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Wize University Chemistry Textbook > Structure Elucidation (IR Spectroscopy, NMR, Mass Spectrometry)

Infrared Spectroscopy

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Infrared Spectroscopy



Infrared (IR) Spectroscopy measures the interaction of IR light with different functional groups found (in particular: carbonyl groups, alkanes/alkenes/alkynes, alcohols and amines) within your sample.

The energy of the IR light is varied to see where the sample absorbs light energy:


IR Spectroscopy

Since we measure absorptions characteristic to certain bonds, IR spectroscopy is a great way to determine which functional groups are present in a molecule.
Wize Concept
We will just focus on memorizing a few functional groups from this table. If you see a more complicated problem on your exam, your prof will provide you with a table similar to the one shown below (usually found on a formula sheet).

Important functional groups to know:
  • OH
  • C=O

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General Trends in IR Peaks

Hybridization affects bond strength! Stronger bonds have vibrate at higher frequencies (they have higher wavenumbers).
sp(C−H)3300 cm−1>sp2(C−H) 3100 cm−1>sp3(C−H) 2900 cm−1\rm sp(C-H) 3300\ cm^{-1} > sp^2(C-H)\ 3100\ cm^{-1} > sp^3(C-H)\ 2900\ cm^{-1}

Vibrations of bonds involving heavier atoms have lower vibrational frequencies (they have lower wavenumbers).
v(C−H)>v(C−C)>v(C−O)>v(C−Br)v({\rm C-H}) > v({\rm C-C}) > v({\rm C-O}) > v({\rm C-Br})

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  • We often use a unit called wavenumber (cm−1\rm cm^{-1}) to represent the energy of vibrational absorbances
ν~=1λ\tilde\nu=\dfrac{1}{\lambda}
  • IR spectroscopy generally covers the 500-4000 cm−1\rm cm^{-1} range.
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Example: IR Spectra

The IR spectrum for cyclohexanone is given below. Which functional groups can you identify from this data? How?

There are sp3 C-H bonds which we can see for the peak(s) around 2900cm-1.
There is a C=O which we can see by the peak at 1700 cm-1
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Which molecule does the IR spectra belong to?




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Which molecule does the IR spectra belong to?



Practice: IR Spectra

Based on the IR spectrum below, what is the most likely empirical formula for this molecule?


Practice: IR Spectra

Identify all the functional groups in the following IR Spectrum:

Practice: IR Spectroscopy

The "Brazilian blowout" hair treatment, now renamed the "keratin smoothing" treatment after bad press, contains the chemical methylene glycol. This treatment was found to release a toxic chemical called formaldehyde due to a reversible reaction between methylene glycol and water. The reaction is detailed below.



checklist
Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.
Which molecule does the IR spectra belong to?





checklist
Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.
Which molecule does the IR spectra belong to?



IR Spectra Cheatsheet

O-H vs C-H vs COOH

OH is broad and to the left of 3000 cm-1
C-H is the sharp band just right of 3000 cm-1

COOH is much more broad than an O-H and overlaps all of the C-H stretch,
We can also see the C=O stretch at ~1800 cm-1 . This is a key feature of all carbonyl compounds.
Do not mistake this for C=C which are small and subtle (and easier to identify using NMR anyway)



Extra Practice
IR Spectroscopy: Structure Elucidation
Prallethrin, a potent insect repellent used for the control of mosquitos, features a number of organic functional groups. Which key peaks would you use IR spectroscopy to detect?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
IR Spectroscopy: Structure Elucidation
Prallethrin, a potent insect repellent used for the control of mosquitos, features a number of organic functional groups. Which key peaks would you use IR spectroscopy to detect?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
Lab techniques: IR Spectroscopy
Which molecule does the IR spectrum belong to?
Lab techniques: IR Spectroscopy
Which molecule does the IR spectrum belong to?
IR Spectroscopy and Structure Determination
Propose the structure of a compound that can produce the following IR spectrum:
IR and dipole moment
How do you distinguish between these two molecules using IR?
IR Spectroscopy
How do you differentiate between the following two molecules by IR?
Structure Determination using IR spectrum
A molecule with molecular formula C4H8O2 has the following IR spectrum. What is the possible structure of the compound?
IR spectroscopy
What is the best option to distinguish between the following two molecules by IR?
IR Question
Which statements are true about the IR spectrum of the following molecule?
IR Spectroscopy
Which molecule fits the best with the IR spectrum provided?
IR Spectrum
Assign all the functional groups for the characteristic peaks in this IR spectrum:
IR, resonance and bond polarity
Match the following C=O stretching frequency values with the right compounds in the picture:
1720 cm-1, 1810 cm-1, 1640 cm-1.
Identifying the Functional Groups using IR Spectrum
Match the following compounds with the proper IR spectra provided below:
I.
II.
The IR spectrum below belongs to which compound?
IR Spectroscopy: Structure Elucidation
Prallethrin, a potent insect repellent used for the control of mosquitos, features a number of organic functional groups. Which key peaks would you use IR spectroscopy to detect?
The IR spectrum below belongs to which compound?
Practice: IR Spectroscopy
Which molecule does the IR spectra belong to?
Practice: IR Spectroscopy
Which molecule does the IR spectra belong to?
The IR spectrum below belongs to which compound?
Hints in structure from IR
Try to identify the functional groups in this IR Spectrum.
Once you do that, propose a structure using the molecular formula: C2H6O
Is there only one isomer (configuration) of this molecule based on the IR spectrum and the formula?
IR Spectroscopy: Structure Elucidation
Prallethrin, a potent insect repellent used for the control of mosquitos, features a number of organic functional groups. Which key peaks would you use IR spectroscopy to detect?
IR functional group assignment
Practice: IR Spectra
Identify all the functional groups in the following IR Spectrum:
Spectroscopy
Analyze the spectroscopic data to properly identify the unknown molecules.
IR Spectroscopy, NMR and Mass spectrometry
From the spectra provided, identify the structure of the unknown molecule.
MS:
IR: