Presentation on theme: "INFRA RED SPECTROSCOPY"— Presentation transcript:
1INFRA RED SPECTROSCOPY A guide for A level studentsKNOCKHARDY PUBLISHING
2KNOCKHARDY PUBLISHING INFRA RED SPECTROSCOPY INTRODUCTIONThis Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...Navigation is achieved by...either clicking on the grey arrows at the foot of each pageor using the left and right arrow keys on the keyboard
3INFRA RED SPECTROSCOPY CONTENTSPrior knowledgeOrigins of infra red spectraVibrations of bonds in moleculesThe Infra Red spectrophotometerUses of IRInterpretation of IR spectraCharacteristic absorption frequenciesCheck list
4INFRA RED SPECTROSCOPY Before you start it would be helpful to… know the names and structures of organic functional groups
5INFRA RED SPECTROSCOPY Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond.As a result, the bonds vibrate at different frequenciesThe frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation.Various types of vibration are possible.
6INFRA RED SPECTROSCOPY Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond.As a result, the bonds vibrate at different frequenciesThe frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation.Various types of vibration are possible.Examples include... STRETCHING and BENDINGSYMMETRIC BENDING ASYMMETRICSTRETCHING STRETCH
7BENDING AND STRETCHING IN WATER MOLECULES SYMMETRIC STRETCHING
8BENDING AND STRETCHING IN WATER MOLECULES ASYMMETRIC STRETCHING
10The Infra-red Spectrophotometer • a beam of infra red radiation is passed through the sample• a similar beam is passed through the reference cell• the frequency of radiation is varied• bonds vibrating with a similar frequency absorb the radiation• the amount of radiation absorbed by the sample is compared with the reference• the results are collected, stored and plotted
11The Infra-red Spectrophotometer vibration having absorbed energy A bond will absorb radiation of a frequency similar to its vibration(s)normal vibrationvibration having absorbed energy
12INFRA RED SPECTRA - USES IDENTIFICATION OF PARTICULAR BONDSIN A MOLECULEThe presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum.
13INFRA RED SPECTRA - USES IDENTIFICATION OF PARTICULAR BONDSIN A MOLECULEThe presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum.IDENTIFICATION OF COMPOUNDS BY DIRECT COMPARISON OF SPECTRAThe only way to completely identify a compound using IR is to compare its spectrum with a known sample. The part of the spectrum known as the ‘Fingerprint Region’ is unique to each compound.
14INFRA RED SPECTRA - INTERPRETATION Infra-red spectra are complex due to the many vibrations in each molecule.Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory.However, the technique is useful when used in conjunction with other methods such as nuclear magnetic resonance (nmr) spectroscopy and mass spectroscopy.Peak position depends on bond strengthmasses of the atoms joined by the bondstrong bonds and light atoms absorb at lower wavenumbersweak bonds and heavy atoms absorb at high wavenumbers
15INFRA RED SPECTRA - INTERPRETATION Vertical axis Absorbance the stronger the absorbance the larger the peakHorizontal axis Frequency wavenumber (waves per centimetre) / cm-1Wavelength microns (m); 1 micron = 1000 nanometres
16FINGERPRINT REGION• organic molecules have a lot of C-C and C-H bonds within their structure• spectra obtained will have peaks in the 1400 cm-1 to 800 cm-1 range• this is referred to as the “fingerprint” region• the pattern obtained is characteristic of a particular compound the frequencyof any absorption is also affected by adjoining atoms or groups.
17IR SPECTRUM OF A CARBONYL COMPOUND • carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm-1• this is due to the presence of the C=O bond
18IR SPECTRUM OF AN ALCOHOL • alcohols show a broad absorption between 3200 and 3600 cm-1• this is due to the presence of the O-H bond
19IR SPECTRUM OF A CARBOXYLIC ACID • carboxylic acids show a broad absorption between 3200 and 3600 cm-1• this is due to the presence of the O-H bond• they also show a strong absorption around 1700 cm-1• this is due to the presence of the C=O bond
20IR SPECTRUM OF AN ESTER• esters show a strong absorption between 1750 cm-1 and 1730 cm-1• this is due to the presence of the C=O bond
21WHAT IS IT! ALCOHOL ALDEHYDE CARBOXYLIC ACID One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra.ALCOHOLO-H STRETCHALDEHYDEC=O STRETCHO-H STRETCHC=O STRETCHANDCARBOXYLIC ACID
24What should you be able to do? REVISION CHECKWhat should you be able to do?Understand the origin of IR spectraIdentify peaks associated with O-H and C=O bondsContrast the spectra of alcohols, carbonyls and carboxylic acidsCAN YOU DO ALL OF THESE? YES NO
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26Try some past paper questions WELL DONE!Try some past paper questions