Ch. 15 - Analytical Techniques: IR, NMR, Mass SpectWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Concept #1: Common IR Frequencies

Example #1: Major IR absorptions

Example #2: Major IR absorptions

Example #3: Major IR absorptions

Example #4: Major IR absorptions

Practice: Answer each of the following questions based on the images below.


a) Which compounds show an intense peak ~ 1700 cm-1


b) Which compound shows an intense, broad peak at ~ 3400 cm-1


c) Which compound has a peak at ~1700 cm-1, but no peaks at 2700 cm-1


d) Which compound has no signal beyond the fingerprint region?

Practice: Identify which carbonyl group will exhibit a signal at a lower wavenumber

Additional Problems
In infrared spectra, the carbonyl group is usually indicated by a single strong and sharp absorption. However, in the case of carboxylic acid anhydrides,two peaks are observed even though the two carbonyl groups are chemically equivalent. Explain this fact, considering what you know about the IR absorption of primary amines.
Match the following structures with the IR spectra shown below.  
Which C=O stretch will occur at the highest frequency (cm  −1) in the IR spectrum?
Predict the key IR absorption bands whose presence would allow each compound in pairs.
Predict the key IR absorption bands whose presence would allow each compound in pairs.  
Predict the key IR absorption bands whose presence would allow each compound in pairs.
Predict the key IR absorption bands whose presence would allow each compound in pairs.
Predict the key IR absorption bands whose presence would allow each compound in pairs
For the following reaction, which of the following is consistent with a comparison of the IR spectra of the starting material vs. the product? a) Absorption at 2250 cm –1 should disappear, new absorption at 2600-2800 cm –1 and 1720 cm –1 should appear. b) Absorption at 2250 cm –1 should disappear and absorption around 3400 cm –1 should appear. c) Absorption at 3200-3400 cm –1 and 1720 cm –1 should appear. d) Absorption at 2250 cm –1 should disappear. e) None of these.
Predict the key IR absorption bands whose presence would allow each compound in pairs
Which of the following bonds would produce the strongest IR absorption? a) C=O b) C–O c) C≡C d) C=N e) sp2 C–H
Rank the indicated bonds in decreasing (highest to lowest) order of wavenumber for IR absorbance. a) III>I>II b) II>I>III c) III>II>I d) I>II>III e) none of these
In what region of an IR spectrum will the triple bond stretch of 1-heptyne be detected? a) 4000-2500 cm –1 b) 2500-2000 cm –1 c) 2000-1500 cm –1 d) 1500-400 cm –1 e) None of the above
A compound contains oxygen but has no IR signals above 3000 cm –1 or between 2000 and 1500 cm –1. This is consistent with the compound being a) an ether. b) an alcohol. c) an aldehyde. d) a ketone. e) none of the above.
Which compound will likely show a significant IR signal between 2000 and 1500 cm –1 ? a) Hexane b) 1-Hexyne c) CH3CH2CH2CH2CH2CH=O d) CH3CH2CH2CH2CH2CH2OH e) None of the above
Which compound will likely NOT show a significant IR signal between 4000 & 2500 cm –1 ? a) CH2Cl2 b) Cyclohexane c) 1-Hexene d) CCl4 e) All of the above are likely to show a significant IR signal between 4000 & 2500 cm  –1 .
A saturated compound (IHD = 0) that contains oxygen and shows strong broadened IR signal above 3000 cm –1 is likely  a) ketone b) an alcohol c) a carboxylic acid d) Any of the above e) None of the above
How would you distinguish the formula by IR spectroscopy? Please list the major difference in the spectra and provide frequencies.
Concentrated alcohols show a _____ absorption in the region of 3200-3600cm  –1, due to _____.  a. sharp, hydrogen bonding b. broad, hydrogen bonding c. sharp, polarity d. broad, polarity
Rank the indicated bonds in decreasing (highest to lowest) order of wavenumber. a) None of these b) III > I > II c) II > I > III d) I > II > III e) III > II > I
Interpret the following infrared spectra and circle the corresponding structure below. Label two peaks present on the spectra with the stretch observed.  
Which compound would be expected to show intense IR absorption at 1746 cm-1? a. CH3CH2OCH2CH3 b. CH3COOCH3 c. CH3CH2CH≡CH d. CH3CH2SCH
Rank IR absorption of the indicated bonds in decreasing (highest to lowest) order of wavenumber. a. III>II>I b. I>II>III c. II>I>III d. III>I>II e. none of these
Below is the IR graph of a compound with the MF C4H8O2. Identify and label peaks a, b and c in the spectrum that are consistent with the molecular formula given. After, say whether the molecule contains a carboxylic acid? Briefly explain your answer as to why or why not.
Which one of the following compounds is consistent with the following IR spectrum? a. I        b. II            c. III         d. IV          e. V
Which of the following compounds will show a broad absorption around 3300 cm-1 and a sharp absorption at 1650 cm-1? a. I  b. II  c. III  d. IV  e. V
Identify the functional groups that are indicated in the following IR spectra.
For the following reaction, which of the following change(s) in the IR spectrum is consistent with conversion of the reactant to the product? a. absorption at 2150 cm -1 should disappear  b. absorption at 3300 cm -1 and 2150 cm -1 should disappear  c. absorption at 2250 cm -1 should disappear, a new absorption at 3300 cm -1 should appear  d. absorption at 1650 cm -1 should disappear, a new absorption at 3300 cm -1 should appear e. absorption at 3300 cm -1 and 2150 cm -1 should disappear, a new absorption at 1720 cm -1 should appear
Answer each of the following questions based on the images below. a) Which compounds show an intense peak ~ 1700 cm -1?   b) Which compound shows an intense, broad peak at ~ 3400 cm -1?   c) Which compound has a peak at ~1700 cm-1, but no peaks at 2700 cm -1?    
Identify which carbonyl group will exhibit a signal at a lower wavenumber. 
Which of the following compounds gives an infrared spectrum with peaks at 3000-3500 cm-1 and ~1750 cm-1? a. 1          b. 2        c. 3       d. 4
Which function group is missing in the following IR spectrum? a) OH           b) sp  2C         c) sp 3C         d) C=O        e) all are present
Which ketone will show a carbonyl absorption at the lowest frequency (cm  –1) in the infrared?
How could IR Spectroscopy be used to distinguish between the following two compounds?
Which of the following carbonyl compounds would have its carbonyl absorption at the lowest wavenumber in its IR spectrum?
Which of the following carbonyl compounds would have its carbonyl absorption at the highest wavenumber in its IR spectrum?
Deuterium (D) is an isotope of hydrogen, in which the nucleus has one proton and one neutron. This nucleus, called a deuteron, behaves very much like a proton, although there are observed differences in the rates of reactions involving either protons or deuterons (an effect called the kinetic isotope effect). Deuterium can be introduced into a compound via the process below:(b) The IR spectrum of compound 4 exhibits a group of signals between 1250 and 1500 cm -1, a signal at 2180 cm-1, and another group of signals between 2800 and 3000 cm -1. Identify the location of the C-D signal in the spectrum and explain your reasoning (J. Chem. Ed. 1981, 58, 79–80).
Which functional group has a characteristic absorption in the 1630-1800 cm -1 region? Select one: a. amine b. alkene c. alcohol d. amide