INFRA RED SPECTROSCOPY A guide for A level

INFRA RED SPECTROSCOPY A guide for A level students KNOCKHARDY PUBLISHING

KNOCKHARDY PUBLISHING INFRA RED SPECTROSCOPY INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A 2 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 A 2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at. . . www. knockhardy. co. uk/sci. htm Navigation is achieved by. . . either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

INFRA RED SPECTROSCOPY CONTENTS • Prior knowledge • Origins of infra red spectra • Vibrations of bonds in molecules • The Infra Red spectrophotometer • Uses of IR • Interpretation of IR spectra • Characteristic absorption frequencies • Check list

INFRA RED SPECTROSCOPY Before you start it would be helpful to… • know the names and structures of organic functional groups

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 frequencies The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible.

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 frequencies The 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 BENDING SYMMETRIC STRETCHING BENDING ASYMMETRIC STRETCH

BENDING AND STRETCHING IN WATER MOLECULES SYMMETRIC STRETCHING

BENDING AND STRETCHING IN WATER MOLECULES ASYMMETRIC STRETCHING

BENDING AND STRETCHING IN WATER MOLECULES BENDING

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

The Infra-red Spectrophotometer A bond will absorb radiation of a frequency similar to its vibration(s) normal vibration having absorbed energy

INFRA RED SPECTRA - USES IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE The 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.

INFRA RED SPECTRA - USES IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE The 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 SPECTRA The 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.

INFRA RED SPECTRA - INTERPRETATION Infra-red spectra are complex due to the many different vibrations taking place in each molecule.

INFRA RED SPECTRA - INTERPRETATION Infra-red spectra are complex due to the many different vibrations taking place 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.

INFRA RED SPECTRA - INTERPRETATION Infra-red spectra are complex due to the many different vibrations taking place 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. The technique is useful when used in conjunction with other methods nuclear magnetic resonance spectroscopy and mass spectroscopy.

INFRA RED SPECTRA - INTERPRETATION Infra-red spectra are complex due to the many different vibrations taking place 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. The technique is useful when used in conjunction with other methods nuclear magnetic resonance spectroscopy and mass spectroscopy. Peak position depends on bond strength masses of the atoms joined by the bond strong bonds and light atoms absorb at lower wavenumbers weak bonds and heavy atoms absorb at high wavenumbers

INFRA RED SPECTRA - INTERPRETATION Vertical axis Absorbance the stronger the absorbance the larger the peak Horizontal axis Frequency Wavelength wavenumber (waves per centimetre) / cm-1 microns (m); 1 micron = 1000 nanometres

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 frequency of any absorption is also affected by adjoining atoms or groups.

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

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

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

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

WHAT IS IT! One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra. O-H STRETCH ALCOHOL C=O STRETCH ALDEHYDE O-H STRETCH AND C=O STRETCH CARBOXYLIC ACID

CHARACTERISTIC FREQUENCIES N-H C N O-H C-Cl C=O C-H C-O Aromatic C-C C=C C-C alkanes

CHARACTERISTIC ABSORPTION FREQUENCIES Bond Class of compound Range / cm-1 Intensity C-H C-C C=C Alkane Alkene 2965 - 2850 1200 - 700 1680 - 1620 strong weak variable C=O Ketone Aldehyde Carboxylic acid Ester Amide Alcohol, ester, acid, ether 1725 - 1705 1740 - 1720 1725 - 1700 1750 - 1730 1700 - 1630 1300 - 1000 strong strong O-H Alcohol (monomer) Alcohol (H-bonded) Carboxylic acid (H-bonded) 3650 - 3590 3420 - 3200 3300 - 3250 variable, sharp strong, broad variable, broad N-H C N Amine, Amide Nitrile 3500 (approx) 2260 - 2240 medium C-X Chloride Bromide Iodide 800 - 600 - 500 (approx) strong C-O

REVISION CHECK What should you be able to do? Understand the origin of IR spectra Identify peaks associated with O-H and C=O bonds Contrast the spectra of alcohols, carbonyls and carboxylic acids CAN YOU DO ALL OF THESE? YES NO

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INFRA RED SPECTROSCOPY THE END © 2005 JONATHAN HOPTON & KNOCKHARDY PUBLISHING