Analysing Structure of Organic Compounds Using Infrared Spectroscopy

This is part of the HSC Chemistry course under Module 8 Section 2: Analysis of Organic Substances. 

HSC Chemistry Syllabus

Investigate the processes used to analyse the structure of simple organic compounds addressed in the course, including but not limited to:

  • proton and carbon-13 NMR

  • mass spectrometry

  • infrared spectroscopy (ACSCH130)


    How to Analyse Organic Compound Structure Using Infrared Spectroscopy

    This video explores how infrared spectroscopy is used to analyse the structure of organic compounds. Infrared spectroscopy is used to identify the presence of specific bonds and functional groups in organic molecules.  

     

    How Infrared (IR) Spectroscopy Works

    • Radiation in the infrared (IR) region of the electromagnetic spectrum has the energy to cause vibrations of covalent bonds. When bonds absorb infrared radiation, they stretch and bend.
      • Stretches correspond to the increasing and decreasing of the bond lengths within a molecule.
      • Bends correspond to the increasing and decreasing of the angle between bonds in a molecule.

     

    • IR spectroscopy is useful for identifying different bonds and functional groups in organic compounds as they absorb different frequencies (wave number) of radiation.

     

    Functional group

    Bond

    Wavenumber

    Characteristic

    Alkane

    C–H

    2850–3300

    Sharp

    Alkene

    C=C

    1620–1680

     

    Alkyne

    C≡C

    2100–2260

     

    Alcohol

    O–H

    3230–3550

    Broad

    C–O

    1000–1300

     

    Carbonyl

    C=O

    1680–1750

     

    Carboxylic acid

    O–H

    2500–3000

    Very broad

    Amine & amide

    N–H

    3300–3500

    Broad

    C–N

    1030–1230

     

    Nitrile

    C≡N

    2220–2260

     

     

     

    Alkane

     
    • Sharp C–H signal at 3100 cm–1.

     

     

    Alkene

     

     

    Alcohol

     

    IR spectrum of ethanol

     
    • Distinct broad O–H signal at 3391 cm–1.
    • Sharp C–H signal at 2981 cm–1.

     

     

    Ketone & aldehyde

     

     

     

    IR spectrum of propanone (acetone)
    • Distinct C=O signal at 1700–1800 cm–1.
    • Sharp C–H signal at 3000 cm–1.

     

    IR spectrum of propanal

     

     

    • Distinct C=O signal at 1700–1800 cm–1.
    • Sharp C–H signal at 3000 cm–1.
    • Ketones and aldehydes are difficult to distinguish using IR spectroscopy alone.

     

    Carboxylic acid

     

    IR spectrum of ethanoic acid (acetic acid)

           

    • Distinct acid O–H signal at 3500 cm–1and C=O signal at 1800 cm–1.
    • Broad O–H signal overlaps a what would be sharp C–H signal. This is common in carboxylic acids but not alcohols.

     

     

    Esters

     

    IR spectrum of ethyl ethanoate
    • Distinct C=O signal at 1752 cm–1.
    • C–O signal at 1055 cm–1 is difficult to identify. This signal is necessary to distinguish esters from ketones and aldehydes as the latter two do not have C–O bonds.
    • Esters can be distinguished from carboxylic acids by the absence of broad O–H signal at 2500–3000 cm–1.

     

    Amines & amides

     

     
    • Distinct N–H signal at 3300–3500 cm–1 for primary and secondary amines and amides.
    • Tertiary amines and amides do not have N–H bonds so no signals will be observed in 3300–3500 cm–1 range.
    • C=O signal at 1680–1750 cm–1distinguishes amides from amines.

    IR spectrum of N-ethylethanamine (secondary amine)

     
    • Weak N–H signal at 3288 cm–1.
    • Primary and secondary amines are difficult to distinguish using IR spectroscopy alone.

    IR spectrum of an amide compound.

     
    • N–H signal at 3300–3500 cm–1 and C=O signal at 1600–1700 cm–1 suggest the presence of an amide functional group.
    • Primary and secondary amides are difficult to distinguish using IR spectroscopy alone.

    Nitriles

     

     

     

     

     

    • Strong sharp C≡N signal at 2250 cm–1.