4. Infrared (IR) sp Ethyl Alcohol C 2 H 6 O Dimethyl Ether C 2 H 6 O Isomers • Molecules that have

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  • 4. Infrared (IR) spectroscopy

    Molecular Structure and Organic Chemistry

    • The structure of a molecule refers to the arrangement of atoms within

    the molecule. The structure of a molecule is critical to the chemical and

    physical properties of a substance. In fact so vital is structure to

    molecular identity that the same molecular formula may represent

    more than one substance based upon their differing structures.

    • Example: Two totally different substances share the same molecular

    formula, C2H6O, but are different because of their differing molecular

    structures.

    Ethyl Alcohol C2H6O Dimethyl Ether C2H6O

    Isomers

    • Molecules that have the same molecular formulas but different

    structures are called isomers

    • There are 2 isomers corresponding to the molecular formula C4H10 ,

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    And 3 corresponding to C5H12 and 39 corresponding to the molecular

    formula C9H20

    The IR Region

    • Just below red in the visible region.

    • Wavelengths usually 2.5-25 mm.

    • More common units are wavenumbers, or cm-1, the reciprocal of the

    wavelength in centimeters.

    Wavenumbers are proportional to frequency and energy.

    Infrared Spectrophotometry

    • using molecular vibrations as a key to structure

    • Although ball and stick models of molecules are very effective at

    approximating the actual shapes of molecules, they do have one

    major flaw they leave you with the false impression that molecules

    are rigid objects

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    Some Vibrational Modes

    Covalent bonds vibrate at only certain allowable frequencies.

    Each type of vibration has a frequency that depends upon the: the mass of

    the vibrating atoms and the nature of the bond between them.

    • For a constant bond type (single, double or triple) the frequency of

    the vibration is low for a bond between heavy atoms. Conversely,

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    for a given bond type the frequency of vibrations is high for light

    atoms.

    • Multiple bonds vibrate at a higher frequency than do single bonds.

    Stretching Frequencies

     Frequency decreases with increasing atomic weight.

     Frequency increases with increasing bond energy.

    Infrared Absorptions

    Vibrations as a key to structure- the entire range of vibrations for all

    organic molecules falls within the Infrared Region of the

    Electromagnetic Spectrum (2500nm – 25000nm). If a beam of IR

    radiation is directed at a molecular sample and if the beam has the

    same frequency as one of the vibrational modes of the molecule then

    the molecule will absorb the energy of the IR radiation and the

    molecular vibration will increase in intensity.

    If in order for absorption to occur, the IR frequency must match the

    frequency of the vibrating atoms, and if the frequency of the vibrating

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    atoms is dependent upon the mass of the atoms and the bond type;

    then the frequency at which absorbance occurs is dependent upon the

    mass of the atoms and the bond type. Therefore, the same two bonded

    atoms, regardless of the molecule that they are in, will have the same

    absorbance frequency in the IR region. This is the major strength of IR

    Spectrophotometry. IR Spec identifies the functional groups present in

    an organic molecule.

    Functional groups

    Functional Group- this is an atom or group of atoms that imparts a unique

    set of chemistry to whatever organic molecule it is bonded to. If the same

    functional group is attached to two different organic molecules then the

    two organic molecules will have similar chemistry’s and have similar

    absorbance values of the IR Specs

    THE FINGERPRINT REGION OF AN INFRA-RED SPECTRUM

    What is the fingerprint region

    This is a typical infra-red spectrum:

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    The region to the right-hand side of the diagram (from about 1500 to 500

    cm-1) usually contains a very complicated series of absorptions. These are

    mainly due to all manner of bending vibrations within the molecule. This is

    called the fingerprint region.

    The importance of the fingerprint region is that each different compound

    produces a different pattern of troughs in this part of the spectrum.

    Using the fingerprint region

    Compare the infra-red spectra of propan-1-ol and propan-2-ol. Both

    compounds contain exactly the same bonds. Both compounds have very

    similar troughs in the area around 3000 cm-1 - but compare them in the

    fingerprint region between 1500 and 500 cm-1.

    The pattern in the fingerprint region is completely different and could

    therefore be used to identify the compound.

    So . . . to positively identify an unknown compound, use its infra-red

    spectrum to identify what sort of compound it is by looking for specific

    bond absorptions. That might tell you, for example, that you had an

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    alcohol because it contained an -OH group. You would then compare the

    fingerprint region of its infra-red spectrum with known spectra measured

    under exactly the same conditions to find out which alcohol (or whatever)

    you had.

    INTERPRETING AN INFRA-RED SPECTRUM

    The infra-red spectrum for a simple carboxylic acid (Ethanoic acid)

    You will see that it contains the following bonds:

    carbon-oxygen double, C=O

    carbon-oxygen single, C-O

    oxygen-hydrogen, O-H

    carbon-hydrogen, C-H

    carbon-carbon single, C-C

    The carbon-carbon bond has absorptions which occur over a wide range of

    wavenumbers in the fingerprint region - that makes it very difficult to pick

    out on an infra-red spectrum.

    The carbon-oxygen single bond also has an absorbtion in the fingerprint

    region, varying between 1000 and 1300 cm-1 depending on the molecule it

    is in. You have to be very wary about picking out a particular trough as

    being due to a C-O bond.

    The other bonds in ethanoic acid have easily recognised absorptions

    outside the fingerprint region.

    The C-H bond (where the hydrogen is attached to a carbon which is singly-

    bonded to everything else) absorbs somewhere in the range from 2853 -

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    2962 cm-1. Because that bond is present in most organic compounds, that's

    not terribly useful! What it means is that you can ignore a trough just under

    3000 cm-1, because that is probably just due to C-H bonds.

    The carbon-oxygen double bond, C=O, is one of the really useful

    absorptions, found in the range 1680 - 1750 cm-1. Its position varies slightly

    depending on what sort of compound it is in.

    The other really useful bond is the O-H bond. This absorbs differently

    depending on its environment. It is easily recognised in an acid because it

    produces a very broad trough in the range 2500 - 3300 cm-1.

    The infra-red spectrum for ethanoic acid looks like this:

    The possible absorption due to the C-O single bond is queried because it

    lies in the fingerprint region. You couldn't be sure that this trough wasn't

    caused by something else.

    The infra-red spectrum for an alcohol (Ethanol) .

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    The O-H bond in an alcohol absorbs at a higher wavenumber than it does in

    an acid - somewhere between 3230 - 3550 cm-1. In fact this absorption

    would be at a higher number still if the alcohol isn't hydrogen bonded - for

    example, in the gas state. All the infra-red spectra on this page are from

    liquids - so that possibility will never apply.

    Notice the absorption due to the C-H bonds just under 3000 cm-1, and also

    the troughs between 1000 and 1100 cm-1 - one of which will be due to the

    C-O bond.

    The infra-red spectrum for an ester (Ethyl ethanoate)

  • Spectroscopy

    Instrumental analysis Dr. Hisham E Abdellatef