Worksheet: Functional Groups in IR Spectra

In this worksheet, we will practice identifying the functional groups in a compound from the nature of the peaks in its infrared spectrum.

Q1:

A common organic chemical exhibits a strong IR absorption at 1,645 cm−1. Which of the following functional groups could have caused this absorption?

  • A C C
  • B C C
  • C C N
  • D C H
  • E N O

Q2:

A pure, colorless liquid with a high boiling point exhibited a strong IR absorption in the 3,3003,400 cm−1 range. The only other strong IR signals outside of the fingerprint region can be assigned to CH resonances. To which of the following chemical families does the liquid most likely belong?

  • AThiols
  • BAlkenes
  • CAlkanes
  • DAlcohols
  • EAldehydes

Q3:

An organic compound that is liquid at room temperature exhibits a strong IR absorption at 1,710 cm−1. It is unreactive to acidified potassium dichromate, but the product of its reaction with lithium aluminum hydride exhibits a strong IR absorption in the 3,3003,400 cm−1 range. To which of the following chemical families does the liquid most likely belong?

  • ANitriles
  • BKetones
  • CAldehydes
  • DAlkenes
  • EAmines

Q4:

Which of the following must a compound undergo in order for it to be IR active?

  • AEmission of an electron
  • BTransfer of an electron
  • CMetal–ligand charge transfer
  • DA change in dipole moment
  • EA change in polarizability

Q5:

Keeping all other variables the same as bond strength decreases, which of the following is true?

  • AThe wave number of the IR stretch decreases.
  • BThe wavelength and wave number of the IR stretch decrease.
  • CThe wavelength of the IR stretch decreases.
  • DThe wave number of the IR stretch increases.

Q6:

Which of the following is in correct order of decreasing IR stretching frequency?

  • AAlkenes > alkynes > alkanes
  • BAlkenes > alkanes > alkynes
  • CAlkynes > alkanes > alkenes
  • DAlkynes > alkenes > alkanes
  • EAlkanes > alkynes > alkenes

Q7:

Keeping all other variables the same, which of the following bonds would have the lowest IR stretching frequency?

  • A C O
  • B C N
  • C C C l
  • D C H

Q8:

Which of the following does not affect the CO stretching frequency in a metal–ligand complex?

  • AThe electron-donating or electron-withdrawing effects of other ligands
  • BThe electron density of the metal center
  • CThe intensity of the infrared source used
  • DThe geometry of the molecule

Q9:

Which of the following statements about IR spectroscopy is false?

  • AAbsorptions due to CO bond stretching are generally weak.
  • BStretching frequencies are higher than corresponding bending frequencies.
  • CSingle bonds have lower stretching frequencies than triple bonds.
  • DConjugation lowers the energy required to cause a bond to stretch.

Q10:

In order for an IR vibration to be allowed, what condition must be met?

  • AThere must be a change in the polarizability of the molecule.
  • BThere must be a change in dipole moment of the molecule.
  • CThe spin state of the molecule must flip.
  • DThere must be a chromophoric species.

Q11:

Which of the following regions would be most useful in determining the substitution patterns of an alkene?

  • AThe CC stretch, occurring at 16601600 cm−1
  • BThe CH out-of-plane bend, occurring between 1000650 cm−1
  • CThe CH stretch, occurring between 31003010 cm−1

Q12:

What does hydrogen bonding cause the hydroxyl IR stretching band to do?

  • AShift to lower frequencies and broaden
  • BShift to higher frequencies and sharpen
  • CShift to lower frequencies and sharpen
  • DShift to higher frequencies and broaden

Q13:

Within the 3,5003,300 cm−1 region, how can primary, secondary, and tertiary amines be distinguished?

  • APrimary amines have two bands, secondary amines have one band, and tertiary amines have no NH stretch.
  • BPrimary amines have one band, secondary amines have two bands, and tertiary amines have three bands.
  • CPrimary amines have no bands, secondary amines have one band, and tertiary amines have two bands.
  • DPrimary and tertiary amines have intense single bands, while secondary amines have two weak bands.

Q14:

Ethers, esters, and alcohols all contain CO stretching bands. How can an ether be distinguished from the other two compounds?

  • AThe absence of CO and OH bands eliminates esters and alcohols.
  • BAlcohols and esters have split CO bands, while ethers show only one band.
  • CAlcohols and esters have very broad CO bands due to hydrogen bonding, while ethers have a very sharp band.
  • DThe CO band of an ether appears in a different region than that of an alcohol or ester.

Q15:

Shown here is the structure of (+)-carvone.

O

Using R/S and E/Z labels, assign the stereochemistry of the chiral center and stereoisomeric alkene group.

  • AChiral center = S, alkene = E
  • BChiral center = S, alkene = Z
  • CChiral center = R, alkene = Z
  • DChiral center = R, alkene = E

Which of the following infrared signals is most likely to correspond to stretching of the carbonyl CO bond?

Q16:

A compound has the following structure:

O O H O N H 2

Which of the following signals is least likely to be observed in the infrared spectrum of this compound?

  • A 1,800 cm−1
  • B 3,200 cm−1 (broad)
  • C 3,500 cm−1
  • D 1,650 cm−1
  • E 2,950 cm−1

Q17:

A compound has the following structure:

O O O H

Which of the following signals is least likely to be observed in the infrared spectrum of this compound?

  • A 1,775 cm−1
  • B 1,500 cm−1
  • C 3,300 cm−1
  • D 2,120 cm−1
  • E 1,850 cm−1

Q18:

A compound has the following structure:

O O H

Which of the following signals is least likely to be observed in the infrared spectrum of this compound?

  • A 3,000 cm−1
  • B 1,720 cm−1
  • C 1,650 cm−1
  • D 1,500 cm−1
  • E 1,850 cm−1

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.