentane (C5H12) 36oC

2,2-dimethylpropane (C5H12) 9oC

7

Hydrogen Bonding

Specific dipole-dipole interaction which occurs when a hydrogen atom covalently bounded to a highly electronegative atom containing lone pair electrons (involving N, O and F) Stronger than dipole-dipole interaction

The attractive force between the lone pair electron of the

electronegative atom and slightly positive charged hydrogen atom in another molecule

8

Evidence of Hydrogen Bonding

In general, the boiling points of hydrogen-containing compounds when moving down the group in molecular size and van der Waals force

However, for Group 15, 16 and 17, the general trend is broken by an abnormally high boiling point of the first member of each group

These 3 hydrogen-containing compounds (NH3, H2O and HF) form intermolecular hydrogen bonds, which are stronger than van der Waals force

9

Examples of Inter- and Intramolecular Hydrogen Bonding

Intermolecular H-bond Intramolecular H-bond

10

A Comparison between Inter- and Intramolecular Force

11

Intermolecular Force in Macromolecules Stabilizing and Maintaining the Structure

DNA

12

Intermolecular Force in Macromolecules Stabilizing and Maintaining the Structure

Protein

13

Classwork

Which of the following substances is/are able to form hydrogen bond? 1) CH3NH2

2) HBr

3) CH2O

4) CH3OH

5) CH3Cl

6) (CH3)3N

H

C

H

O

H3C

N

CH3

CH3

14

Reference

1. Bettelheim FA, Brown WH, Campbell MK & Farrell SO (2010). Introduction to general, organic & biochemistry, 9th edition: Thomson Brooks/Cole.

2. McMurry J, Castellion ME, Ballantine DS, Hoeger CA & Peterson VE (2010). Fundamentals of general, organic & biological chemistry, 6th edition: Pearson Prentice Hall.

15