Spectral Regions
Home Up Molecular Motion Spectral Regions Alkanes Alkyl Halides

 

Spectral Regions

First, let's take a quick look at an infrared spectrum just to see what they look like. The spectra shown on these pages are simplified for display purposes. Copies of actual IR spectra can be found in your workbook located after the examples and before the laboratory experiments.

61ir02.JPG (2672 bytes)

You can see that the spectra consist of a squiggle that runs across the top of the graph that drops down in places. Wherever that line drops down, it means that energy is absorbed by a particular functional group or bond to allow for faster or more energetic vibration. The location of the absorption correlates with the amount of energy absorbed and thus with the type of bond that is absorbing the energy. Each spectrum is, in total, different from all of the others. However, some parts are the same or similar from one to another.

Now we want to focus on which parts of the spectra correspond to which bonds.

Find the blank spectrum, #0, in your workbook. It looks like this without the red lettering. Let's divide it into different sections where the absorptions of different kinds of bonds are found.

Blank IR spectrum with regions marked.[61irsp00.JPG (9638 bytes)]

Across the top there is a scale that says "wavelength in microns". Ignore that. Across the bottom is a scale that says "wavenumber cm-1". That is the one we will use. If you want to know, wavenumber is the inverse of wavelength, but let's not dwell on that.

The sections into which we will divide these spectra are arbitrary, but convenient. The dividing lines that are shown in this section have no physical significance, but using them will help you interpret the IR spectra of compound and allow you to determine the class (alkane, alcohol, ketone, for example) of many organic compounds.

Using the wavenumber scale, draw a solid vertical line at 1500 cm-1.

The section to the right of 1500 cm-1 is called the fingerprint region, and it is used for detailed analysis which is beyond the scope of this course. We won't use it.

The section to the left of 1500 cm-1 is referred to as the diagnostic region. That is the part we will use. The absorptions that are characteristic of certain functional groups or bond types show up here.

Divide the diagnostic region by drawing another vertical line at 2500 cm-1. The section between 1500 and 2500 cm-1 is the multiple bond section. Let's divide it into two parts by drawing a dashed line at 2000 cm-1. Double bonds show up in the right half of this section (between 1500 and 2000 cm-1) and triple bonds show up in the left half of this section (between 2000 and 2500 cm-1). We will deal with these in the next lesson when we work with alkenes, alkynes and aromatic compounds.

The section to the left of 2500 cm-1 is where the bonds to hydrogen atoms show up. We can divide this section into two parts also by drawing a dashed line at 3000 cm-1. To the right of 3000 cm-1 is where the hydrogen-to-carbon bonds will show up if the carbon atom has only single bonds. To the left of 3000 cm-1 is where the hydrogen-to-carbon bonds will show up if the carbon atom has any multiple bonds.

These useful generalities, with specific information about the compounds we are studying in this lesson, are summarized in tabular form below (and in example 25 in your workbook). These tables contains the bond types, wavenumber regions, and specific wavenumber ranges in brackets.

BOND TYPE WAVENUMBER REGION
general [detailed]
| |
-C-C-
| |
in fingerprint region only
|
-C-H
|
in 2500-3000 region [2853-2962]
and in fingerprint region

-C-Cl
in fingerprint region only

 

COMPOUND TYPE BOND TYPES WAVENUMBER REGIONS
alkane C-C, C-H 2500-3000
and fingerprint
alkyl halide C-C, C-Cl,
(and C-H)
fingerprint
(and 2500-3000)

The C-C bond absorbs infrared light in the fingerprint region only so we won't pay attention to it. It is important to note that the C-C bond does not absorb in the central multiple bond region. The C-H bond absorbs infrared light just to the right of the 3000 cm-1 mark in the 2500-3000 cm-1 region, as well as in the fingerprint region. The C-Cl bond (and other  carbon-to-halogen bonds as well) absorbs infrared light only in the fingerprint region. One consequence of this is that we will not be able to use infrared spectra to help us distinguish between alkanes and alkyl halides.

 

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E-mail instructor: Eden Francis

Clackamas Community College
2001, 2003 Clackamas Community College, Hal Bender