Introduction to Infrared spectroscopy

Use the information in the following charts to identify

peaks/bands in the Infrared Spectra of the

activity and exercises which follow.

 Acknowledgements: All chemical structures were drawn using ACD Labs ChemSketch version12.0 freeware.
All spectra were taken on a Thermo Nicolet AVATAR model 370 FT-IR--Department of Physical Sciences, Prince George's Community College.
This exercise was created in SoftChalk Lesson Builder 4.

Chart 1.

The following is a list of important infrared chromophores, their frequencies of absorption and descriptions of the peaks/signals they give rise to:

Functional group/chromophore

Frequency of absorption & description of peak(s)

Alcohols and carboxylic acids: O-H stretch

3600 ~ 3200cm-1  

If H-bonded a broad band appears.

If not H-bonded, a sharp band appears.

Amines: N-H stretch

3500 -3300 cm-1

H-bonded broad band

If not H-bonded, a sharp band appears.

C-H stretch

~3300 - 2900 cm-1 (the greater the hydrocarbon portion of the molecule, the larger this peak will be)

Unsymmetrical triple bond                

C≡C stretch                                        

C≡N stretch

2300 - 1950 cm-1 (usually small/weak but sharp band)

 

C=O stretches for anhydrides, acid halides, esters, aldehydes,

ketones, carboxylic acids, and amides

1850 - 1630 cm-1 strong and sharp band.

 

C=C  

1640-1680 cm-1  medium

C-O

1000-1200 cm-1  strong band

C-O-C

1150 -1085 cm-1  strong band

C-Cl,                                                  

C-Br                                                   

C-I

600-800 cm-1 strong

500-600 cm-1 strong

500 cm-1 strong

 

Chart 2.

The information in this table was obtained from:

http://www.chem.ucla.edu/~webspectra/irintro.html

IR Range

Generalized chromophore

3600cm-1 - 2700cm-1

X-H

2700cm-1 – 1900cm-1

X≡Y

1900cm-1 – 1500cm-1

X=Y

1500cm-1 – 500cm-1

X-Y

 

Infrared Spectroscopy Exercises

 

Activity: In order to view the infrared spectra of some compounds

and to simulate the vibration that occurs when a chromophore absorbs

infrared energy, hyperlink to the following sites. If CHIME is not loaded

on your computer, you will not be able to see the molecules and you

won't be able to perform the simulations. In order to see which vibration

causes a peak on the infrared spectrum, click on that peak.

 

Ethanol  

Triphenylmethanol

Pentan-2-one

Maleicanhydride

Acetophenone

Methyldinitrobenzoate

The above links are subsites of:

www.chem.umass.edu/~nermmw/Spectra/vibtutor.htm

 

 

In the following exercises, for each of the twelve structures

shown below determine which of the twelve IR spectra

(graphs a through l shown in the slide presentation below)

corresponds to that compound.

 

IRsoyoilstruct.gif

 

 

This slide show contains, in a scrambled order, the spectrum of each of the twelve compounds that are shown above. Perform Exercises 2, 3 and 4 in order to match each spectrum to the compound it corresponds to. Change slides by using the arrows or drop-down menu.
 
 

 

 

Exercise 1. For each of the spectra in Table 1. below, complete the

Labeling Activity beneath it. In each of these activities you are asked

to label the important peaks/bands/functional groups/bonds.


Table 1.

a)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

  

b)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

 

c)

  

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

d)

  

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

e)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

f) 

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

g)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

 

h)

 A pleasant smelling water insoluble compound

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

i)

  

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

j)

  

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

k)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

l)

 

Label the peaks/bands appropriately. Place labels that don't apply in order of appearance from top to bottom on the right hand side:
 
 

  

Exercise 2. In the following Labeling Activity match each compound
to its IR spectrum.

 

Match the compound name (labeled with a number) to the corresponding spectrum (labeled with a letter).
 
 

 

Exercise 3. In the following Labeling Activity match each compound
to its IR spectrum.

 

Match the compound name (labeled with a number) to the corresponding spectrum (labeled with a letter).
 
 

 

Explanation of Answers:

Soy oil (compound 1, spectrum l) is a tri-ester. Its spectrum has very prominent bands/peaks ~3100-2800 cm-1, corresponding to the large proportion of C-H bonds in the molecule. It has a strong C=O band ~1750 cm-1. A prominent set of unresolved bands appears ~1450-1000 cm-1; the C-O and C-O-C bands are a part of this. Ethyl ethanoate (compound 5, spectrum h) has a similar spectrum to that of soy oil; however, the proportion of C-H is small, hence the C-H band is small. The spectrum of propan-2-one (compound 6, spectrum d) has a minimal C-H band ~3000 cm-1, a prominent C=O stretch band at ~ 1700 cm-1, and a prominent C-O band at ~1200 cm-1. It is a challenge to distinguish between ethyl ethanoate and propan-2-one using IR alone. Therefore the hint was given--a pleasant smelling water insoluble compound. Esters have pleasant odors, and an ethyl ethanoate-water mixture has two layers; whereas, propan-2-one (acetone) is totally miscible in water.

1-chloropentane (compound 2, spectrum a) gives prominent C-H bands ~3000-2800 cm-1, and a prominent C-Cl band ~600-700 cm-1-1. Tetrachloromethane (compound 3, spectrum e) gives only a prominent C-Cl ~750 cm-1; its spectrum shows no C-H stretches or bends. Trichloromethane (compound 8, spectrum f) gives a very small C-H stretch band ~3050cm-1 and a very prominent C-Cl band ~750cm-1. This is in contrast to tetrachloromethane which gives only a prominent C-Cl band.


Ethanol (compound 7, spectrum i) gives a broad and prominent O-H Stretch band at~ 3600-3000cm-1, significant C-H stretch bands~3000-2800cm-1, and a prominent C-O band ~1050 cm-1. Hexadecan-1-ol (compound 11, spectrum b) has a similar spectrum, however, the ratio of C-H stretch to O-H stretch is vastly increased relative to the same ratio in the ethanol spectrum (33 C-H bonds in hexadecan-1-ol versus 5 C-H bonds in ethanol).

Hexane (compound 9, spectrum j) gives prominent C-H stretch bands ~3000-2800cm-1. The spectrum of ethyl benzene (compound 4, spectrum g) shows only C-H and weak C=C bonds (that would be easy to overlook), and no other functional groups. In hexane the ratio of H to C is greater than that in ethylbenzene; that is, there proportion of C-H bonds in hexane is greater than that of ethyl benzene; subsequently, the C-H stretch bands in the hexane spectrum are more prominent than those in the ethyl benzene spectrum.

The spectrum of tert-butyl methyl ether (compound 10, spectrum k) shows medium C-H stretch bands ~3000-2800cm-1 and prominent C-O and/or C-O-C bands ~1200cm-1 and ~1100cm-1.


The spectrum of ethanoic acid (compound 12, spectrum c) shows a broad and prominent O-H stretch band overlapping C-H stretch bands ~3700- 2500cm-1, and a very prominent C=O stretch band ~1700cm-1.

 


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