1. AS 2.12 SPECTROSCOPIC TECHNIQUES Infra-red spectra
b. use infrared spectra, or data from infrared spectra, to deduce functional
groups present in organic compounds and predict infrared absorptions, given wave number data, due to familiar functional groups. This will be limited to:
i. C-H stretching absorptions in alkanes, alkenes and aldehydes
ii. O-H stretching absorption in alcohols and carboxylic acids
iii. N-H stretching absorption in amines
iv. C=O stretching absorption in aldehydes and ketones
v. C-X stretching absorption in halogenoalkanes
vi. as an analytical tool to show the change in functional groups during the oxidation of an alcohol to a carbonyl
c. demonstrate an understanding that only molecules which change their polarity
as they vibrate can absorb infrared radiation
d. demonstrate an understanding that H2O, CO2, CH4 and NO molecules
absorb IR radiation and are greenhouse gases, whilst O2 and N2 are not.
Connector:
Draw the displayed formulae of H2O, CO2, CH4, NO, O2 and N2.
Then use the symbols δ+ and δ-, to show the polarity of the bonds in these molecules.
Crowe2009

2. δ+ δ- δ- δ+ δ- δ+ δ+ δ- δ+ δ+ δ+ δ+ δ-
Non-polar molecules

3. INFRA-RED SPECTROSCOPY
All organic molecules absorb in the infra-red range of the electromagnetic spectrum
These absorptions tend to be narrow and easy to interpret
The absorption spectra produced correspond to vibrations of the bonds in the molecule:

4. Obtaining an IR spectrum
A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in.
The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained.

5. The resulting IR spectrum shows where certain IR frequencies were absorbed by the sample
Note: the absorption peaks are measured in wavenumbers (cm-1)

6. USING INFRA-RED SPECTROSCOPY
Certain bonds give characteristic absorption frequencies

7. INFRA-RED SPECTROSCOPY
Infra-red spectra are used to identify the functional groups in a substance:
Absorption peak (cm-1) Functional group
C-H
C=O
O-H
N-H
C-O
C-X
These are usually found in the Band Region of the spectrum

8. INFRA-RED SPECTROSCOPY
The fingerprint region is used to compare the spectrum of the unknown with that of a known substance

9. INFRA-RED SPECTROSCOPY
Q’s ac4u 286/7

10. Broad*
C-H stretch
O-H stretch
* broad due to hydrogen bonding

11. Infrared spectrum of ethanal

12. *
broad due to hydrogen bonding between O-H groups

13. Identify similarities and differences between the spectra.

14. See word doc “AS 2.12 IR spectroscopy work sheet” for suitable practise exercises