Presentation on theme: "INFRA RED SPECTROSCOPY"— Presentation transcript:
1 INFRA RED SPECTROSCOPY A guide for A level studentsKNOCKHARDY PUBLISHING
2 KNOCKHARDY 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
3 INFRA RED SPECTROSCOPY CONTENTSPrior knowledgeOrigins of infra red spectraVibrations of bonds in moleculesThe Infra Red spectrophotometerUses of IRInterpretation of IR spectraCharacteristic absorption frequenciesCheck list
4 INFRA RED SPECTROSCOPY Before you start it would be helpful to… know the names and structures of organic functional groups
5 INFRA 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.
6 INFRA 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
7 BENDING AND STRETCHING IN WATER MOLECULES SYMMETRIC STRETCHING
8 BENDING AND STRETCHING IN WATER MOLECULES ASYMMETRIC STRETCHING
10 The 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
11 The 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
12 INFRA 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.
13 INFRA 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.
14 INFRA 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
15 INFRA 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
16 FINGERPRINT 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.
17 IR 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
18 IR 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
19 IR 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
20 IR 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
21 WHAT 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
24 What 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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26 Try some past paper questions WELL DONE!Try some past paper questions