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Lesson: Functional Groups in IR Spectra

In this lesson, we will learn how to identify the functional groups in a compound from the nature of the peaks in its infrared spectrum.

Worksheet • 15 Questions

Q1:

Within the 3 5 0 0 – 3 3 0 0 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 N H stretch.
  • BPrimary amines have one band, secondary amines have two bands, and tertiary amines have three bands.
  • CPrimary and tertiary amines have intense single bands, while secondary amines have two weak bands.
  • DPrimary amines have no bands, secondary amines have one band, and tertiary amines have two bands.

Q2:

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 = Z
  • BChiral center = S, alkene = E
  • 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 C O bond?

Q3:

In order for a compound to be IR active, it must undergo which of the following?

  • AA change in dipole moment
  • BMetal-ligand charge transfer
  • CA change in polarizability
  • DTransfer of an electon
  • EEmission of an electron

Q4:

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

  • AAlkynes > alkenes > alkanes
  • BAlkenes > alkanes > alkynes
  • CAlkanes > alkynes > alkenes
  • DAlkynes > alkanes > alkenes
  • EAlkenes > alkynes > alkanes

Q5:

An organic compound that is liquid at room temperature exhibits a strong IR absorption at 1 7 1 0 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 3 0 0 – 3 4 0 0 cm−1 range. To which of the following chemical families does the liquid most likely belong?

  • AKetones
  • BNitriles
  • CAldehydes
  • DAmines
  • EAlkenes

Q6:

A pure, colorless liquid with a high boiling point exhibited a strong IR absorption in the 3 3 0 0 – 3 4 0 0 cm−1 range. The only other strong IR signals outside of the fingerprint region can be assigned to C H resonances. To which of the following chemical families does the liquid most likely belong?

  • AAlcohols
  • BAlkenes
  • CThiols
  • DAlkanes
  • EAldehydes

Q7:

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

  • AThe C H stretch, occurring between 3 1 0 0 – 3 0 1 0 cm−1
  • BThe C H out-of-plane bend, occurring between 1 0 0 0 – 6 5 0 cm−1
  • CThe C C stretch, occurring at 1 6 6 0 – 1 6 0 0 cm−1

Q8:

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

  • AWavenumber of the IR stretch decreases
  • BWavenumber of the IR stretch increases
  • CWavelength of the IR stretch decreases
  • DWavelength and wavenumber of the IR stretch decrease

Q9:

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

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

Q10:

What of the following does not affect the C O stretching frequency in a metal-ligand complex?

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

Q11:

Which of the following statements about IR spectroscopy is false?

  • AAbsorptions due to C O bond stretching are generally weak.
  • BStretching frequencies are higher than corresponding bending frequencies.
  • CConjugation lowers the energy required to cause a bond to stretch.
  • DSingle bonds have lower stretching frequencies than triple bonds.

Q12:

A common organic chemical exhibits a strong IR absorption at 1 6 4 5 cm−1. Which of the following functional groups could have caused this absorption?

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

Q13:

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

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

Q14:

Hydrogen bonding causes the hydroxyl IR stretching band to do what?

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

Q15:

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

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