Infrared Spectroscopy (OCR A Level Chemistry)

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Introduction to Infrared Spectroscopy

  • All covalent bonds act rather like springs, as opposed to rigid bars
  • Like springs, the bonds can vibrate in a number of different ways
  • The frequency of vibration occurs in the infra-red region of the electromagnetic spectrum
  • If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases
  • This is known as resonance

Different modes of vibration in molecules. Each mode has a characteristic frequency of vibration

Infrared (IR) spectroscopy

  • Infrared (IR) spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies
  • A spectrophotometer irradiates the sample with IR radiation and then detects the intensity of IR radiation absorbed by the molecule
  • IR energy is absorbed only if a molecule has a permanent dipole that changes as it vibrates
    • Symmetrical molecules such as O2 or H2, are therefore IR inactive

  • The resonance frequency is the specific frequency at which the bonds will vibrate
  • Rather than displaying frequency, an IR spectrum shows a unit called wavenumber
    • Wavenumber is the reciprocal of the wavelength and has units of cm-1

  • Characteristic absorptions can be matched to specific bonds in molecules
    • This enables chemists to determine the functional groups present

Absorption values for infrared spectroscopy analysis table

Bond  Location  Wavenumber / cm−1
C–C  Alkanes, alkyl chains  750 – 1100
C–X  Haloalkanes (X = Cl, Br, I)  500 – 800
C–F  Fluoroalkanes  1000 – 1350
C–O  Alcohols, esters, carboxylic acids  1000 – 1300
C=C  Alkenes  1620 – 1680
C=O  Aldehydes, ketones, carboxylic acids, esters, amides, acyl chlorides and acid anhydrides  1630 – 1820
aromatic
C=C 
Arenes  Several peaks in the range
1450 – 1650
C=N Nitriles  2220 – 2260
C–H  Alkyl groups, alkenes, arenes  2850 – 3100
O–H  Carboxylic acids  2500 – 3300 (broad)
N–H  Amines, amides  3300 – 3500
O–H  Alcohols, phenols  3200 – 3600

  • Due to some absorption bands overlapping each other, other analytical techniques such as mass spectroscopy should be used alongside IR spectroscopy to identify an unknown compound

Interpreting & Predicting Infrared Spectra

  • The best way to understand how to interpret an IR spectrum is by looking at examples and becoming familiar with the characteristic features of an IR spectrum

Worked example

Examine the two spectra shown and determine which one belongs to propan-2-ol and which one belongs to propanoneAnalytical Techniques Question Worked Example - Analysing IR Spectra, downloadable AS & A Level Chemistry revision notes

Answer:

    • IR spectrum A is propanone
      • In IR spectrum A the presence of a strong, sharp absorption around 1710 cm-1 corresponds to the characteristic C=O, carbonyl, group in a ketone.
    • IR spectrum B is propan-2-ol.
      • In spectrum B the presence of a strong, broad absorption around 3200-3600 cm-1 suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2-ol.

Examiner Tip

You can be asked to interpret or predict infrared spectra of both familiar and unfamiliar substances

Three of the key peaks to be aware of are:
  1. The narrow scoop caused by the O-H bond of an alcohol at between 3200 and 3600 cm-1 
  2. The sharp spike caused by the carbonyl C=O bond that belongs to many compounds as listed in the data booklet and table above
  3. The broad scoop caused by the O-H bond of a carboxylic acid between 2500 and 3300 cm-1, this right hand side of this peak is often distorted by the peaks from C-H bonds

Uses of Infrared Spectroscopy

  • Infrared spectroscopy is used to identify pollutants in vehicle emissions in the air
    • Sensors detect and measure the amount of pollutants such as carbon monoxide, carbon dioxide and unburnt hydrocarbons
    • This commonly occurs on motorways and in busy town centres to monitor localised pollution

  • Infrared spectroscopy can be used to measure alcohol levels using roadside breathalysers
    • A ray of infrared radiation is passed through the breath that is exhaled into the breathalyser chamber
    • The characteristic bonds of ethanol are detected and measured - the higher the absorbance of infrared radiation, the more ethanol in the person's breath

Fingerprint Region

  • The region below about 1500 cm-1 is called the fingerprint region and is unique to every molecule
  • It has many peaks that can be difficult to assign
  • These peaks represent the complex vibrational interactions that occur between different bonds within a molecule
  • The value of the fingerprint region is in being able to compare the IR spectrum to a known compound from a database and coming up with an exact match
  • This is particularly useful, for example, in identifying a specific member of a homologous series
    • All members of the series will show the same type of bonds present, but no two molecules will have the same fingerprint region

  • Infrared spectroscopy is one of a number of techniques used to determine the structure of organic molecules
    • It is most commonly used in combination with other analytical techniques such as:
      • Elemental analysis - to determine the empirical formula
      • Mass spectrometry - to determine the molecular mass and fragment ions from the whole molecule
      • (NMR spectroscopy is also included in the techniques but this is not covered as part of the AS course)

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.