1. LI: Understand how IR Spectroscopy works
Explain how infrared spectroscopy can be used to identify types of organic bonds
Identify main peaks on an IR spectrum

2. Bonding recap – match the words to phrases
Covalent bond Polar bond Non- polar bond Molecular dipole Instantaneous dipole Dipole moment
A mathematical value for the centre of positive and negative charges in a molecule Uneven sharing of electrons in a colvalent bond due to electronegativity differences Electrons are shared between atoms and the nucleii move closer A dipole that is induced by distortion of the electron cloud on a non-polar bond by a neighbouring molecule When bond dipoles are assymetrically arranged around a central molecule Even sharing of electrons in a covalent bond

3. Infra-red light Longer wavelengths that visible light
Radiation with wavelengths nm x10-6 m

4. Covalent bonds are not really fixed in position as they are in our model kits
They are more like springs between atoms and can be stretched in length and bent or twisted.

5. Infra-red light
Radiation in the infrared (IR) region of the electromagnetic spectrum has the energy to excite vibrations of covalent bonds. The absorption of IR radiation causes bonds to stretch and bend.
Stretches correspond to the increasing and decreasing of the bond lengths within a molecule. Bends correspond to the increasing and decreasing of the angle between bonds in a molecule.

7. Creating the spectrum
When IR radiation is shone on a molecule, it can only absorb it if:
The energy of the radiation corresponds to the energy required to vibrate the molecule in a certain way, and
The vibration leads to the dipole moment of the molecule changing.
If radiation is absorbed at a particular wavelength, we see a peak on the spectrum.

8. When a ketone carbonyl bond is stretched, the vibration frequency required to stretch it is the same as one of the frequencies of IR light.

10. Wavenumber
Infrared frequencies are recorded in wavenumber (cm -1) This number is inversely proprotional to wavelength (ie the bigger the wavenumber, the shorter and higher energy wavelength. Organic molecules have many bonds with many possible vibrations so infra – red spectra have many peaks. However some certain functional groups show at particular wavenumbers which can allow us to identify functional groups. Note that the spectrum is upside down as we are recording how much radiation is not transmitted through the sample (100% – absorption)

11. Fingerprint region - ignore
Example: Butanone
C-H stretch
C=O stretch
Fingerprint region - ignore

12. Work time:
Use page 12 of your workbooks to take notes on how infra red spectra are created and used to identify molecules. Then have a go at questions on worksheet. Your notes should include keywords: Wavenumber and % transmission Bond stretching and bond bending Dipole moment Vibrations Functional groups and fingerprint region