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

δ+ δ- δ- δ+ δ- δ+ δ+ δ- δ+ δ+ δ+ δ+ δ- Non-polar molecules

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: http://en.wikipedia.org/wiki/Infrared_spectroscopy#Typical_method

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.

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

USING INFRA-RED SPECTROSCOPY Certain bonds give characteristic absorption frequencies

INFRA-RED SPECTROSCOPY Infra-red spectra are used to identify the functional groups in a substance: Absorption peak (cm-1) Functional group 2850-3010 C-H 1650-1750 C=O 2500-3500 O-H 3300-3500 N-H 1000-1300 C-O 500-1400 C-X These are usually found in the Band Region of the spectrum

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

INFRA-RED SPECTROSCOPY Q’s ac4u 286/7

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

Infrared spectrum of ethanal

* broad due to hydrogen bonding between O-H groups

Identify similarities and differences between the spectra.

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